Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Deprecated spellings have been preserved.
Typographical and punctuation errors have been silently corrected.
ANCIENT
AND
MODERN ENGINEERING
AND
THE ISTHMIAN CANAL.
BY
WILLIAM H. BURR, C.E.,
_Professor of Civil-Engineering in Columbia University;
Member of the American Society of Civil Engineers and of
the Institution of Civil Engineers of Great Britain._
_FIRST EDITION._
FIRST THOUSAND.
NEW YORK:
JOHN WILEY & SONS.
LONDON: CHAPMAN & HALL, LIMITED.
1903.
Copyright, 1902,
BY
WILLIAM H. BURR
ROBERT DRUMMOND, PRINTER, NEW YORK.
INTRODUCTION.
This book is the outcome of a course of six lectures delivered at the
Cooper Union in the city of New York in February and March, 1902,
under the auspices of Columbia University. It seemed desirable by the
President of the University that the subject-matter of the lectures
should be prepared for ultimate publication. The six Parts of the
book, therefore, comprise the substance of the six lectures, suitably
expanded for the purposes of publication.
It may be interesting to state that the half-tone illustrations
have, with scarcely an exception, been prepared from photographs of
the actual subjects illustrated. All such illustrations in Parts V
and VI devoted to the Nicaragua and Panama Canal routes are made
from photographs at the various locations by members of the force of
the Isthmian Canal Commission; they are, therefore, absolutely true
representations of the actual localities to which they apply.
For other illustrations the author wishes to express his indebtedness
to Messrs. G. P. Putnam’s Sons, Messrs. Turneaure and Russell, John
Wiley & Sons, The Morrison-Jewell Filtration Company, Mr. H. M. Sperry,
Signal Engineer, _The Engineering News_, _The Railroad Gazette_, The
American Society of Civil Engineers, The Standard Switch and Signal
Company, The Baldwin Locomotive Works, the American Locomotive Works,
Mr. Clemens Herschel, and the International Pump Company, and to
others from whom the author has received courtesies which he deeply
appreciates.
The classification or division of the matter of the text, and the table
of contents, have been made so complete, with a view to convenience
even of the desultory reader in seeking any particular subject or
paragraph, that no index has been prepared, as it is believed that the
table of contents, as arranged, practically supplies the information
ordinarily given by a comprehensive index.
Complete and detailed treatments of the purely technical matters
covered by Part II will be found in the author’s “Elasticity and
Resistance of Materials” and in his “Stresses in Bridge and Roof
Trusses, Arched Ribs and Suspension Bridges.”
W. H. B.
COLUMBIA UNIVERSITY,
October 24, 1902.
CONTENTS.
PART I.
_ANCIENT CIVIL-ENGINEERING WORKS._
CHAPTER I.
ART. PAGE
1. Introductory 1
2. Hydraulic Works of Chaldea and Egypt 2
3. Structural Works in Chaldea and Egypt 4
4. Ancient Maritime Commerce 7
5. The Change of the Nile Channel at Memphis 8
6. The Pyramids 8
7. Obelisks, Labyrinths, and Temples 12
8. Nile Irrigation 13
9. Prehistoric Bridge-building 14
10. Ancient Brick-making 15
11. Ancient Arches 16
CHAPTER II.
12. The Beginnings of Engineering Works of Record 19
13. The Appian Way and other Roman Roads 20
14. Natural Advantages of Rome in Structural Stones 22
15. Pozzuolana Hydraulic Cement 24
16. Roman Bricks and Masonry 25
17. Roman Building Laws 27
18. Old Roman Walls 27
19. The Servian Wall 28
20. Old Roman Sewers 29
21. Early Roman Bridges 31
22. Bridge of Alcantara 35
23. Military Bridges of the Romans 35
24. The Roman Arch 36
CHAPTER III.
25. The Roman Water-supply 37
26. The Roman Aqueducts 38
27. Anio Vetus 39
28. Tepula 40
29. Virgo 40
30. Alsietina 40
31. Claudia 41
32. Anio Novus 42
33. Lengths and Dates of Aqueducts 42
34. Intakes and Settling-basins 43
35. Delivery-tanks 44
36. Leakage and Lining of Aqueducts 44
37. Grade of Aqueduct Channels 45
38. Qualities of Roman Waters 46
39. Combined Aqueducts 46
40. Property Rights in Roman Waters 46
41. Ajutages and Unit of Measurement 47
42. The Stealing of Water 49
43. Aqueduct Alignment and Design of Siphons 49
CHAPTER IV.
44. Antiquity of Masonry Aqueducts 52
45. Pont du Gard 52
46. Aqueducts at Segovia, Metz, and other Places 53
47. Tunnels 54
48. Ostia, the Harbor of Rome 56
49. Harbors of Claudius and Trajan 58
CHAPTER V.
50. Ancient Engineering Science 60
51. Ancient Views of the Physical Properties of Materials 61
52. Roman Civil Engineers Searching for Water 62
53. Locating and Designing Conduits 63
54. Siphons 64
55. Healthful Sites for Cities 65
56. Foundations of Structures 65
57. Pozzuolana and Sand 66
58. Lime Mortar 66
59. Roman Bricks according to Vitruvius 66
60. Roman Timber 67
61. The Rules of Vitruvius for Harbors 67
62. The Thrusts of Arches and Earth;
Retaining-walls and Pavements 68
63. The Professional Spirit of Vitruvius 68
64. Mechanical Appliances of the Ancients 69
65. Unlimited Forces and Time 69
PART II.
_BRIDGES._
CHAPTER VI.
66. Introductory 70
67. First Cast-iron Arch 70
68. Early Timber Bridges in America 71
69. Town Lattice Bridge 72
70. Howe Truss 74
71. Pratt Truss 76
72. Squire Whipple’s Work 77
73. Character of Work of Early Builders 77
CHAPTER VII.
74. Modern Bridge Theory 78
75. The Stresses in Beams 79
76. Vertical and Horizontal Shearing Stresses 80
77. Law of Variation of Stresses of Tension and Compression 82
78. Fundamental Formulæ of Theory of Beams 83
79. Practical Applications 85
80. Deflection 86
81. Bending Moments and Shears with Single Load 87
82. Bending Moments and Shears with any System of Loads 89
83. Bending Moments and Shears with Uniform Loads 92
84. Greatest Shear for Uniform Moving Load 94
85. Bending Moments and Shears for Cantilever Beams 96
86. Greatest Bending Moment with any System of Loading 97
87. Applications to Rolled Beams 99
CHAPTER VIII.
88. The Truss Element or Triangle of Bracing 100
89. Simple Trusses 101
90. The Pratt Truss Type 102
91. The Howe Truss Type 105
92. The Simple Triangular Truss 106
93. Through- and Deck-Bridges 108
94. Multiple Systems of Triangulation 108
95. Influence of Mill and Shop Capacity on Length of Span 109
96. Trusses with Broken or Inclined Chords 109
97. Position of any Moving Load for Greatest Webb Stress 110
98. Application of Criterions for both Chord and Web Stresses 111
99. Influence Lines 112
100. Influence Lines for Moments both for Beams and Trusses 113
101. Influence Lines for Shears both for Beams and Trusses 115
102. Application of Influence-line Method to Trusses 118
CHAPTER IX.
103. Lateral Wind Pressure on Trusses 122
104. Upper and Lower Lateral Bracing 124
105. Bridge Plans and Shopwork 125
106. Erection of Bridges 126
107. Statically Determinate Trusses 126
108. Continuous Beams and Trusses—Theorem of Three Moments 128
109. Application to Draw- or Swing-bridges 130
110. Special Method for Deflection of Trusses 130
111. Application of Method for Deflection of Triangular Frame 133
112. Application of Method for Deflection to Truss 134
113. Method of Least Work 137
114. Application of Method of Least Work to General Problem 138
115. Application of Method of Least Work to Trussed Beam 139
116. Removal of Indetermination by Methods of Least Work
and Deflection 141
CHAPTER X.
117. The Arched Rib, of both Steel and Masonry 142
118. Arched Rib with Ends Fixed 144
119. Arched Rib with Ends Jointed 144
120. Arched Rib with Crown and Ends Jointed 145
121. Relative Stiffness of Arched Ribs 145
122. General Conditions of Analysis of Arched Ribs 146
CHAPTER XI.
123. Beams of Combined Steel and Concrete 149
CHAPTER XII.
124. The Masonry Arch 154
125. Old and New Theories of the Arch 155
126. Stress Conditions in the Arch-ring 158
127. Applications to an Actual Arch 158
128. Intensities of Pressure in the Arch-ring 162
129. Permissible Working Pressures 163
130. Largest Arch Spans 163
CHAPTER XIII.
131. Cantilever and Stiffened Suspension Bridges 166
132. Cantilever Bridges 166
133. Stiffened Suspension Bridges 168
134. The Stiffening Truss 170
135. Location and Arrangement of Stiffening Trusses 171
136. Division of Load between Cables and Stiffening Truss 173
137. Stresses in Cables and Moments and Shears in Trusses 174
138. Thermal Stresses and Moments in Stiffened
Suspension Bridges 175
139. Formation of the Cables 176
140. Economical Limits of Spans 177
PART III.
_WATER-WORKS FOR CITIES AND TOWNS._
CHAPTER XIV.
141. Introductory 179
142. First Steam-pumps 180
143. Water-supply of Paris and London 181
144. Early Water-pipes 181
145. Earliest Water-supplies in the United States 182
146. Quality and Uses of Public Water-supply 182
147. Amount of Public Water-supply 183
148. Increase of Daily Consumption and the Division
of that Consumption 183
149. Waste of Public Water 186
150. Analysis of Reasonable Daily Supply per Head of Population 188
151. Actual Daily Consumption in Cities of the United States 189
152. Actual Daily Consumption in Foreign Cities 191
153. Variations in Rate of Daily Consumption 192
154. Supply of Fire-streams 193
CHAPTER XV.
155. Waste of Water, Particularly in the City of New York 196
156. Division of Daily Consumption in the City of New York 197
157. Daily Domestic Consumption 198
158. Incurable and Curable Wastes 199
159. Needless and Incurable Waste in City of New York 200
160. Increase in Population 200
161. Sources of Public Water-supplies 202
162. Rain-gauges and their Records 204
163. Elements of Annual and Monthly Rainfall 204
164. Hourly or Less Rates of Rainfall 207
165. Extent of Heavy Rain-storms 207
166. Provision for Low Rainfall Years 208
167. Available Portion of Rainfall or Run-off of Watersheds 209
168. Run-off of Sudbury Watershed 211
169. Run-off of Croton Watershed 211
170. Evaporation from Reservoirs 213
171. Evaporation from the Earth’s Surface 215
CHAPTER XVI.
172. Application of Fitzgerald’s Results to the Croton Watershed 216
173. The Capacity of the Croton Watershed 217
174. Necessary Storage for New York Supply to Compensate
for Deficiency 218
175. No Exact Rule for Storage Capacity 220
176. The Color of Water 221
177. Stripping Reservoir Sites 222
178. Average Depth of Reservoirs should be as
Great as Practicable 224
179. Overturn of Contents of Reservoirs Due to
Seasonal Changes of Temperature 224
180. The Construction of Reservoirs 225
181. Gate-houses, and Pipe-lines in Embankments 229
182. High Masonry Dams 230
CHAPTER XVII.
183. Gravity Supplies 234
184. Masonry Conduits 234
185. Metal Conduits 236
186. General Formula for Discharge of Conduits—Chezy’s Formula 237
187. Kutter’s Formula 239
188. Hydraulic Gradient 241
189. Flow of Water in Large Masonry Conduits 244
190. Flow of Water through Large Closed Pipes 245
191. Change of Hydraulic Gradient by Changing Diameter of Pip 250
192. Control of Flow by Gates at Upper End of Pipe-line 251
193. Flow in Old and New Cast-iron Pipes—Tubercles 251
194. Timber-stave Pipes 253
CHAPTER XVIII.
195. Pumping and Pumps 254
196. Resistances of Pumps and Main—Dynamic Head 258
197. Duty of Pumping-engines 260
198. Data to be Observed in Pumping-engine Tests 261
199. Basis of Computations for Duty 262
200. Heat-units and Ash in 100 Pounds of Coal,
and Amount of Work Equivalent to a Heat-unit 262
201. Three Methods of Estimating Duty 265
202. Trial Test and Duty of Allis Pumping-engine 265
203. Conditions Affecting Duty of Pumping-engines 266
204. Speeds and Duties of Modern Pumping-engines 266
CHAPTER XIX.
205. Distributing-reservoirs and their Capacities 267
206. System of Distributing Mains and Pipes 268
207. Diameters of and Velocities in Distributing Mains and Pipes 269
208. Required Pressures in Mains and Pipes 270
209. Fire-hydrants 270
210. Elements of Distributing Systems 270
CHAPTER XX.
211. Sanitary Improvement of Public Water-supplies 276
212. Improvement by Sedimentation 277
213. Sedimentation Aided by Chemicals 279
214. Amount of Solid Matter Removed by Sedimentation 279
215. Two Methods of Operating Sedimentation-basins 279
216. Sizes and Construction of Settling-basins 280
217. Two Methods of Filtration 281
218. Conditions Necessary for Reduction of Organic Matter 282
219. Slow Filtration through Sand—Intermittent Filtration 283
220. Removal of Bacteria in the Filter 286
221. Preliminary Treatment—Sizes of Sand Grains 286
222. Most Effective Sizes of Sand Grains 288
223. Air and Water Capacities 288
224. Bacterial Efficiency and Purification—Hygienic Efficiency 290
225. Bacterial Activity near Top of Filter 290
226. Rate of Filtration 291
227. Effective Head on Filter 291
228. Constant Rate of Filtration Necessary 292
229. Scraping of Filters 293
230. Introduction of Water to Intermittent Filters 294
231. Effect of Low Temperature 294
232. Choice of Intermittent or Continuous Filtration 294
233. Size and Arrangement of Slow Sand Filters 295
234. Design of Filter-beds 296
235. Covered Filters 299
236. Clear-water Drain-pipes of Filters 299
237. Arrangement of the Sand at Lawrence and Albany 300
238. Velocity of Flow through Sand 302
239. Frequency of Scraping and Amount Filtered between Scrapings 303
240. Cleaning the Clogged Sand 303
241. Controlling or Regulating Apparatus 305
242. Cost of Slow Sand Filters 307
243. Cost of Operation of Albany Filter 308
244. Operation and Cost of Operation of Lawrence Filter 309
245. Sanitary Results of Operation of Lawrence
and Albany Filters 310
246. Rapid Filtration with Coagulants 311
247. Operation of Coagulants 312
248. Principal Parts of Mechanical Filter-plant—Coagulation
and Subsidence 313
249. Amount of Coagulant—Advantageous Effect of Alum
on Organic Matter 314
250. High Heads and Rates for Rapid Filtration 315
251. Types and General Arrangement of Mechanical Filters 316
252. Cost of Mechanical Filters 318
253. Relative Features of Slow and Rapid Filtration 318
PART IV.
_SOME FEATURES OF RAILROAD ENGINEERING._
CHAPTER XXI.
254. Introductory 320
255. Train Resistances 322
256. Grades 322
257. Curves 324
258. Resistance of Curves and Compensation in Grades 324
259. Transition Curves 325
260. Road-bed, including Ties 327
261. Mountain Locations of Railroad Lines 328
262. The Georgetown Loop 331
263. Tunnel-loop Location, Rhætian Railways, Switzerland 331
CHAPTER XXII.
264. Railroad Signalling 335
265. The Pilot Guard 335
266. The Train-Staff 335
267. First Basis of Railroad Signalling 336
268. Code of American Railway Association 337
268_a_. The Block 338
269. Three Classes of Railroad Signals 338
270. The Banner Signal 338
271. The Semaphore 340
272. Colors for Signalling 340
273. Indications of the Semaphore 341
274. General Character of Block System 342
275. Block Systems in Use 343
276. Locations of Signals 344
277. Home, Distant, and Advance Signals 344
278. Typical Working of Auto-controlled Manual System 345
279. General Results 348
280. Distant Signals 349
281. Function of Advance Signals 349
282. Signalling at a Single-track Crossing 350
283. Signalling at a Double-track Crossing 352
284. Signalling for Double-track Junction and Cross-over 352
285. General Observations 353
286. Interlocking-machines 354
287. Methods of Applying Power in Systems of Signalling 357
288. Train-staff Signalling 358
CHAPTER XXIII.
289. Evolution of the Locomotive 363
290. Increase of Locomotive Weight and Rate
of Combustion of Fuel 365
291. Principal Parts of a Modern Locomotive 366
292. The Wootten Fire-box and Boiler 367
293. Locomotives with Wootten Boilers 370
294. Recent Improvements in Locomotive Design 372
295. Compound Locomotives with Tandem Cylinders 373
296. Evaporative Efficiency of Different Rates of Combustion 375
296_a_. Tractive Force of a Locomotive 376
297. Central Atlantic Type of Locomotive 378
298. Consolidation Engine, N. Y. C. & H. R. R. R. 379
299. P., B. & L. E. Consolidation Engine 380
300. L. S. & M. S. Fast Passenger Engine 381
301. Northern Pacific Tandem Compound Locomotive 382
302. Union Pacific Vauclain Compound Locomotive 384
303. Southern Pacific Mogul with Vanderbilt Boiler 384
304. The “Soo” Decapod Locomotive 385
305. The A., T. & S. F. Decapod, the Heaviest Locomotive
yet Built 386
306. Comparison of Some of the Heaviest Locomotives in Use 389
PART V.
_THE NICARAGUA ROUTE FOR A SHIP-CANAL._
307. Feasibility of Nicaragua Route 390
308. Discovery of Lake Nicaragua 390
309. Early Maritime Commerce with Lake Nicaragua 391
310. Early Examination of Nicaragua Route 392
311. English Invasion of Nicaragua 392
312. Atlantic and Pacific Ship-canal Company 392
313. Survey and Project of Col. O. W. Childs 393
314. The Project of the Maritime Canal Company 393
315. The Work of the Ludlow and Nicaragua Canal Commissions 394
316. The Route of the Isthmian Canal Commission 395
317. Standard Dimensions of Canal Prism 396
318. The San Juan Delta 397
319. The San Carlos and Serapiqui Rivers 398
320. The Rapids and Castillo Viejo 399
321. The Upper San Juan 399
322. The Rainfall from Greytown to the Lake 399
323. Lake-surface Elevation and Slope of the River 400
324. Discharges of the San Juan, San Carlos, Serapiqui 401
325. Navigation on the San Juan 401
326. The Canal Line through the Lake and Across the West Side 402
327. Character of the Country West of the Lake 403
328. Granada to Managua, thence to Corinto 404
329. General Features of the Route 404
330. Artificial Harbor at Greytown 405
331. Artificial Harbor at Brito 407
332. From Greytown Harbor to Lock No. 2 408
333. From Lock No. 2 to the Lake 409
334. Fort San Carlos to Brito 410
335. Examinations by Borings 411
336. Classification and Estimate of Quantities 412
337. Classification and Unit Prices 413
338. Curvature of the Route 413
339. The Conchuda Dam and Wasteway 414
340. Regulation of the Lake Level 417
341. Evaporation and Lockage 418
342. The Required Slope of the Canalized River Surface 419
343. All Surplus Water to be Discharged over the Conchuda Dam 419
344. Control of the Surface Elevation of the Lake 420
345. Greatest Velocities in Canalized River 425
346. Wasteways or Overflows 427
347. Temporary Harbors and Service Railroad 427
348. Itemized Statement of Length and Cost 427
PART VI.
_THE PANAMA ROUTE FOR A SHIP-CANAL._
349. The First Panama Transit Line 429
350. Harbor of Porto Bello Established in 1597 429
351. First Traffic along the Chagres River, and
the Importance of the Isthmian Commerce 431
352. First Survey for Isthmian Canal Ordered in 1520 431
353. Old Panama Sacked by Morgan and the Present City Founded 431
354. The Beginnings of the French Enterprise 432
355. The Wyse Concession and the International Congress of 1870 432
356. The Plan without Locks of the Old Panama Canal Company 433
357. The Control of the Floods in the Chagres 434
358. Estimate of Time and Cost—Appointment of Liquidators 435
359. The “Commission d’Etude” 435
360. Extensions of Time for Completion 436
361. Organization of the New Panama Canal Company, 1894 437
362. Priority of the Panama Railroad Concession 437
363. Resumption of Work by the New Company—The Engineering
Commission and the Comité Technique 438
364. Plan of the New Company 439
365. Alternative Plan of the New Panama Canal Company 440
366. The Isthmian Canal Commission and its Work 441
367. The Route of the Isthmian Canal Commission that of
the New Panama Canal Company 441
368. Plan for a Sea-level Canal 443
369. Colon Harbor and Canal Entrance 443
370. Panama Harbor and Entrance to Canal 444
371. The Route from Colon to Bohio 445
372. The Bohio Dam 446
373. Variation in Surface Elevation of Lake 448
374. The Extent of Lake Bohio and the Canal Line in It 448
375. The Floods of the Chagres 449
376. The Gigante Spillway or Waste-weir 450
377. Storage in Lake Bohio for Driest Dry Season 451
378. Lake Bohio as a Flood Controller 452
379. Effect of Highest Floods on Current in Channel
in Lake Bohio 453
380. Alhajuela Reservoir not Needed at Opening of Canal 453
381. Locks on Panama Route 454
382. The Bohio Locks 454
383. The Pedro Miguel and Miraflores Locks 454
384. Guard-gates near Obispo 455
385. Character and Stability of the Culebra Cut 455
386. Length and Curvature 456
387. Small Diversion-channels 457
388. Principal Items of Work to be Performed 457
389. Lengths of Sections and Elements of Total Cost 458
390. The Twenty Per Cent Allowances for Exigencies 459
391. Value of Plant, Property, and Rights on the Isthmus 460
392. Offer of New Panama Coal Company to Sell for $40,000,000 461
393. Annual Costs of Operation and Maintenance 462
394. Volcanoes and Earthquakes 463
395. Hygienic Conditions on the Two Routes 464
396. Time of Passage Through the Canal 465
397. Time for Completion on the Two Routes 466
398. Industrial and Commercial Value of the Canal 469
399. Comparison of Routes 471
PART I.
_ANCIENT CIVIL-ENGINEERING WORKS._
CHAPTER I.
=1. Introductory.=—It is a common impression even among civil engineers
that their profession is of modern origin, and it is frequently called
the youngest of the professions. That impression is erroneous from
every point of view. Many engineering works of magnitude and of great
importance to the people whom they served were executed in the very
dawn of history, and they have been followed by many other works of at
least equal magnitude and under circumstances scarcely less noteworthy,
of which we have either remains or records. During the lapse of the
arts and of almost every process of civilization throughout the
darkness of the Middle Ages there was little if any progress made
in the art of the engineer, and what little was done was executed
almost entirely under the name of architecture. With the revival of
intellectual activity and with the development of science the value
of its practical application to the growing nations of the civilized
world caused the modern profession of civil-engineering to take
definite shape and to be known by the name which it now carries, but
which was not known to ancient peoples. Unfortunately the beginnings
of engineering cannot be traced; there is no historical record running
back far enough to render account of the earliest engineering works
whose ruins remain as enduring evidence of what was then accomplished.
It is probably correct to state that the material progress of any
people has always been concurrent with the development of the
art of civil-engineering, and, hence, that the practice of civil
engineering began among the people who made the earliest progress in
civilization, to whom “the art of directing the Great Sources of Power
in Nature for the use and convenience of man” became an early and
imperative necessity. Indeed that conclusion is confirmed by the most
ancient ruins of what may be termed public works that archæological
investigations have revealed to us, among which are those to be found
in the Chaldean region, in India, and in Egypt. Obviously, anything
like a detailed account of the structural and other works of such
ancient character must be lacking, as some of them were built before
even the beginnings of history. Our only data, therefore, are the
remains of such works, and unfortunately they have too frequently been
subject to the destructive operations of both man and nature.
=2. Hydraulic Works of Chaldea and Egypt.=—It is absolutely certain
that the populous centres of prehistoric times could not have existed
nor have been served with those means of communication imperatively
necessary to their welfare without the practice of the art of
engineering, under whatever name they may have applied to it. It is
known beyond any doubt that the anciently populous and prosperous
country at the head of the Persian Gulf and watered by the Euphrates
and the Tigris was irrigated and served by a most complete system
of canals, and the same observation can be made in reference to the
valley of the Nile. It is not possible at this period of that country’s
history to determine to what extent irrigation was practised or how
extensively the former country was served by water transportation
conducted along artificial channels; but hydraulic works, including
dams and sluices with other regulating appliances designed to bring
waters from the rivers on to the land, were certainly among the
earliest executed for the benefit of the communities inhabiting those
regions. The remains of those works, spread over a large territory
in the vicinity of ancient Babylon, Nippur, and other centres of
population, show beyond the slightest doubt that there existed a
network of water communication throughout what was in those days
a country rich in agricultural products and which supported the
operations of a most prosperous commerce. These canals were of ample
dimensions to float boats of no mean size, although much smaller than
those occupied in our larger systems of canal transportation. They
were many miles in length, frequently interlacing among themselves
and intersecting both the Tigris and the Euphrates. The remains of
these canals, some of them still containing water, show that they must
originally have been filled to depths varying from five or six to
fifteen or twenty feet, and that their widths may have been twenty-five
or thirty feet or more. Another curious feature is their occasional
arrangement in twos and threes alongside of each other with embankments
only between. The entire Euphrates-Tigris valley from the head of the
Persian Gulf at least to modern Baghdad (i.e., Babylonia) and possibly
to ancient Nineveh was served by these artificial waterways. Later,
when Alexander the Great made one of his victorious expeditions through
the Assyrian country, he found in the Tigris obstructions to the
passage of his ships down-stream in the shape of masonry dams. This was
between 356 and 322 B.C. These substantial dams were built across the
river for the purpose of intakes to irrigating-canals for the benefit
of the adjacent country. These canals, like those of Egypt, were fitted
with all the necessary regulating-devices of sluices or gates, both
of a crude character, but evidently sufficiently effective for their
purpose.
[Illustration]
It is known that there were in those early days interchanges of large
amounts and varieties of commodities, and it is almost if not quite
certain that the countries tributary to the Persian Gulf not only
produced sufficient grain for their own needs, but also carried on
considerable commerce with the Asiatic coast. We have no means of
ascertaining either the volume or the precise character of the traffic,
but there is little or no doubt of its existence. It is established
also that the waters of the Red Sea and the Nile were connected by a
canal about 1450 B.C. Recent investigations about Nippur and other
sites of ancient cities in that region confirm other indications that
the practice of some branches of hydraulic engineering had received
material development from possibly two to four thousand years before
the Christian era.
=3. Structural Works in Chaldea and Egypt.=—The ruins of ancient
buildings which have been unearthed by excavations in the same vicinity
show with the same degree of certainty that the art of constructing
buildings of considerable dimensions had also made material progress
at the same time, and in many cases must have involved engineering
considerations of a decided character both as to structural materials
and to foundations. Bricks were manufactured and used. Stones were
quarried and dressed for building purposes and applied so as to
produce structural results of considerable excellence. Even the arch
was probably used to some extent in that locality in those early
days, but stone and timber beams were constantly employed. In the
prehistoric masonry constructions of both the Egyptians and Chaldeans
and probably other prehistoric peoples, lime or cement mortar was not
employed, but came into use at a subsequent period when the properties
of lime and cement as cementing materials began to be recognized. The
first cementing material probably used in Egypt was a sticky clay, or
possibly a calcareous clay or earth. The same material was also used
in the valley of the Euphrates, but in the latter country there are
springs of bitumen, where that material exudes from the earth in large
quantities. The use of this asphaltic cement at times possibly involved
that of sand or gravel in some of the early constructions. Later,
lime mortar and possibly a weak hydraulic cement came to be employed,
although there is little if any evidence of the latter material.
Iron was manufactured and used at least in small quantities, and for
some structural purposes, even though in a crude manner. Bituminous
or other asphaltic material was found as a natural product at various
points, and its value for certain structural purposes was well known;
it was used both for waterproofing and for cement. It is practically
certain that the construction of engineering works whose interesting
ruins still remain involved a considerable number of affiliated
engineering operations of which no evidence has yet been found, and of
the employment of tools and appliances of which we have no record. So
far as these works were of a public character they were constructed
by the aid of a very different labor system from that now existing.
The kings or ruling potentates of those early times were clothed with
the most arbitrary authority, sometimes exercised wisely in the best
interests of their people, but at other times the ruling motive was
selfishness actuated by the most intense egotism and brutal tyranny.
Hence all public works were executed practically as royal enterprises
and chiefly by forced labor, perhaps generally without compensation
except mere sustenance. Under such conditions it was possible to
construct works on a scale out of all proportion to national usefulness
and without structural economy. When it is remembered that these
conditions existed without even the shadow of engineering science, it
is obvious that structural economy or the adaptation of well-considered
means to an end will not be found to characterize engineering
operations of prehistoric times. Nevertheless there are evidences of
good judgment and reasonable engineering design found in connection
with some of these works, particularly with those of an hydraulic
character. Water was lifted or pumped by spiral or screw machines and
by water-wheels, and it is not improbable that other appliances of
power served the purposes of many industrial and crude manufacturing
operations which it is now impossible for us to determine.
[Illustration: FIG. 1.—Home Built on Piles in the Land of Punt.]
It is an interesting fact that while many ancient works were
exceedingly massive, like the pyramids, the largest of those of which
the ruins have been preserved seldom seem to show little or any
evidence of serious settlement. Whether the ancients had unusually
sound ideas as to the design of foundation works, or whether those only
have come down to us that were founded directly upon rock, we have
scarcely any means of deciding. Nor can we determine at this time what
special recourses were available for foundation work on soft ground.
Probably one of the earliest recognized instances, if not the earliest,
of the building of structures on piles is that given by Sir George
Rawlinson, when he states that a fleet of merchant vessels sent down
the northeast African coast by the Egyptian queen Hatasu, probably 1700
B.C. or 1600 B.C., found a people whose huts were supported on piles in
order to raise them above the marshy ground and possibly for additional
safety. A representation (Fig. 1) of one of these native homes on piles
is found among Egyptian hieroglyphics of the period of Queen Hatasu.
=4. Ancient Maritime Commerce.=—It is well known that both the Chaldean
region and the Nile valley and delta, at least from Ethiopia to the
Mediterranean Sea, were densely populated during the period of two to
four or five thousand years before the Christian era. By means of the
irrigation works to which reference has already been made both lands
became highly productive, and it is also well known that those peoples
carried on a considerable commerce with other countries, as did the
Phœnicians also, at least between the innumerable wars which seemed
to be the main business of states in those days. These commercial
operations required not only the construction of fleets of what seem
to us small vessels for such purposes, but also harbor-works at least
suitable to the vessels then in use. The marine activity of the
Phœnicians is undoubted, and there is strong reason to believe that
there was also similar activity between Babylonian ports and those east
of them along the shores of the Indian Ocean, perhaps even as far as
ancient Cathay, and possibly also to the eastern coast of Africa.
Investigations in the early history of Egypt have shown that a
Phœnician fleet, constructed at some Egyptian port on the Red Sea,
undoubtedly made the complete circuit of Africa and returned to Egypt
through the Mediterranean Sea the third year after setting out, over
2100 years (about 600 B.C.) before the historic fleet of the Portuguese
explorer Vasco da Gama sailed the same circuit in the opposite
direction. It is therefore probable, in view of these facts, that at
least simple harbor-works of sufficient efficiency for those early days
found place in the public works of the ancient kingdoms bordering upon
the Mediterranean and Red seas and the Persian Gulf.
=5. The Change of the Nile Channel at Memphis.=—Although such obscure
accounts as can be gathered in connection with the founding of the
city of Memphis are so shadowy as to be largely legendary, it has been
established beyond much if any doubt that prior to its building the
reigning Egyptian monarch determined to change the course of the Nile
so as to make it flow on the easterly side of the valley instead of the
westerly. This was for the purpose of securing ample space for his city
on the west of the river, and, also, that the latter might furnish a
defence towards the east, from which direction invading enemies usually
approached. He accordingly formed an immense dam or dike across the
Nile as it then existed, and compelled it to change its course near
the foot of the Libyan Hills on the west and seek a new channel nearer
the easterly side of the valley. This must have been an engineering
work of almost appalling magnitude in those early times, yet even with
the crude means and limited resources of that early period, possibly,
if not probably, at least 5000 B.C., the work was successfully
accomplished.
=6. The Pyramids.=—Among the most prominent ancient structural works
are the pyramids of Egypt, those royal tombs of which so much has been
written. These are found chiefly in the immediate vicinity of Memphis
on the Nile. There are sixty or seventy of them in all, the first of
which was built by the Egyptian king Khufu and is known as the “Great
Pyramid” or the “First Pyramid of Ghizeh.” They have been called “the
most prodigious of all human constructions.” Their ages are uncertain,
but they probably date from about 4000 B.C. to about 2500 B.C. These
are antedated, however, by two Egyptian pyramidal constructions of
still more ancient character whose ages cannot be determined, one at
Meydoum and the other at Saccarah.
[Illustration: A Corner of the Great Pyramid.
(Copyright by S. S. McClure Co., 1902. Courtesy of _McClure’s
Magazine_.)]
[Illustration: FIG. 2.—Section of the Great Pyramid.]
[Illustration: FIG. 3.]
The pyramids at Memphis are constructed of limestone and granite, the
latter being the prominent material and used entirely for certain
portions of the pyramids where the stone would be subjected to severe
duty. The great mass of most of the pyramids consists of roughly hewn
or squared blocks with little of any material properly considered
mortar. The interior portions, especially of the later pyramids, were
sometimes partially composed of chips, rough stones, mud bricks, or
even mud, cellular retaining-walls being used in the latter cases for
the main structural features. In all pyramids, however, the outer or
exposed surfaces and the walls and roofs of all interior chambers were
finished with finely jointed large stones, perhaps usually polished.
The Great Pyramid has a square base, which was originally 764 feet on a
side, with a height of apex above the surface of the ground of over 480
feet. This great mass of masonry contains about 3,500,000 cubic yards
and weighs nearly 7,000,000 tons. The area of its base is 13.4 acres.
The Greek historian Herodotus states that its construction required the
labor of 100,000 men for twenty years. An enormous quantity of granite
was required to be transported about 500 miles down the Nile from the
quarries at Syene. Some of the blocks at the base are 30 feet long with
a cross-section of 5 feet by 4 or 5 feet. The bulk of the entire mass
is of comparatively small stones, although so squared and dressed as
to fit closely together. Familiar descriptions of this work have told
us that the small passages leading from the exterior to the sepulchral
chambers are placed nearly in a vertical plane through the apex. The
highest or king’s chamber, as it is called, measures 34 feet by 17
feet and is 19 feet high, and in it is placed the sarcophagus of King
Khufu. It is composed entirely of granite most exactly cut and fitted
and beautifully polished. The construction of the roof is remarkable,
as it is composed of nine great blocks “each nearly 19 feet long and 4
feet wide, which are laid side by side upon the walls so as to form a
complete ceiling.” There is a singular feature of construction of this
ceiling designed to remove all pressure from it and consisting of five
alternate open spaces and blocks of granite placed in vertical series,
the highest open space being roofed over with inclined granite slabs
leaning or strutted against each other like the letter V inverted.
This arrangement relieves the ceiling of the sepulchral chamber from
all pressure; indeed only the inclined highest set of granite blocks
or slabs carry any load besides their own weight. There are two small
ventilating- or air-shafts running in about equally inclined directions
upward from the king’s chamber to the north and south faces of the
pyramid. These air-shafts are square and vary between 6 and 9 inches on
a side. The age of this pyramid is probably not far from 5000 years.
[Illustration: Entrance to the Great Pyramid.]
The second pyramid is not much inferior in size to the Great Pyramid,
its base being a square of about 707 feet on a side, and its height
about 454 feet. The remaining pyramids are much inferior in size,
diminishing to comparatively small dimensions, and of materials much
inferior to those used in the earlier and larger pyramids.
=7. Obelisks, Labyrinths, and Temples.=—Among other constructions of
the Egyptians which may be called engineering in character, as well
as architectural, are the obelisks, the “Labyrinth” so called, on the
shore of Lake Mœris, and the magnificent temples at the ancient capital
Thebes, which are the most remarkable architectural creations probably
that the world has ever known. These latter were not completed by one
king, as was each of the pyramids. They were sometimes despoiled and
largely wrecked by invading hosts from Assyria, and then reconstructed
in following periods by successive Egyptian kings and again added
to by still subsequent monarchs, whose reigns were characterized
by statesmanship, success in war, and prosperity in the country.
Their construction conclusively indicates laborious operations and
transportation of great blocks of stone characteristic of engineering
development of the highest order for the days in which they took place.
The dates of these constructions are by no means well defined, but they
extend over the period running from probably about 2500 B.C. to about
400 B.C., with the summit of excellence about midway between.
Another class of ancient structures which can receive but a passing
notice, although it deserves more, is the elaborate rock tombs of some
of the old Egyptian monarchs in the rocks of the Libyan Hills. They
were very extensive constructions and contained numerous successions
of “passages, chambers, corridors, staircases, and pillared halls,
each further removed from the entrance than the last, and all covered
with an infinite number of brilliant paintings.” These tombs really
constituted rock tunnels with complicated ramifications which must have
added much to the difficulty of the work and required the exercise of
engineering skill and resources of a high order.
[Illustration]
=8. Nile Irrigation.=—The value of the waters of the Nile for
irrigation and fertilization were fully appreciated by the ancient
Egyptians. They also apparently realized the national value of some
means of equalizing the overflow, although the annual régimen of
the Nile was unusually uniform. There were, however, periods of
great depression throughout the whole Nile valley consequent upon
the phenomenal failure of overflow to the normal extent. One of the
earliest monarchs who was actuated by a fine public spirit undertook to
solve the problem of providing against such depressions by diverting
a portion of the flood-waters of the Nile into an enormous reservoir,
so that during seasons of insufficient inundation the reservoir-waters
could be drawn upon for the purpose of irrigation. This monarch is
known as the good Amenemhat, although the Greeks call him Mœris. In
the Nile valley, less than a hundred miles above Memphis, on the
left side or to the west of the river, there is a gap in the Libyan
Hills leading to an immense depression, the lower parts of which are
much below the level of the water in the Nile. This topographical
depression, perhaps 50 miles in length by 30 in breadth, with an area
between 600 and 700 square miles, now contains two bodies of water or
lakes, one known as the Birket Keroun and the other as Lake Mœris.
The vicinity of this depression is called the Fayoum. A narrow rocky
gorge connects it with the west branch of the Nile, known as Bahr el
Yousuf, and it is probable that during extreme high water in the Nile
there was a natural overflow into the Fayoum. The good Amenemhat, with
the judgment of an engineer, or guided by advisers who possessed that
judgment, appreciated the potential value of this natural depression
as a possible reservoir for the surplus Nile waters and excavated a
channel, possibly a natural channel enlarged, of suitable depth from it
to the Bahr el Yousuf. As a consequence he secured a storage-reservoir
of enormous capacity and which proved of inestimable value to the
lowlands along the Nile in times of shortage in the river-floods.
Investigators have differed much in their conclusions as to the extent
of this reservoir. Some have maintained that only the lower depressions
of the Fayoum were filled for reservoir purposes, while others, like
Mr. Cope Whitehouse, believe that the entire depression of the Fayoum
was utilized with the exception of a few very high points, and that
the depth of water might have been as much as 300 feet in some places.
In the latter case the circuit of the lake would have been from 300 to
500 miles. Whatever may have been the size of the lake, however, its
construction and use with its regulating-works was a piece of hydraulic
engineering of the highest type, and it indicates an extraordinary
development of that class of operations for the period in which it was
executed. The exact date of this construction cannot be determined, but
it may have been as early as 2000 B.C., or perhaps earlier.
=9. Prehistoric Bridge-building.=—The development of the art of
bridge-building seems to have lagged somewhat in the prehistoric
period. The use of rafts and boats prevented the need of bridges for
crossing streams from being pressing. It is not improbable that some
small and crude pile or other timber structures of short spans were
employed, but no remains of this class of construction have been found.
Large quantities of timber and much of an excellent quality were used
in the construction of buildings. That much is known, but there is
practically no evidence leading to the belief that timber bridges of
any magnitude were used by prehistoric people. It is highly probable
that single-timber-beam crossings of small streams were used, but that
must be considered the limit of ancient bridging until other evidence
than that now available is found.
=10. Ancient Brick-making.=—It has already been seen that stone as a
building material has been used since the most ancient periods, and the
use of brick goes back almost as far. Fortunately it was frequently
a custom of the ancient brick-makers to stamp proprietary marks upon
their bricks, and we know by these marks that bricks were made in the
Chaldean regions certainly from 3000 to 4000 years before the Christian
era. In Egypt also the manufacture of brick dates back nearly or quite
as far. Some of these Chaldean bricks, as well as those in other parts
of the ancient world, were of poor quality, readily destroyed by water
or even a heavy storm of rain when driving upon them. Other bricks,
however, were manufactured of good quality of material and by such
methods as to produce results which compare favorably with our modern
building-bricks. The ruins of cities, at least in Assyria and Chaldea,
show that enormous buildings, many of them palaces of kings, were
constructed largely of these bricks, although they were elaborately
decorated with other material. The walls were heavy, indeed so massive
that many of the ruin-mounds are frequently formed almost entirely
of the disintegrated brick of poorer quality. These old builders not
only executed their work on a large scale, but did not hesitate to
pile up practically an artificial mountain of earth, or other suitable
material, on which to construct a palace or temple. The danger of
water to these native bricks was so well known and recognized that
elaborate and very excellent systems of subsurface drains or sewers
were frequently constructed to carry off the storm-water as fast as it
fell.
=11. Ancient Arches.=—In the practice of these building operations it
became necessary to form many openings and to construct roofs for the
sewers or drains, and the arch, both true and false, came to be used in
the Euphrates valley, in that of the Nile, and in other portions of the
ancient world. Pointed sewer-arches of brick have been found in what
is supposed to be the palace of Nimrod on the Tigris River, possibly
of the date about 1300 B.C. Excavations at Nippur have revealed a
mud-brick pointed arch supposed to date back to possibly 4000 B.C.
Also semicircular voussoir arches have been discovered at the ruins
of Khorsabad near Nineveh with spans of 12 to 15 feet. These arches
are supposed to belong to the reign of Sargon, an Assyrian king who
flourished about 705 to 722 B.C. Again, the ancient so-called treasury
of Atreus at Mycenæ in Greece, although a dome, exhibits an excellent
example of the method of forming the false arch, the date of the
construction being probably about 1000 B.C. The main portion of this
structure consists of a pointed dome, the diameter of the base being 48
feet and the interior central height 49 feet. A central section shows a
beehive shape, as in Fig. 6.
[Illustration: VAULTED DRAIN, KHORSABAD FIG. 4.]
[Illustration: VAULTED DRAINS, KHORSABAD. FIG. 5.]
The exterior approach is between two walls 20 feet apart, the
intermediate entrance to the dome or main chamber being a passage
9 feet 6 inches wide at the bottom and 7 feet 10 inches at the top
and about 19 feet high. At right angles to the entrance there is a
chamber 27 feet by 20 feet cut into the adjacent rock, entered through
a doorway about 4 feet 6 inches wide and 9 feet 6 inches high. Both
the main entrance to the dome and the doorway to the adjacent chamber
are covered or roofed with large flat lintel-stones, over which
are the triangular relieving (false) arches, so common in ancient
construction, by which the lintels are relieved of load, the triangular
openings being closed by single, great upright flat stones. There are a
considerable number of these in Greece. The stone used is a “hard and
beautiful breccia” from the neighboring hills and Mount Eubora near
by. The courses of stone are about two feet thick and closely fitted
without cement.
[Illustration: FIG. 6.—Plan and Section of the Treasury of Atreus at
Mycenæ.]
1. Plan of the Treasury of Atreus: _A_, rock-cut chamber, probably a
tomb; _B_, doorway; _C_, approach.
2. Section of the above: _B_, doorway; _C_, approach filled up with
earth; _D_, slope of the ground; _E_, wall on north side of approach;
_F_, lintel stone, weight 133 tons; _G_, door to rock-cut chamber.
The great majority, or perhaps all, of the Assyrian true arches, so
far discovered, are formed of wedge-shaped bricks, most of them being
semicircular, although some are pointed, the span being not over
about 15 feet. The most of the arches found at Nineveh and Babylon
belong to a period reaching possibly from 1300 to 800 B.C., but some
of the Egyptian arches are still older. Egyptians, Assyrians, Greeks,
and other ancient people used false arches formed by projecting each
horizontal course of stones or bricks over that below it on either side
of an opening. The repetition of this procedure at last brings both
sides of the opening together at the top of the arch, and they are
surmounted at that point with a single flat stone, brick, or tile. It
has been supposed by some that these false arches, whose sides may be
formed either straight or curved, exhibit the oldest form of the arch,
and that the true arch with its ring or rings of wedge-shaped voussoirs
was a subsequent development. It is possible that this is true, but
the complete proof certainly is lacking. In Egypt and Chaldea both
styles of arches were used concurrently, and it is probably impossible
to determine which preceded the other. Again, some engineers have
contended that two flat slabs of stone leaning against each other,
each inclined like the rafters of a roof, was the original form of the
arch, as found in the pyramids of Egypt; but it is probable that the
true arch was used in Chaldea prior to the time of the pyramids. Indeed
crude arches of brick have been found at Thebes in Egypt dating back
possibly to 2500 B.C., or still earlier. Aside from that, however,
such an arrangement of two stones is not an arch at all, either true
or false. The arrangement is simply a combination of two beams. A
condition of stress characteristic of that in the true arch is lacking.
The ancient character of the engineering works whose ruins are found
in Chaldea and Assyria is shown by the simple facts that Babylon was
destroyed about the year 690 B.C. and Nineveh about the year 606 B.C.
CHAPTER II.
=12. The Beginnings of Engineering Works of Record.=—In a later
period of the world’s history we reach a stage in the development of
engineering works of which we have both records and remains in such
well-defined shape that the characteristics of the profession may
be realized in a definite manner. This is particularly true of the
civil-engineering works of the Romans. In their sturdy and unyielding
character, with their limitless energy and resolution, the conditions
requisite for the execution of engineering works of great magnitude are
found. An effeminate or generally æsthetic nation like the Greeks would
furnish but indifferent opportunity for the inception and development
of great engineering works, but the resolute and vigorous Roman nation
offered precisely the conditions needed. They appreciated among other
things the absolute necessity of the freest possible communication
with the countries which they conquered and made part of their own
empire. They recognized water transportation as the most economical
and effective, and used it wherever possible. They also realized the
advantages of roads of the highest degree of solidity and excellence.
No other roads have ever been constructed so direct, so solid, and so
admirably adapted to their purposes as those built by the Romans. They
virtually ignored all obstacles and built their highways in the most
direct line practicable, making deep cuts and fills with apparently
little regard for those features which we consider obstacles of
sufficient magnitude to be avoided. They regarded this system of land
communication so highly that they made it radiate from the Golden
Mile-stone in the Roman Forum. The point from which radiated these
roads was therefore in the very centre of Roman life and authority,
and it fitly indicated the importance which the Roman government gave
to the system of communication that bound together with the strongest
bonds all parts of the republic and of the empire.
The design and construction of these roads must have been a matter to
which their constructors gave the most careful attention and study.
They were works involving principles deduced from the most careful
thought and extended experience. There were incorporated in them
the most effective materials of construction then known, and it was
evidently the purpose of their constructors that they should possess
indefinite endurance. The existence of some of them at the present
time, with no other attention given to them than required for ordinary
maintenance, demonstrates that the confidence of the builders was not
misplaced.
[Illustration: Street Fountain and Watering-trough in Pompeii. Called
the Fountain of Plenty, from the figure with Horn of Plenty on the
perforated upright post.]
=13. The Appian Way and other Roman Roads.=—Probably the oldest and
most celebrated of these old Roman roads is the Appian Way. It was the
most substantially built, and the breadth of roadway varied from 14
to 18 feet exclusive of the footwalks. Statius called it the Queen of
Roads. It was begun by Appius Claudius Cæcus, 312 years before the
Christian era. He carried its construction from the Roman gate called
Porta Capena to Capua, but it was not entirely completed till about the
year 30 B.C. Its total length was three hundred and fifty miles, and
it formed a perfect highway from Rome to Brundisium, an important port
on what may be called the southeastern point of Italy. It was built in
such an enduring manner that it appears to have been in perfect repair
as late as 500 to 565 A.D.
The plan of construction of these roads was so varied as to suit
local conditions, but only as required by sound engineering judgment.
They wisely employed local materials wherever possible, but did not
hesitate to transport proper material from distant points wherever
necessary. This seemed to be one of their fundamental principles of
road construction. In this respect the old Romans exhibited more
engineering and business wisdom than some of the American states in
the beginnings of improved road construction in this country. An
examination of the remains of some Roman roads now existing appears to
indicate that in earth the bottom of the requisite excavation was first
suitably compacted, apparently by ramming, although rollers may have
been used. On this compacted subgrade were laid two or three courses
of flat stones on their beds and generally in mortar. The second layer
placed on the preceding was rubble masonry of small stones or of coarse
concrete. On the latter was placed the third layer of finer concrete.
The fourth or surface course, consisting of close and nicely jointed
polygonal blocks, was then put in place, and formed an excellent
unyielding pavement. This resulted in a most substantial roadway,
sometimes exceeding 3 feet in total thickness. It is difficult to
conceive of a more substantial and enduring type of road construction.
The two lower layers were omitted when the road was constructed in
rock. Obviously the finer concrete constituting the second layer from
the top surface was a binder between the pavement surface and the
foundation of the roadway structure.
The paved part of a great road was usually about 16 feet in width,
and raised stone causeways or walls separated it from an unpaved way
on each side having half the width of the main or paved portion. This
seemed to be the type of the great or main Roman roads. Other highways
of less important character were constructed of inferior materials,
earth or clay sometimes being used instead of mortar; but in such cases
greater crowning was employed, and the road was more elevated, possibly
for better drainage. Then, as now, adequate drainage was considered one
of the first features of good road design. City streets were paved with
the nicely jointed polygonal blocks to which reference has already been
made, while the footways were paved with rectangular slabs much like
our modern sidewalks.
[Illustration: EXAMPLE OF EARLY BASALT ROAD.
BY THE TEMPLE OF SATURN ON THE CLIVUS CAPITOLINUS.
FIG. 7.]
The smooth polygonal pavements of the old Romans put to the keenest
shame the barbarous cobblestone street surfaces with which the people
of American cities have been and are still so tortured.
The beneficial influence of these old Roman highways has extended down
even to the present time in France, where some of them were built.
The unnecessarily elaborate construction has not been followed, but
the recognition of the public benefits of excellent roads has been
maintained. The lower course of the foundation-stones apparently began
to be set on edge toward the latter part of the eighteenth century, the
French engineer Tresaguet having adopted that practice in 1764. At the
same time he reduced the thickness of the upper layers. His methods
were but modifications of the old Roman system, and they prevailed in
France until the influence of the English engineers Macadam and Telford
began to be felt.
=14. Natural Advantages of Rome in Structural Stones.=—Although the
ancient Romans were born engineers, possessing the mental qualities and
sturdy character requisite for the analytic treatment and execution of
engineering problems, it is doubtful whether they would have attained
to such an advanced position in structural matters had not the city of
Rome been so favorably located.
The geological character of the great Roman plain and the Roman hills
certainly contributed most materially to the early development of
some of the most prominent of the Roman engineering works. The plain
surrounding the city of Rome is composed largely of alluvial and sandy
deposits, or of the emissions of neighboring volcanoes, of which the
Alban Hills form a group. While these and other volcanic hills in the
vicinity are, and have been for a long period, quiescent, they were
formerly in a very active state. The scoriæ, or matter emitted in
volcanic eruptions, is found there in all possible degrees of coherence
or solidity, from pulverulent masses to hard rock. The characteristic
Roman material called tufa is a mixture of volcanic ash and sand, loose
and friable, as dropped from the eruptions in large quantities or again
compressed into masses with all degrees of hardness. The hard varieties
of yellow or brown tufa form building material much used, although a
considerable percentage of it would not be considered fit building
material for structures of even moderate height at the present time.
The most of it weathers easily, but forms a fairly good building-stone
when protected by a coating of plaster or stucco.
Another class of building-stones found at or in the vicinity of Rome
is the so-called “peperino,” consisting chiefly of two varieties of
conglomerate of ash, gravel, broken pieces of lava, and pieces of
limestone, some possessing good weathering qualities, while others
do not. Ancient quarries of these stones exist whence millions of
cubic yards have been removed, and are still being worked. The better
varieties of “peperino” possess good resisting qualities, and were much
used in those portions of masonry construction where high resistance
was needed, as in the ring-stones of arches, heavily loaded points of
foundations, and other similar situations.
Some of the prehistoric masonry remains of the Romans show that their
earliest constructors appreciated intelligently the qualities of this
stone for portions of works where the duty was most severe.
Lava from the extinct volcanoes of the Alban Hills called “silex” was
used for paving roads and for making concrete. It was hard and of gray
color. At times considerable quantities of this stone were employed.
A species of pure limestone called “travertine,” of a creamy white
color, was quarried at Tibur or Tivoli, and began to be used about
the second century B.C. Vitruvius speaks of its having good weathering
qualities, but naturally it is easily calcined. Its structure is
crystalline, and it is strong in consequence of that quality only when
it is laid on its bed.
=15. Pozzuolana Hydraulic Cement.=—The most valuable of all building
materials of old Rome was the “pozzuolana,” as it furnished the basis
of a strong, enduring, and economic concrete, and permitted almost
an indefinite development of masonry construction. Had there not
been at Rome the materials ready at hand to be manufactured into an
excellent cementing product, it is highly probable that neither the
structural advance nor the commercial supremacy of the Roman people
could have been attained. It is at least certain that the majority
of the great masonry works constructed by the Romans could not have
been built without the hydraulic cementing material produced with so
little difficulty and in such large quantities from the volcanic earth
called pozzuolana. The name is believed to have its origin from the
large masses of this material at Pozzuoli near Naples. Great beds are
also found at and near Rome. The earliest date of its use cannot be
determined, but it has given that strong and durable character to Roman
concrete which has enabled Roman masonry to stand throughout centuries,
to the admiration of engineers.
It is a volcanic ash, generally pulverulent, of a reddish color, but
differs somewhat in appearance and texture according to the locality
from which it is taken. It consists chiefly of silicate of alumina, but
contains a little oxide of iron, alkali, and possibly other components.
The Romans therefore pulverized the pozzuolana and mixed it with lime
to make hydraulic cement. This in turn was mixed with sand and gravel
and broken stone to form mortar and concrete, and that process is
carried on to this day. The concrete was hand-mixed, and treated about
as it is at present. After having been well mixed the Romans frequently
deposited it in layers of 6 to 9 or 10 inches thick, and subjected
it to ramming. In connection with this matter of mortar and concrete
production, Vitruvius observes that pit-sand is preferable to either
sea or river sand.
=16. Roman Bricks and Masonry.=—The Romans produced bricks both by
sun-baking and by burning, although there are now remaining apparently
no specimens of the former in Rome. Bricks were used very largely for
facing purposes, such as a veneer for concrete work. The failure to
recognize this fact has led some investigators and writers into error.
As matter of fact bricks were used as a covering for concrete work, the
latter performing all the structural functions.
The old Roman aqueducts were frequently lined with concrete, made of a
mixture of pozzuolana, lime, and crushed (pounded) bricks or potsherds.
The same material was also used for floors under the fine mortar in
which the mosaics were imbedded.
Marble came into use in Rome about 100 B.C., from Luna, near modern
Carrara, Mt. Hymettus, and Mt. Pentelicus, near Athens and the Isle of
Paros, nearly all being for sculpture purposes. Colored and structural
marbles were brought from quarries in various parts of Italy, Greece,
Phrygia, Egypt, near Thebes (oriental alabaster or “onyx”), Arabia, and
near Damascus.
From the latter part of the first century B.C. the hard building-stones
like granites and basalts were brought to Rome in large quantities.
Most of the granites came from Philæ on the Nile. The basalts came both
from Lacedæmonia and Egypt. Both emery (from the island of Naxos in the
Ægean Sea) and diamond-dust drills were used in quarrying or working
these stones. Ships among the largest, if not the largest, of those
days, were built to transport obelisks and other large monoliths.
The quality of ancient Roman mortar varies considerably as it is now
found. That of the first and second centuries is remarkably hard, and
made with red pozzuolana. In the third century it began to be inferior
in quality, brown pozzuolana sometimes being used. The reason for this
difference in quality cannot be confidently assigned. The deterioration
noted in the third century work may be due to the introduction of bad
materials, or to the wrong manipulation of material intrinsically good,
or it is not unlikely the deterioration is due to a combination of
these two influences. The use of mortar indicates a class of early
construction; it is found in the Servian wall on the Aventine, of date
700 B.C., or possibly earlier.
[Illustration: Dovetail Wooden Tenon. Wooden Dowel. FIG. 8.]
Under the empire (27 B.C. to A.D. 475) large blocks of tufa, limestone
(travertine), or marble were set with very close joints, with either
no mortar or, if any, as thin as paper; end, top, and bottom clamps of
iron were used to bond such stones together. It was also customary, in
laying such large, nicely finished blocks of stone without mortar, to
use double dovetailed wooden ties, or, as in the case of columns, a
continuous central dowel of wood, as shown in the figures.
The joints were frequently so close as to give the impression that the
stones might have been fitted by grinding together. In rectangular
dimension stonework (ashlar) great care was taken, as at present, to
secure a good bond by the use of judiciously proportioned headers and
stretchers. Foundation courses were made thicker than the body of
the superincumbent wall, apparently to distribute foundation weights
precisely as done at present. Weaker stone was used in thicker portions
of walls, and strong stone in thinner portions. Also at points of
concentrated loading, piers or columns of strong stone are found built
into the bodies of walls of softer or weaker stone. Quarry chips,
broken lava, broken bricks, or other suitable refuse fragments were
used for concrete in the interest of economy, the broken material
always being so chosen as to possess a sharp surface to which the
cement would attach itself in the strongest possible bond.
At the quarries where the stones were cut the latter were marked
apparently to identify their places in the complete structure, or
for other purposes. The remains of the quarries themselves as seen
at present are remarkable both for their enormous extent and for the
system on which the quarrying was conducted. It appears that the
systems employed were admirably adapted to the character of the stone
worked, and that the quarrying operations were executed as efficiently
and with as sound engineering judgment as those employed in great
modern quarries.
=17. Roman Building Laws.=—So much depended upon the excellence of the
building in Rome, and upon the materials and methods employed, that
building laws or municipal regulations were enacted in the ancient
city, prescribing kind and quality of material, thickness of walls,
maximum height of buildings, minimum width of streets, and many other
provisions quite similar to those enacted in our modern cities. The
differences appear to arise from the different local conditions to
be dealt with, rather than from any failure on the part of the old
Romans to reach an adequate conception of the general plans suitable
for the masses of buildings in a great city. Prior to the great fire
A.D. 64 in Nero’s reign, an act prescribing fire-proof exterior
coverings of buildings was under consideration, and subsequently to
that conflagration it was enacted into law. Many of the city roads or
streets were paved with closely fitting irregular polygonal blocks of
basalt, laid on concrete foundations, and with limestone (travertine)
curbs and gutters, producing an effect not unlike our modern streets.
=18. Old Roman Walls.=—In no class of works did the ancient Romans show
greater engineering skill or development than in the massive masonry
structures that were built not only in and about the city of Rome, but
also in distant provinces under Roman jurisdiction. Among the home
structures various walls, constituting strong defences against the
attacks of enemies, stand in particular prominence. Some of these great
structures had their origin prior even to historic times. The so-called
“Wall of Romulus,” around the famous Roma Quadrata of the Palentine,
is among the latter. It is supposed by many that this wall formed the
primitive circuit of the legendary city of Romulus. That, however, is
an archæological and not an engineering question, and, whatever its
correct answer may be, the wall itself is a great engineering work;
it demonstrates that the early Romans, whatever may have been their
origin, had attained no little skill in quarrying and in the building
of dry masonry, no mortar being used in this ancient wall. Portions
of it 40 feet high and 10 feet thick at bottom, built against a rocky
hill, are still standing. The courses are 22 to 24 inches thick, and
they are laid as alternate headers and stretchers; the lengths of the
blocks being 3 to 5 feet, and the width from 19 to 22 inches. The ends
of the blocks are carefully worked and true, as are the vertical joints
in much of the wall, although some of the latter, on the other hand,
are left as much as 2 inches open.
Civil engineers, who are familiar with the difficulties frequently
experienced in laying up dry walls of considerable height, as evidenced
by many instances of failure probably within the knowledge of every
experienced engineer, will realize that this great dry masonry
structure must have been put in place by men of no little engineering
capacity. The rock is soft tufa, and marks on the blocks indicate that
chisels from ¼ to ¾ inch in width were used, as well as sharp-pointed
picks. In all cases the faces of the blocks were left undressed, i.e.,
in modern terms they were “quarry-faced.”
=19. The Servian Wall.=—Later in the history of Rome the great Servian
Wall, built chiefly by Servius Tullius to enclose the seven hills
of Rome, occupies a most prominent position as an engineering work.
Part of the wall, all of which belongs to the regal period (753 to
509 B.C.), is supposed to be earlier than Servius, and may have been
planned and executed by Tarquinius Priscus. A part only of the stones
of this wall were laid in cement mortar, and concrete was used, to
some extent at least, in its foundation and backing. The presence of
cement mortar in this structure differentiates it radically from the
wall of Romulus. Probably the discovery of pozzuolana cement, and
the fabrication of mortar and concrete from it, had been made in the
intervening period between the two constructions. Tufa, usually the
softer varieties but of varying degrees of hardness, was mostly used
in this wall, and the blocks were placed, as in the previous instance,
as alternate headers and stretchers in courses about two feet thick.
Portions of the wall 45 feet high and about 12 feet thick have been
uncovered. At points it was pierced with arched openings of 11 feet 5
inches span, possibly as embrasures for catapults or other engines of
war. The upper parts of these openings are circular arches with the
usual wedge-like ring-stones. The voussoirs were cut from peperino
stone. This wall, like that of Romulus, was constructed as a military
work of defence, and at some points it was built up from the bottom
of a wide foss 30 feet deep. At such places it was counterforted or
buttressed, a portion of wall 11 feet 6 inches long being found between
two counterforts, each of the latter being 9 feet wide and projecting 7
feet 9 inches out from the wall.
[Illustration: FIG. 9.—Part of Servian Wall on Aventine.]
[Illustration: FIG. 10.—Wall and Agger of Servius.]
=20. Old Roman Sewers.=—It is demonstrable by the writings of Vitruvius
and others that the old Romans, or at any rate the better educated of
them, possessed a correct general idea of some portions of the science
of Sanitary Engineering, so far as anything of the nature of science
could then be known. Their sanitary views were certainly abreast of
the scientific knowledge of that early day. The existence of the
“cloacæ,” or great sewers, of the ancient city of Rome showed that
its people, or at least its rulers, not only appreciated the value
of draining and sewering their city, but also that they knew how to
secure the construction of efficient and enduring sewers or drains. It
has been stated, and it is probably true, that this system of cloacæ,
or sewers, was so complete that every street of the ancient city was
drained through its members into the Tiber. They were undoubtedly the
result of a gradual growth in sewer construction and did not spring
at once into existence, but they date back certainly to the beginning
of the period of the kings (753 B.C.). The famous Cloaca Maxima, as
great as any sewer in the system, and certainly the most noted, is
still in use, much of it being in good order. The mouth of the latter
where it discharges into the Tiber is 11 feet wide and 12 feet high,
constituting a large arch opening with three rings of voussoirs of
peperino stone. Many other sewers of this system are also built with
arch tops of the same stone, with neatly cut and closely fitting
voussoirs. We do not find, unfortunately, any detailed accounts of the
procedures involved in the design of these sewers, yet it is altogether
probable that the old Roman civil engineers formed the cross-sections,
grades, and other physical features of their sewer system by rational
processes, although they would doubtless appear crude and elementary at
the present time. It would not be strange if they made many failures in
the course of their structural experiences, but they certainly left in
the old Roman sewers examples of enduring work of its kind.
Some portions of this ancient sewer system are built with tops that are
not true arches, and it is not impossible that they antedate the regal
period. These tops are false arches formed of horizontal courses of
tufa or peperino, each projecting over that below until the two sides
thus formed meet at the top. The outline of the crowns of such sewers
may therefore be triangular, curved, or polygonal; they were usually
triangular. Smaller drains forming feeders to the larger members of
the system were formed with tops composed of two flat stones laid with
equal inclination to a vertical line so as to lean against each other
at their upper edges and over the axis of the sewer. This method of
forming the tops of the drains by two inclined flat stones was a crude
but effective way of accomplishing the desired purpose.
The main members of this great sewer system seem to have followed the
meandering courses of small rivers or streams, constituting the natural
drainage-courses of the site of the city. The Cloaca Maxima has an
exceedingly crooked course and it, along with others, was probably
first formed by walling up the sides of a stream and subsequently
closing in the top. Modern engineers know that such an alignment for
a sewer is viciously bad, and while this complicated system of drains
is admirably constructed in many ways for its date, it cannot be
considered a perfect piece of engineering work in the light of present
engineering knowledge. It is probable that the walling in of the sides
of the original streams began to be done in Rome at least as early as
the advent of the Tarquins, possibly as early as 800 B.C. or earlier.
We know little about the original outfalls or points of discharge into
the Tiber, except that, as previously stated, these points were made
through the massive quay-walls constructed during the period of the
kings along both shores of the Tiber, probably largely for defence as
originally built. The discharge of the old Roman sewers through the
face of this quay-wall and into the river is precisely the manner in
which the sewers of New York City in many places are discharged into
the North, East, and Harlem rivers.
The Cloaca Maxima is not the only great ancient sewer thus far
discovered. There are at least two others equal to it, and some of the
single stones with which they are built contain as much as 45 cubic
feet each. These cloacæ were not mere sewers; indeed they were more
drains than sewers, for they carried off flood-waters and the natural
drainage as well as the sewerage. They were therefore combined sewers
and drains closely akin to the sewers of our “combined” systems. The
openings into them were made along the streets of Rome and in public
buildings or some other public places. There is no evidence that
they were ventilated except through these openings, and from each
noxious gases were constantly rising to be taken into the lungs of
the passers-by. It is a rather curious as well as important fact that
so far as excavations have been made there is practically no evidence
that a private residence in Rome was connected with the sewers. The
“latrines” were generally located adjacent to the Roman kitchens and
discharged into the cloacæ.
=21. Early Roman Bridges.=—The early Romans were excellent
bridge-builders as well as constructors in other lines of engineering
work. Although the ancient city was first located on the left bank of
the Tiber, apparently it was but a comparatively short time before
the need of means for readily crossing from bank to bank was felt.
The capacity of the Roman engineers was equal to the demands of the
occasion, and it is now known that seven or eight ancient bridges
connected the two shores of the river Tiber. The oldest bridge is that
known as Pons Sublicius. No iron was used in its construction, as
bronze was the chief metal employed in that early day. The structure
was probably all of timber except possibly the abutments and the piers.
A French engineer, Colonel Emy, has exhibited in his “Traité de l’Art
de la Charpenterie” a plan of this structure restored as an all-timber
bridge with pile foundations. Lanciani, on the other hand, believes
that the abutments and piers must have been of masonry. The masonry
structures, however, known to exist at a later day may have been parts
of the work of rebuilding after the two destructions by floods. The
date of its construction is not known, but tradition places it in
the time of Ancus Marcius. This may or may not be correct. A flood
destroyed the bridge in 23 B.C., and again in the time of Antoninus
Pius, but on both occasions it was rebuilt. The structure has long
since disappeared. The piers only remained for a number of centuries,
and the last traces of them were removed in 1877 in order to clear the
bed of the river.
Fig. 11 shows Colonel Emy’s restoration of the plan for the pile bridge
which Julius Cæsar built across the Rhine in ten days for military
purposes. This plan may or may not include accurate features of the
structure, but it is certain that such a timber bridge was built, and
well preserved pieces of the piles have been taken from under water
at the site little the worse for wear after two thousand years of
submersion.
The censor Ælius Scaurus built a masonry arch across the Tiber about
a mile and a half from Rome in the year 100 B.C. This bridge is now
known as the Ponte Molle, and some parts of the original structure are
supposed to be included in it, having been retained in the repeated
alterations. The arches vary in span from 51 to 79 feet, and the width
of the structure is a little less than 29 feet.
In or about the year 104 A.D. the emperor Trajan constructed what
is supposed to be a wooden arch bridge with masonry piers across the
Danube just below the rapids of the Iron Gate.
[Illustration: Cross-section at Pier.]
[Illustration: Plan at Pier.
FIG. 11.—Bridge thrown across the Rhine by Julius Cæsar.]
A _bas relief_ on the Trajan Column at Rome exhibits the timber arches,
but fails to give the span lengths, which have been the subject of much
controversy, some supposing them to have been as much as 170 feet.
The ancient Pons Fabricius, now known as Ponte Quattiro Capi, still
exists, and it is the only one which remains intact after an expiration
of nearly two thousand years. It has three arches, the fourth being
concealed by the modern embankment at one end; a small arch pierces the
pier between the other two arches. This structure is divided into two
parts by the island of Æsculapius. It is known that a wooden bridge
must have joined that island with the left bank of the Tiber as early
as 192 B.C., and a similar structure on the other side of the island
is supposed to have completed the structure. While Lucius Fabricius
was Commissioner of Roads in the year 62 B.C. he reconstructed the
first-named portion into a masonry structure of arches. An engraved
inscription below the parapets shows that the work was duly and
satisfactorily completed, and further that it was the custom to require
the constructors or builders of bridges to guarantee their work for the
period of forty years. Possession of the last deposit, made in advance
as a guarantee of the satisfactory fulfilment of the contract, could
not be regained until the forty-first year after completion.
[Illustration: FIG. 12.—Trajan’s Bridge.]
The Pons Cestius is a bridge since known as the Pons Gratianus and
Ponte di S. Bartolomeo. Its first construction is supposed to have been
completed in or about 46 B.C., and it was rebuilt for the first time in
A.D. 365. A third restoration took place in the eleventh century. The
modern reconstruction in 1886-89 was so complete that only the middle
arch remains as an ancient portion of the structure. The island divides
the bridge into two parts, the Ship of Æsculapius lying between the
two, but it is not known when or by whom the island was turned into
that form.
Another old Roman bridge, of which but a small portion is now standing,
is Pons Æmilius, the piers of which were founded in 181 B.C., but the
arches were added and the bridge completed only in 143 B.C. It was
badly placed, so that the current of the river in times of high water
exerted a heavy pressure upon the piers, and in consequence it was at
least four times carried away by floods, the first time in the year
A.D. 280.
The discovery of what appears to be a row of three or four ruins of
piers nearly 340 feet up-stream from the Ponte Sisto seems to indicate
that a bridge was once located at that point, although little or
nothing is known of it as a bridge structure. Some suppose it to be the
bridge of Agrippa.
The most historical of all the old Roman bridges is that which was
called Pons Ælius, now known as Ponte S. Angelo, built by Hadrian A.D.
136. Before the reconstruction of the bridge in 1892 six masonry
arches were visible, and the discovery of two more since that date
makes a total of eight, of which it is supposed that only three were
needed in a dry season. The pavement of the approach to this bridge as
it existed in 1892 was the ancient roadway surface. Its condition at
that time was an evidence of the substantial character of the old Roman
pavement.
Below the latter bridge remains of another can be seen at low water. It
is supposed that this structure was the work of Nero, although its name
is not known.
The modern Ponte Sisto is a reconstruction of the old Pons
Valentinianus or bridge of Valentinian I. The latter was an old Roman
bridge, and it was regarded as one of the most impressive of all the
structures crossing the river. It was rebuilt in A.D. 366-67.
The most of these bridges were built of masonry and are of the usual
substantial type characteristic of the early Romans. They were
ornamented by masonry features in the main portions and by ornate
balustrades along either side of the roadway and sidewalks. The roadway
pavements were of the usual irregular polygonal old Roman type, the
sidewalk surfaces being composed of the large slabs or stones commonly
used in the early days of Rome for that purpose.
=22. Bridge of Alcantara.=—Among the old Roman bridges should be
mentioned that constructed at Alcantara in Spain, supposedly by Trajan,
about A.D. 105. It is 670 feet long and its greatest height is 210
feet. One of its spans is partially destroyed. The structure is built
of blocks of stone without cementing material. In this case the number
of arches is even, there being six in all, the central two having
larger spans than those which flank them. It is a bridge of no little
impressiveness and beauty and is a most successful design.
=23. Military Bridges of the Romans.=—In the old Roman military
expeditions the art of constructing temporary timber structures along
lines of communication was well known and practised with a high degree
of ability. Just what system of construction was employed cannot be
determined, but piles were constantly used. At least some of these
timber military bridges, and possibly all, were constructed with
comparatively short spans, the trusses being composed of such braces
and beams as might be put in place between bents of piles. As already
observed, some of the sticks of these bridges have been found in the
beds of German rivers, and at other places, perfectly preserved after
an immersion of about two thousand years. These instances furnish
conclusive evidence of the enduring qualities of timber always
saturated with water.
=24. The Roman Arch.=—The Romans developed the semicircular arch to a
high degree of excellence, and used it most extensively in many sewers,
roads, and aqueducts. While the aqueduct spans were usually made with
a length of about 18 or 20 feet, they built arches with span lengths
as much as 120 feet or more, comparing favorably with our modern
arch-bridge work. They seldom used any other curve for their arches
than the circular, and when they built bridges an odd number of spans
was usually employed, with the central opening the largest, possibly in
obedience to the well-known esthetic law that an odd number of openings
is more agreeable to the eye than an even number. Apparently they were
apprehensive of the safety of the piers from which their arches sprang,
and it was not an uncommon rule to make the thickness of the piers one
third of the clear span. Nearly one fourth of the entire length of the
structure would thus be occupied by the pier thicknesses. Although
the use of mortar, both lime and cement, early came into use with the
Romans, they usually laid up the ring-stones of their arches dry, i.e.,
with out the interposition of mortar joints.
CHAPTER III.
=25. The Roman Water-supply.=—There is no stronger evidence of
engineering development in ancient Rome, nor of the advanced state of
civilization which characterized its people, than its famous system of
water-supply, which was remarkable both for the volume of water daily
supplied to the city and for the extensive aqueducts, many of whose
ruins still stand, as impressive monuments of the vast public works
completed by the Romans. These ruins, and those of many other works,
would of themselves assure us of the elaborate system of supply, but
fortunately there has been preserved a most admirable description of
it, the laws regulating consumption, the manner of administering the
water department of the government of the ancient city, and much other
collateral information of a most interesting character. In the work
entitled, in English, “The Two Books on the Water-supply of the City
of Rome,” by (Sextus) Julius Frontinus, an eminent old Roman citizen,
who, besides having filled the office of water commissioner[1] of
the city, was governor of Britain and three times consul, as well as
having enjoyed the dignity of being augur. He may properly be called
a Roman engineer, although he evidently was a man of many public
affairs, and so esteemed by the emperors who ruled during his time
that he accompanied them in various wars as a military man of high
rank. He wrote seven books at least, viz., “A Treatise on Surveying,”
“Art of War,” “Strategematics,” “Essays on Farming,” “Treatise on
Boundaries, Roads, etc.,” “A Work on Roman Colonies,” and his account
of the water-works of Rome, entitled “De Aquis.” It is the latter
book in which engineers are particularly interested. The translation
of this book from the original Latin is made from what is termed the
“Montecassino Manuscript,” an account of which with the translation is
given by Mr. Clemens Herschel in his entertaining work, “Frontinus, and
the Water-supply of the City of Rome.”
[1] The first permanent water commissioner in Rome was M. Agrippa,
son-in-law of Cæsar Augustus, who took office B.C. 34. He was one of
the greatest Roman engineers and constructors, if indeed he was not the
first in rank.
As near as can be determined Frontinus lived from about A.D. 35 to
A.D. 103 or 104. Judging from the offices which Frontinus held and
the honors which he enjoyed throughout his life, it would appear that
he was a patrician; he was certainly a man of excellent executive
capacity, of intellectual vigor and refined taste, and a conscientious
public servant. The water-supply of the city was held by the Romans
to be one of the most important of all its public works, and its
administration during the life of Frontinus was entrusted to what
we should call a water commissioner, appointed by the emperor. It
was considered to be an office of dignity and honor, and the proper
discharge of its responsibilities was a public duty which required a
high order of talent, as well as great integrity of character.
=26. The Roman Aqueducts.=—Frontinus states that from the foundation
of the city of Rome until 313 B.C., i.e., for a period of 441 years,
the only water-supply was that drawn either from the river Tiber or
from wells or springs. The veneration of the Romans for springs is a
well-known feature of their religious tenets. They were preserved with
the greatest care, and hedged about with careful safeguards against
irreverent treatment or polluting conditions. Apparently after this
date the people of Rome began to feel the need of a public water-supply
adequate to meet the requirements of a great city. At any rate, in
the year 313 B.C. the first aqueduct, called the Appia, for bringing
public water into the city of Rome was attempted by Censors Appius
Claudius, Crassus, and C. Plautius, the former having constructed the
aqueduct, and the latter having found the springs. Appius must have
been an engineer of no mean capacity, for it was he who constructed
the first portion of the Appian Way. The origin of this water-supply
is some springs about 10 miles from Rome, and they may now be seen at
the bottom of stone quarries in the valley of the Anio River. This
aqueduct, Aqua Appia, is mostly an underground waterway, only about
300 feet of it being carried on masonry arches. At the point where
it enters the city it was over 50 feet below the surface; its clear
cross-section is given as 2½ feet wide by 5 feet high. The elevation of
its water surface in Rome was probably under 60 feet above sea-level.
[Illustration: Claudia, of dimension stone, and Anio Novus, of brick
and concrete, on top of it.]
=27. Anio Vetus.=—The next aqueduct built for the water-supply of Rome
was called Anio Vetus. It was built 272-269 B.C., and is about 43 miles
long; it took its water from the river Anio. About 1100 feet of its
length was carried above ground on an artificial structure. It also
was a low-level aqueduct, the elevation at which it delivered water
at Rome being about 150 feet above sea-level. It was built of heavy
blocks of masonry, laid in cement, and the cross-section of its channel
was about 3.7 feet wide by 8 feet high. In the year 144 B.C. the Roman
senate made an appropriation equal to about $400,000 of our money to
repair the two aqueducts already constructed, and to construct a new
one called Aqua Marcia, to deliver water to the city at an elevation
of about 195 feet above sea-level. This aqueduct was finished 140
B.C.; it is nearly 58 miles long, and carried water of most excellent
quality through a channel which, at the head of the aqueduct, was
5⅞ feet wide by 8³/₁₀ feet high, but farther down the structure was
reduced to 3 feet wide by 5⁷/₁₀ feet high. The excellent water of these
springs is used for the present supply of Rome, and is brought in the
Aqua Pia, built in 1869, as a reconstruction of the old Aqua Marcia.
This aqueduct, like its two predecessors, is built of dimension stone,
18 inches by 18 inches by 42 inches, or larger, laid in cement; but
concrete and brick were used in the later aqueducts, with the exception
of Claudia.
=28. Tepula.=—The aqueduct called Aqua Tepula, about 11 miles in
length, and completed 125 B.C., was constructed to bring into the
city of Rome a slightly warm water from the volcanic springs situated
on the hill called Monte Albani (Alban Hills) southeast of Rome. The
temperature of these springs is about 63° Fahr. In the year B.C. 33
Agrippa caused the water from some springs high up the same valley to
be brought in over the aqueduct Aqua Julia, 14 miles long. This latter
water was considerably colder than that of the Tepula Springs. The two
waters were united before reaching Rome and allowed to flow together
far enough to be thoroughly mixed. They were then divided and carried
into Rome in two conduits. The volume of water carried in the Aqua
Julia was about three times that taken from the Tepula Springs, the
cross-section of the latter being only 2.7 feet wide by 3.3 feet high,
while that of Julia was 2.3 feet by 4.6 feet. The water from Aqua Julia
entered Rome at an elevation of about 212 feet above sea-level, and
that from Aqua Tepula about 11 feet lower.
=29. Virgo.=—The sixth aqueduct in chronological order was called
Virgo, and it was completed 19 B.C. It takes water from springs about 8
miles from Rome and only about 80 feet above sea-level, but the length
of the aqueduct is about 13 miles. The delivery of water in the city by
this aqueduct is about 67 feet above that level. The cross-section of
this channel is about 1.6 feet wide and 6.6 feet high.
=30. Alsietina.=—The preceding aqueducts are all located on the left or
easterly bank of the Tiber, but one early structure was located on the
right bank of the Tiber to supply what was called the Trans-Tiberine
section of the city, and it was known as Aqua Alsietina. The emperor
Augustus had this aqueduct constructed during his reign, and it was
finished in the year A.D. 10. Its source is a small lake of the same
name with itself, about 20 miles from Rome. The elevation of this lake
is about 680 feet above sea-level, while the water was delivered at
an elevation of about 55 feet above the same level. The water carried
by this aqueduct was of such a poor quality that Frontinus could not
“conceive why such a wise prince as Augustus should have brought to
Rome such a discreditable and unwholesome water as the Alsietina,
unless it was for the use of Naumachia.” The latter was a small
artificial lake or pond in which sham naval fights were conducted.
[Illustration:
Sand and Pebble Catch-tanks near Tivoli. Dimension-stone aqueducts of
Marcia at either end of the tank built of small stone; _opus incretum_.
The arches are chambers of the tanks.]
=31. Claudia.=—The eighth aqueduct described by Frontinus is the Aqua
Claudia, built of dimension stone, which he calls a magnificent work
on account of the large volume of water which it supplied, its good
quality, and the impressive character of considerable portions of the
aqueduct itself, between 9 and 10 miles being carried on arches. It was
built in 38-52 A.D. and is forty-three miles long. The sources of its
supply are found in the valley of the Anio, and consequently it belongs
to the system on the left bank of the Tiber. The cross-section of its
channel was about 3.3 feet wide by 6.6 feet high. It was a work greatly
admired by the Roman people, as is evidenced by the praise “given to
it by Roman authors who wrote at that time.” It delivered water at the
Palatine 185 feet above sea-level. According to Pliny, the combined
cost of it and the Aqua Anio Novus was 55,500,000 sestertii, or nearly
$3,000,000. This aqueduct probably belongs to the highest type of Roman
hydraulic engineering. It follows closely the location of the Aqua
Marcia, although its alignment now includes a cut-off tunnel about 3
miles long, the latter having been constructed about thirty-six years
after the aqueduct was opened. Mr. Clemens Herschel observes that the
total sum expended for these two aqueducts makes a cost of about $6 per
lineal foot for the two. The arches of this aqueduct and those of the
Anio Novus have clear spans of 18 to 20 feet, with a thickness at the
crown of about 3 feet.
=32. Anio Novus.=—The ninth aqueduct described by Frontinus is called
Anio Novus. It was also constructed in the years A.D. 38-52. This
aqueduct has a length of about 54 miles and takes its supply from
artificial reservoirs constructed by Nero at his country-seat in the
valley of the Anio near modern Subiaco. This structure is built of
brick masonry lined with concrete. That portion of the Aqua Claudia
which is located on the Campagna carries for 7 miles the Anio Novus,
and it forms the long line of aqueduct ruins near Roma Vecchia. The
upper surface of the arch-ring at the crown forms the bottom of the
channel of the aqueduct. The cross-section of the channel of the Anio
Novus was 3.3 feet wide by 9 feet high. The elevation of the water in
this, as in the Claudia, when it reached the Palatine was about 185
feet above sea-level. The Anio Novus in some respects would seem to be
a scarcely less notable work than the Claudia. About 8 miles of its
length is carried on arches, some of them reaching a height of about
105 feet from the ground.
=33. Lengths and Dates of Aqueducts.=—These nine aqueducts constituted
all those described by Frontinus, as no others were completed prior to
his time. Five others were, however, subsequently completed between
the years 109 A.D. and 306 A.D., but enough has already been shown in
connection with the older structures to show the character of the
water-supply of ancient Rome.
The following tabular statement is a part of that given by Mr. F. W.
Blackford in “The Journal of the Association of Engineering Societies,”
December, 1896. It shows the dates and lengths of the ancient aqueducts
of Rome between the years 312 B.C. and 226 A.D., with the length of
the arch portions. The list includes those built up to the end of the
Empire. It will be observed that the total length of the aqueducts
is 346 miles, and that of the arch portions 44 miles. The figures
vary a little from those given by Lanciani and others, but they are
essentially accurate.
+------------+--------+----------+----------+
| | | Total |Length of |
| Name. | Date. |Length in |Arches in |
| | B.C. | Miles. | Miles. |
+------------+--------+----------+----------+
|Appia | 312 | 11 | Little |
|Vetus | 272-264| 43 | ” |
|Marcia | 145 | 61 | 12 |
|Tepula | 126 | 13 | Little |
|Julia | 34 | 15 | 6 |
|Virgo | 21 | 14 | Little |
| | | | |
| | A.D. | | |
|Alsietina | 10 | 22 | Little |
|Augusta | 10 | 6 | ” |
|Claudia | 50 | 46 | 10 |
|Anio Novus | 52 | 58 | 9 |
|Triana | 109 | 42 | Little |
|Alexandrina | 226 | 15 | 7 |
| +--------+----------+----------+
| Totals | 346 | 44 |
+------------+--------+----------+----------+
=34. Intakes and Settling-basins.=—The preceding brief descriptions
of the old Roman aqueducts give but a superficial idea of the real
features of those great works and of the system of water-supply of
which they were such essential portions. Enough has been shown,
however, to demonstrate conclusively that the engineers and
constructors of old Rome were men who, on the one hand, possessed a
high order of engineering talent and, on the other, ability to put in
place great structures whose proportions and physical characteristics
have commanded the admiration of engineers and others from the time
of their completion to the present day. If a detailed statement were
to be made in regard to the water-supply of ancient Rome, it would
appear that much care was taken to insure wholesome and potable water.
At the intakes of a number of the aqueducts, reservoirs or basins
were constructed in which the waters were first received and which
acted as settling-basins, so that as much sedimentation as possible
might take place. Similar basins (picinæ) were also constructed at
different points along the aqueducts for the same purpose and for
such other purposes as the preservation of the water in a portion
of the aqueduct in case another portion had to be repaired or met
with an accident which for the time being might put it out of use.
These basins were usually constructed of a number of apartments, the
water flowing from one to the other, very much as sewage in some
sewage-disposal works flows at the present time through a series of
settling-basins. The object of these picinæ was the clearing of the
water by sedimentation. Indeed there was in some cases a use of salt in
the water to aid in clarifying it. This is an early type of the modern
process of clarifying water by chemical precipitation, not the best of
potable water practice, but one that is sometimes permissible.
=35. Delivery-tanks.=—The aqueducts brought the water to castellæ
or delivery-tanks, i.e., small reservoirs, both inside the city and
outside of it, and from these users were obliged by law to take
their supplies; that is, for baths, for fountains, for public uses,
for irrigation, and for private uses. When Frontinus wrote his “De
Aquis” a little less than three tenths of all the water brought to
Rome by the aqueducts was used outside of the city. The remainder was
distributed in the city from 247 delivery-tanks or small reservoirs,
about one sixth of it being consumed by 39 ornamental fountains and 591
water-basins.
=36. Leakage and Lining of Aqueducts.=—These aqueducts were by no
means water-tight. Indeed they were subject to serious leakage, and
Frontinus shows that forces of laborers were constantly employed in
maintaining and repairing them. As has been stated, the older aqueducts
were built of dimension stones, while the later were constructed of
concrete or bricks and concrete. The channels of these aqueducts, as
well as reservoirs and other similar structures, were made as nearly
water-tight as possible by lining them with a concrete in which
pottery, broken into fine fragments, was mixed with mortar.
[Illustration: Claudia and Anio Novus near Porta Furba. Repairs in
brickwork and in a composite of concrete and brickwork.]
=37. Grade of Aqueduct Channels.=—The fall of the water surface in
these aqueducts cannot be exactly determined. The levelling-instruments
used by the Romans were simple and, as we should regard them, crude,
although they served fairly well the purposes to which they were
applied. They were not sufficiently accurate to determine closely the
slope or grade of the water surface in the aqueduct channels. The
deposition of the lime from the water along the water surface on the
sides of the channels in many cases would enable that slope to be
determined at the present time, but sufficiently careful examinations
have not yet been made for that purpose. Lanciani states that the
slopes in the Aqua Anio Vetus vary from about one in one thousand to
four in one thousand. An examination of the incrustation on the sides
of the Aqua Marcia near its intake makes it appear that the slope of
the surface was about .06 foot per 100 feet, which would produce a
velocity, according to the formula of Darcy, of about 3.3 feet per
second. In some aqueducts built in Roman provinces it would appear that
slopes have been found ranging from one in six hundred to one in three
thousand.
=38. Qualities of Roman Waters.=—The chief characteristic in most of
the old Roman waters was their extreme hardness. They range from 11°
to 48° of hardness, the latter belonging to the water of the Anio,
while the potable waters in this country scarcely reach 5°. The old
Romans recognized these characteristics of their waters and, as has
been intimated, used the best of them for table purposes, while the
less wholesome were employed for fountains, flushing sewers, and other
purposes not affected by undesirable qualities. The water from Claudia,
for instance, was used for the imperial table. The water from the Aqua
Marcia was also of excellent quality, while that brought in by the Aqua
Alsietina was probably not used for potable purposes at all.
=39. Combined Aqueducts.=—In several cases a number of aqueduct
channels were carried in one aqueduct. A marked instance of this kind
was that of Julia, Tepula, and Marcia, all being carried in vertical
series in one structure. Numerous instances of this sort occurred.
=40. Property Rights in Roman Waters.=—In reading the two books of
Frontinus one will be impressed by the property values which the old
Romans created in water rights. The laws of Rome were exceedingly
explicit as to the rights of water-users and as to the manner in which
water should be taken from the aqueducts and from the pipes leading
from the reservoirs in and about the city. The proper methods for
taking the water and using it were carefully set forth, and penalties
were prescribed for violations of the laws pertaining to the use of
water. There were many abuses in old Rome in the administration of
the public water-supply, and one of the most troublesome duties which
Frontinus had to perform lay in reforming those abuses and preventing
the stealing of water. The unit of use of water (a “quinaria,” whose
value is not now determinable) was the volume which would flow from
an orifice .907 inch in diameter and having an area of about .63 of
a square inch. Mr. Herschel shows that in consequence of the failure
of the Romans to understand the laws of the discharge of water under
varying heads, the quinaria may have ranged from .0143 cubic foot to
.0044 cubic foot per second or between even wider limits.
=41. Ajutages and Unit of Measurement.=—Frontinus describes twenty-five
ajutages of different diameter, officially approved in connection with
the Roman system of public water-supply; but only fifteen of these were
actually used in his day. All of these were circular in form, although
two others had been used prior to that time. They varied in diameter
from .907 to 8.964 English inches and were originally made of lead, but
that soft metal lent itself too easily to the efforts of unscrupulous
water-users to enlarge them by thinning the metal. In his time they
were made of bronze, which was a hard metal and could not be tampered
with so as to enlarge its cross-section. The discharge through the
smallest of these ajutages was the quinaria, the unit in the scale of
water rights. The largest of the above ajutages had a capacity of a
little over 97 quinariæ.
This unit (the quinaria) was based wholly on superficial area, and had
no relation whatever to the head over the orifice or to the velocity
corresponding to that head. Although Frontinus refers in several cases
to the fact that the deeper the ajutage is placed below the water
surface the greater will be the discharge through it, also to the fact
that a channel or pipe of a given area of cross-section will pass
more water when the latter flows through it with a high velocity, he
and other Roman engineers seem to have failed completely to connect
the idea of volume of discharge to the product of area of section by
velocity. In the Roman mind of his day, and for perhaps several hundred
years after that, the area of the cross-section of the prism of water
in motion was the only measure of the volume of discharge. This seems
actually preposterous at the present time, and yet, as observed by Mr.
Herschel, possibly a majority of people now living have no clearer idea
of the volume of water flowing in either a closed or open channel.
Existing statutes even respecting water rights bear out this statement,
improbable as it may at first sight appear. This early Roman view of
the discharge is, however, in some respects inexplicable, for Hero of
Alexandria wrote, probably in the period 100-50 B.C., that the section
of flow only was not sufficient to determine the quantity of water
furnished by a spring. He proceeded to set forth that it was also
necessary to know the velocity of the current, and further explained
that by forming a reservoir into which a stream would discharge for an
hour the flow or discharge of that stream for the same length of time
would be equal to the volume of water received by the reservoir. His
ideas as to the discharge of a stream of water were apparently as clear
as those of a hydraulic engineer of the present time. Indeed the method
which he outlines is one which is now used wherever practicable.
It has been a question with some whether Frontinus and other Roman
engineers were acquainted with the fact that a flaring or outward
ajutage would increase the flow or discharge through the orifice.
The evidence seems insufficient to establish completely that degree
of knowledge on their part. At the same time, in the CXII chapter of
Frontinus’ book on the “Water-supply of the City of Rome,” he states
that in some cases pipes of greater diameter than that of the orifice
were improperly attached to legal ajutages. He then states: “As a
consequence the water, not being held together for the lawful distance,
and being on the contrary forced through the short restricted distance,
easily filled the adjoining larger pipe.” He was convinced that the
use of a pipe with increased diameter under such circumstances would
give the user of the water a larger supply than that to which he was
entitled, and he was certainly right in at least most cases.
The actual unit orifice through which the unit volume of water called
the quinaria was discharged was usually of bronze stamped by a proper
official, thus making its use legal for a given amount of water. The
Roman engineers understood that such an orifice should be inserted
accurately at right angles to the side of the vessel or orifice, and
that was the only legal way to make the insertion. Furthermore, the law
required that there should be no change in the diameter of the pipe
within 50 feet of the orifice. It was well known that a flaring pipe
of increased diameter applied immediately at the orifice would largely
increase the discharge, and unscrupulous people resorted to that means
for increasing the amount of water to be obtained for a given price.
=42. The Stealing of Water.=—It appears also that Frontinus experienced
much trouble from clandestine abstraction of water from reservoirs and
water-pipes. The administration of the water commissioner’s office
had been exceedingly corrupt prior to his induction into office, and
some of his most troublesome official work arose from his efforts to
detect water-thieves, and to guard the supply system from being tapped
irregularly or illegally. We occasionally hear of similar instances of
water-stealing at the present time, which shows that human nature has
not altogether changed since the time of Frontinus.
=43. Aqueduct Alignment and Design of Siphons.=—The alignment of some
of the Roman aqueducts followed closely the contours of the hills
around the heads of valleys, while others took a more direct line
across the valleys on suitable structures, frequently series of arches.
Judging from our own point of view it may not be clear at first sight
why such extensive masonry constructions were used when the aqueduct
could have been kept in excavation by following more closely the
topography of the country. There is little doubt that the Romans knew
perfectly well what they were about. Indeed it is definitely stated in
some of the old Roman writings that the structures were built across
valleys for the specific purpose of saving distance which, in most
instances at least, meant saving in cost.
These masonry structures, it must be remembered, were built of material
immediately at hand. Furthermore, these aqueducts were generally only
made of sufficient width for the purpose of carrying water-channels.
They were not wide structures. In some cases they were not more than
8 feet or 9 feet wide for a height of nearly 100 feet. The cost of
construction was thus largely reduced below that of wide structures.
[Illustration: Old Roman Lead and Terra-cotta Pipe.]
The Romans were perfectly familiar with the construction of inverted
siphons. As a matter of fact Vitruvius, in Chapter VII of his Eighth
book, describes in detail how they should be designed. His specific
descriptions relate to lead pipes, but it is clear from what he states
at other points that he considered earthenware pipes equally available.
He sets forth how the pipes should be carried down one slope, along
the bottom of the valley, and up the other slope, the lowest portion
being called the “venter.” He realized the necessity of guarding all
elbows in the pipe by using a single piece of stone as a detail for the
elbow, a hole being cut in it in each direction in which the adjoining
sections of pipe should be inserted, the sections of lead pipe being
10 feet long, and even goes so far as to describe the stand-pipes that
should be inserted for the purpose of allowing air to escape. Vitruvius
also advises that the water should not only be admitted to inverted
siphons in a gradual manner, but that ashes should be thrown into the
water when the siphon is first used in order that they may settle into
the joints or open places so as to close any existing leaks. Lead pipe
siphons, 12 to 18 inches in diameter, with 1 inch thickness of metal
under 200 feet head, built in ancient times, have been found at Lyons
in France. Also a drain-pipe siphon with masonry reinforcement was
built at Alatri in Italy 125 B.C. to carry water under a head of
about 340 feet. There are other notable instances of inverted siphons
constructed and used during the ancient Roman period, some of them
being of lead pipe imbedded in concrete.
CHAPTER IV.
=44. Antiquity of Masonry Aqueducts.=—Masonry aqueducts, either solid
or with open arches, were not first constructed by the city of Rome;
their origin was much farther back in antiquity than that. The Greeks
at least used them before the Roman engineers, and it is not unlikely
that the latter drew their original ideas from the former, if indeed
they were not instructed by them. Nor during the times of the Romans
was the construction of aqueducts confined to Rome. Wherever Roman
colonies were created it would appear that vast sums were expended in
the construction of aqueducts for the purpose of suitably supplying
cities with water. Such constructions are found at many points in
Spain, France, and other countries which were in ancient times Roman
colonies. It is probable that there are not less than one hundred, and
perhaps many more, of such structures in existence at the present time.
=45. Pont du Gard.=—Among the more prominent aqueducts constructed
during the old Roman period and outside of Italy were the Pont du Gard
at Nismes in the south of France, and those at Segovia and Tarragona in
Spain. The Pont du Gard has three tiers of arches with a single channel
at the top. The greatest height above the river Gardon is about 180
feet, and the length of the structure along the second tier of arches
is 885 feet. The arches in the lowest tier are 51 feet, 63 feet, and
80.5 feet in span, while the arches in the highest tier are uniformly
15 feet 9 inches in span. The thickness of the masonry at the top of
the structure from face to face is 11 feet 9 inches, and 20 feet 9
inches at the lower tier of arches, the thickness at the intermediate
tier being 15 feet.
The largest arch has a depth of keystone of 5 feet 3 inches, while the
other arches of the lower tier have a depth of keystone of 5 feet. The
depth of the ring-stones of the small upper arches is 2 feet 7 inches.
This structure forms a sort of composite construction, the lower arches
constituting four separate arch-rings placed side by side, making a
total thickness of 20 feet 9 inches. The intermediate arches consist
of three similar series of narrow arches placed side by side, but the
masonry of the upper tier is continuous throughout from face to face.
The three and four parallel series of arches of the middle and lowest
tiers are in no way bonded or connected with each other. There is no
cementing material in any of the arch-rings, but cement mortar was used
in rubble masonry or concrete around the channel through which the
water flowed above the upper tier of small arches. This structure is
supposed to have been built between the years 31 B.C. and 14 A.D.
=46. Aqueducts at Segovia, Metz, and Other Places.=—The Segovia
aqueduct was built by the emperor Trajan about A.D. 100-115. It is
built without mortar, and has 109 arches, but 30 are modern, being
reproductions of the old. It has a length of over 2400 feet, and in
places its height is about 100 feet. The old Tarragona aqueduct is
built with two series of arches, 25 being in the upper series and 11
in the lower. It is 876 feet long and has a maximum height of over 80
feet. At Mayence there are ruins of an aqueduct about 16,000 feet long.
In Dacia, Africa, and Greece there are other similar ruins. Near Metz
are the remains of a large old Roman aqueduct. It consisted of a single
row of arches, and had no features of particular prominence. This
latter observation, however, could not be made of one of the bridges in
the aqueduct at Antioch. Although the masonry and design of this latter
structure were crude, its greatest height is 200 feet, and its length
700 feet. The lower portion of this structure was a solid wall with the
exception of two openings, the arches extending in a single row along
its upper portion. On the island of Mytilene are the ruins of another
old aqueduct about 500 feet long, with a maximum height of about 80 feet.
The building of these remarkable aqueducts was practised at least
down to the later periods of the Roman empire, that of Pyrgos, near
Constantinople,—built not earlier than the tenth century,—being an
excellent example. It consists of two branches at right angles to each
other. The greater branch is 670 feet long, and its greatest height
106 feet. There are three tiers of arches, the two upper being of
semicircular and the lower of Gothic outline. The number in each tier
for a given height is the same, but with an increasing length of span
in rising from the lowest to the highest tier. Thus the highest tier of
piers is the lightest, relieving the top of the structure of weight.
The lowest row of piers is reinforced by counterforts or buttresses.
At the top of the structure the width or thickness is 11 feet, but the
thickness increases uniformly to 21 feet at the bottom. The smaller
branch of the aqueduct is 300 feet long, and was built with twelve
semicircular arches.
=47. Tunnels.=—The construction of tunnels, especially in connection
with the building of aqueducts, constituting a branch of engineering
procedure, was frequently practised by the ancient nations. Large
tunnel-works were executed many times by the ancient Greeks and Romans.
It would seem that the Greeks were the instructors of the Romans in
this line of engineering operations. As early as B.C. 625 we are told
that the Greek engineer Eupalinus constructed a tunnel 8 feet broad, 8
feet high, and 4200 feet long, through which was built a channel for
carrying water to the city of Athens.
Sixty-five years later a similar work was constructed for the same
Grecian city. Indeed it appears that tunnels were constructed in the
time of the earliest history of aqueducts built to supply ancient Greek
and Roman cities with water.
It is certain that at the beginning of the Christian era tunnelling
processes were well known among the Romans. Vitruvius writes, in
speaking of the construction of aqueducts, in Chapter VII of the Eighth
Book: “If hills intervene between the city wall and spring head,
tunnels underground must be made, preserving the fall above assigned;
if the ground cut through be sandstone or stone, the channel may be cut
therein; but if the soil be earth or gravel, side walls must be built,
and an arch turned over, and through this the water may be conducted.
The distance between the shafts over the tunnelled part is to be 120
feet.”
The Romans pierced rock in their tunnel-work, not only by chiselling,
but sometimes by building fire against the rock so as to heat it as
hot as possible. The heated rock was then drenched with cold water, so
that it might be cracked and disintegrated to as great an extent as
practicable. According to Pliny vinegar was used instead of water in
some cases, under the impression that it was more efficacious.
[Illustration: Roman water-pipe made of bored-out blocks of stone.]
One of the methods mentioned by Vitruvius is plainly “the cut and
cover” procedure of the present day. In Duruy’s history of Rome a
tunnel over three miles long is mentioned on a line of an aqueduct at
Antibes in France, as well as another constructed to drain Lake Fucinus
in Italy, about A.D. 50. It is there stated that the latter required
eleven years’ labor of 30,000 men to build a rock tunnel with a section
of 86 to 96 square feet 18,000 feet long.
Lanciani, in his “Ancient Rome,” states that about A.D. 152 a Roman
engineer (Nonius Datus) began the construction of a tunnel in Algeria,
and after having carefully laid out the axis of the tunnel across the
ridge “by surveying, and taking the levels of the mountains,” left the
progress of the work in the hands of the contractor and his workmen.
After the rather long absence from such a work of four years he was
called back by the Roman governor to ascertain why the two opposite
sections of the tunnel, as constructed, would not meet, and to take
the requisite measures for the completion of the work through which
water was to be conducted to Saldæ in a suitable channel. He explains
that there should have been no difficulty, and that the failure of the
two headings to meet was due to the negligence of the contractor and
his assistant, whom he states “had committed blunder upon blunder,”
although he writes, “As always happens in these cases, the fault was
attributed to the engineer.” He solved the problem by connecting the
two approximately parallel tunnels by a transverse tunnel, so that
water was finally brought to the city of Saldæ.
The art of tunnel construction has been one of the most widely
practised branches of Civil-Engineering from the times of the ancient
Assyrians, Egyptians, Greeks, Romans, and other ancient nations down to
the present.
=48. Ostia, the Harbor of Rome.=—The capacity of the ancient Romans
to build harbor-works is shown by what they did at Ostia, which was
then at the mouth of the Tiber, but is now not less than four miles
inland from the present shore line. At the Ostia mouth of the river the
present annual average advance seaward is not less than 30 feet, and at
the Fiumicino mouth about one third of that amount.
[Illustration: FIG. 13.—Plan of Ostia and Porto.]
The ancient port of Ostia is supposed to have been founded during the
reign of the fourth king Ancus Marcius, but it attained its period
of greatest importance during the reign of Claudius and Trajanus. At
that time the fertile portions of the Campania had been so largely
taken up by the country-places of the wealthy Romans that it was no
longer possible for the peasantry to cultivate sufficient ground to
yield the grain required by the home market of the Romans. Large
fleets were consequently engaged in the foreign grain-trade of Rome.
The wheat and other grain required in great quantities was grown
mostly in Egypt, although Carthage and other countries supplied large
amounts. The great fleets occupied in this trade made ancient Ostia
their Roman port. At the present time it has no inhabitants, but is a
group of complete ruins, with its streets of tombs, baths, palaces,
and temples, deeply covered with the accumulations of many centuries.
Enough excavations have been made along the shores of the Tiber at
this point to show that the river was bordered with continuous and
substantial masonry quays, flanked on the land side by successions of
great warehouses, obviously designed to receive grain, wine, oil, and
other products of the time. The entrance to this harbor was difficult,
as the mouth of the river was shallow, with bars apparently obstructing
its approach. There were no jetties, or other seaward works for the
protection of vessels desiring to make the harbor. It is stated that
during one storm nearly or quite two hundred vessels were destroyed
while they were actually in the harbor.
=49. Harbors of Claudius and Trajan.=—The difficulty in entering the
mouth of the Tiber prompted the emperor Claudius to construct another
harbor to accommodate the vast commerce then centring at the port of
Rome. Instead of increasing the capacity of Ostia and opening the mouth
of the river by deepening it, he constructed a new harbor on what was
then the seashore, a short distance from Ostia, and connected it with
the Tiber by a canal, the extension of which by the natural forces of
the river has become the Fiumicino, the only present navigable entrance
to the river. This harbor was enclosed by two walls stretching out
from the shore, and converging on the sea side to a suitable opening
left for the entrance of ships. The superficial area of this harbor
was about 175 acres, but it became insufficient during the time of
Trajan. He then proceeded to excavate inland a hexagonal harbor with
a superficial area of about 100 acres, which was connected both with
the harbor of Claudius and the canal connecting the latter with the
Tiber. These harbor-works were elaborate in their fittings for the
accommodation of ships, and were built most substantially of masonry.
They showed that at least in some branches of harbor-work the old
Romans were as good engineers as in the construction of aqueducts,
bridges, and other internal public works. The harbors at ancient
Ostia, including those of Claudius and Trajan, were not the only works
of their class constructed by the Romans, but they are sufficient to
show as great advancement in harbor and dock work as in other lines of
engineering.
These harbors were practically defenceless and exposed to the
incursions of pirates, which came to be frequently and successfully
made in the days of the declining power of Rome. It was therefore
rather early in the Christian era that these attacks discouraged, and
ultimately drove away, first, the maritime business of the Romans and,
subsequently, all the inhabitants of these ports, leaving the pillaged
remnants of the vast harbor-works, warehouses, palaces, temples, and
other buildings in the ruined condition in which they are now found.
CHAPTER V.
=50. Ancient Engineering Science.=—The state of what may be called the
philosophy or science of engineering construction in ancient Rome is
admirably illustrated by the work on Architecture by Marcus Vitruvius
Pollio, who is ordinarily known as Vitruvius, and who wrote probably a
little more than two thousand years ago. He calls himself an architect,
and his work is a classic in that profession of which he claims to be
a member. Although much of his work was purely architectural, a great
portion of it, on the other hand, was not architecture as we now know
it, but civil-engineering in the best sense of the term. It must be
remembered, therefore, that what is here written applies to that large
portion of his work which is purely civil-engineering.
It will be seen that although he understood really little or nothing
about the science of civil-engineering as we now comprehend it, he
perceived many of the general and fundamental principles of the best
practice of that profession and frequently applied them in a manner
which would do credit to a modern civil engineer. He not only laid
down axioms to govern the design of civil-engineering structures and
machinery for the transmission of power, but he also set forth many
considerations bearing upon public and private health and the practice
of sanitary engineering in a way that was highly creditable to the
state of scientific knowledge in his day. Speaking of the general
qualifications of an architect, remembering that that word as he
understood it includes the civil engineer, he states: “An architect
should be ingenious, and apt in the acquisition of knowledge; ... he
should be a good writer, a skilful draughtsman, versed in geometry and
optics, expert at figures, acquainted with history, informed on the
principles of natural and moral philosophy, somewhat of a musician,
not ignorant of the sciences both of law and physics, nor of the
motions, laws, and relations to each other of the heavenly bodies.”
Again he adds: “Moral philosophy will teach the architect to be above
meanness in his dealings and to avoid arrogance; it will make him just,
compliant, and faithful to his employer; and, what is of the highest
importance, it will prevent avarice gaining an ascendency over him; for
he should not be occupied with the thoughts of filling his coffers, nor
with the desire of grasping everything in the shape of gain, but by
the gravity of his manners and a good character should be careful to
preserve his dignity.”
These quaint statements of the desirable qualities of a professional
man are worthy to be considered rules of good professional living at
this time fully as much as they were in the days of old Rome. His
esteem for his profession was evidently high, but not higher than the
value which every civil engineer should put upon his professional
life. The need of a general education for a civil engineer is greater
now even than in his day, although musical accomplishments need not
be considered as essential in modern engineering practice. That
qualification, it is interesting to observe in passing, was inserted by
Vitruvius in order to illustrate the wide range of engineering practice
in those days when the architect-engineer was called upon, among other
things, to construct catapults and other engines of war, in which
a nice adjustment of gut ropes was determined by the musical tones
emitted under the desired tension.
=51. Ancient Views of the Physical Properties of Materials.=—When it
is remembered that the chemical constitution of materials used in
engineering was absolutely unknown, that no quantitative determination
of physical qualities had been made, and that the first correct
conception of engineering science had yet to be acquired, it is
a matter of wonder that there had been attained the engineering
development evidenced both by ancient writings like those of Vitruvius
and great engineering works like those of Rome, in the Babylonian Plain
and in Egypt. In discussing the problem of water-supply, he mentions
that certain learned ancients, “physiologists and philosophers,
maintained that there are four elements—air, fire, water, and earth—and
that their mixture, according to the difference of the species,
formed a natural mode of different qualities. We must recollect that
not only from these elements are all things generated, but that they
can neither be nourished nor grow without their assistance.” This
view of the construction of material things was not conducive to a
clear comprehension of those physical laws which lie at the foundation
of engineering science, and it is absolutely essential that these
elementary considerations be kept constantly in view in considering the
engineering attainments of the Romans and other ancient peoples.
=52. Roman Civil Engineers Searching for Water.=—In ancient times, as
at present, it was very important in many cases to know where to look
for water, and how to make what might promise to be a successful search
for it. Vitruvius states that the sources of water for a supply may
easily be found “if the springs are open and flowing above ground.” If
the sources are not so evident, but are more obscure, he recommends
that “before sunrise one must lie down prostrate in the spot where he
seeks to find it, and, with his chin placed on the ground and fixed,
look around the place; for, the chin being fixed, the eye cannot range
upwards further than it ought and is confined to the level of the
place. Then where the vapors are seen curling together and rising into
the air, there dig, because those appearances are not discovered in dry
places.” This method of discovering water-supply would be considered
by modern engineers at least somewhat awkward as well as damp and
disagreeable in the early morning hours. It is not more fantastic,
however, or less philosophical than the use of the divining-rod, which
has been practised in modern times as well as ancient, and is used even
in some country districts at the present time.
Vitruvius does not forget that the local features, including both
those of soil and of an artificial character, may affect the quality
of the water and possibly make it dangerous. He, therefore, sets forth
general directions by which good potable water may be found and that
of a dangerous nature avoided. The necessity of distinguishing between
good and bad water was as present to his mind and to the minds of the
old Roman engineers as to civil engineers of the present day, but the
means for making a successful discrimination were crude and obviously
faulty, and very often unsuccessful. He set forth, what is well known,
that rain-water when collected from an uncontaminated atmosphere is
most wholesome, but proceeds to give reasons which would not now be
considered in the highest degree scientific.
In Chapter V of his Eighth Book there are described some “means of
judging water” so quaint and amusing that they may now well be quoted
even though no civil engineer would be bold enough to cite them in
modern hydraulic practice. He says: “If it be of an open and running
stream, before we lay it on, the shape of the limbs of the inhabitants
of the neighborhood should be looked to and considered. If they are
strongly formed, of fresh color, with sound legs and without blear
eyes, the supply is of good quality.” At another point he comes rather
closely to our modern requirements which look to the exclusion of
minute and elementary vegetable growths, when he says: “Moreover, if
the water itself, when in the spring, is limpid and transparent, and
the places over which it runs do not generate moss, nor reeds, nor
other filth be near it, everything about it having a clean appearance,
it will be manifest by these signs that such water is light and
exceedingly wholesome.”
=53. Locating and Designing Conduits.=—In treating of the manner
of conducting water in pipes or other conduits, he adverts to the
necessity of accurate levelling and the instruments that were used for
that purpose. The three instruments which he mentions as being used are
called the dioptra, the level (_libra aquaria_), and the chorobates,
the latter consisting of a rod about 20 feet in length, having two
legs at its extremities of equal length and at right angles to it.
Cross-pieces were fastened between the rod and the legs with vertical
lines accurately marked on them. These vertical lines were placed in
a truly vertical position by means of plumb-lines so that the top of
the rod was perfectly level, and the work could thus be made level in
reference to it.
In Rome the water was generally conducted either by means of open
channels, usually built in masonry for the purpose, or in lead pipes,
or in “earthen tubes.” Vitruvius states that the open channels should
be as solid as possible, and have a fall of not less than one half a
foot in 100 feet. The open channels were covered with an arch top, so
that the sun might be kept from striking the water. After bringing the
water to the city it was divided into three parts. One was for the
supply of pools and fountains, another for the supply of baths, and
a third for the supply of private houses. A charge was made for the
use of water for the pools, fountains, and baths, and in this way a
yearly revenue was obtained. A further charge was also made for the
water used in private houses, the revenue from which was applied for
the maintenance of the aqueduct which supplied the water. The treatment
to be given to the different soils, rocks, and other materials through
which the conduit was built which brought the supply to Rome is duly
set forth by Vitruvius, and he describes the conditions under which
tunnels were constructed. He also described the methods of classifying
the lead pipes through which water was conducted from the reservoirs
to the various points in the city after stating that they must be made
in lengths of not less than 10 feet. The sheets of lead employed in
the manufacture of the pipes he describes as ranging in width from 5
inches to 100 inches. The diameter of the pipe would obviously equal
very closely the width of the sheet divided by the ratio between the
circumference and the diameter of the corresponding circle.
=54. Siphons.=—He speaks of passing valleys in the construction of the
conduits by means of what we now call siphons, and prescribes a method
for relieving it of the accumulated air. In speaking of earthen tubes
or pipes he says that they are to be provided not less than 2 inches
thick and “tongued at one end so that they may fit into one another,”
the joints being coated with quicklime and oil. He further observes
that water conducted through earthen pipes is more wholesome than that
through lead, and that water conveyed in lead must be injurious because
from it white lead is obtained, which is said to be injurious to the
human system. Indeed the effects of lead-poisoning were recognized in
those early days, and its avoidance was attempted. In the digging of
wells he wisely states that “the utmost ingenuity and discrimination”
must be used in the examination of the conditions under which wells
were to be dug. He also appreciated the advantage of sedimentation,
for he advises that reservoirs be made in compartments so that, as the
water flows from one to another, sedimentation may take place and the
water be made more wholesome.
=55. Healthful Sites for Cities.=—In the location of cities, as well as
of private residences, Vitruvius lays down the general principle that
the greatest care should be taken to select sites which are healthy
and subject only to clean and sanitary surroundings. Marshy places and
those subject to fogs, especially those “charged with the exhalations
of the fenny animals,” are to be avoided. Apparently this reference to
“fenny animals” may have beneath it the fundamental idea of bacteria,
but that is not certain. The main point of all these directions for
the securing of sanitary conditions of living is that, so far as his
technical knowledge permitted him to go, he insists on the same class
of wholesome conditions that would be prescribed by a modern sanitary
engineer.
=56. Foundations of Structures.=—Similarly in Chapter V of his
First Book, on “Foundations of Walls and Towers,” Vitruvius shows a
realization of the principal conditions needful and requisite for
the suitable founding of heavy buildings. After a sanitary site for
a city is determined and one that can be put in communication with
other people “by good roads, and river or sea navigation for the
transportation of merchandise,” he proceeds to state that “foundations
should be carried down to solid bottom, if such can be found, and
that they should be built thereon of such thickness as may be
necessary for the proper support of that part of the wall standing
above the natural level of the ground. They should be of the soundest
workmanship, and materials of greater thickness than the walls above.”
Again, in speaking of the foundations supporting columns, he states:
“The intervals between the foundations brought up under the columns
should be either rammed down hard, or arched, so as to prevent the
foundation-piers from swerving. If solid ground cannot be come to,
and the ground be loose or marshy, the place must be excavated,
cleared, and either alder, olive, or oak piles, previously charred,
must be driven with a machine as close to each other as possible
and the intervals between the piles filled with ashes. The heaviest
foundations may be laid on such a base.” It is thus seen that pile
foundations were used by the Romans, and that the piles were driven
with a machine. It would be difficult to give sounder general rules of
practice even after more than two thousand years’ additional experience.
=57. Pozzuolana and Sand.=—Of all the materials which were useful to
the Romans in their various classes of construction, including the
foundations of roads, “pozzuolana” must have been the most useful,
and that which contributed more to the development of successful
construction in Rome than any other single agent. Vitruvius speaks
of it frequently and gives rules not only for the use of it in the
production of mortar and concrete, but also lays down at considerable
length the treatment which should be given to lime in order to produce
the best results. It was common, according to his statements, to use
two measures of “pozzuolana” with one of lime in order to obtain
a suitable cementing material. This mixture was used in varying
proportions with sand and gravel or broken stone to produce concrete.
He describes the various grades of sands to be found about Rome and the
manner of using them. The statement is made that sand should be free of
earth and that the best of it was such as to yield a “grating sound”
when “rubbed between the fingers.” This is certainly a good engineering
test of sand. He prefers pit-sand to either river- or sea-sand; indeed
throughout all his directions regarding this particular class of
construction his rules might be used at the present time with perfect
propriety.
=58. Lime Mortar.=—The old Romans had also discovered the advisability
of allowing lime to stand for a considerable period of time after
slaking. This insured the slaking of all those small portions which
were possibly a little hydraulic and therefore slaked very slowly. He
prescribes as a good proportion two parts of sand to one of lime, and
also mentions the proportion of three to one. He attempts to explain
the setting, as we term it, of lime, but his explanation in obscure
terms, involving qualities of the elements of fire and air, is not very
satisfactory.
=59. Roman Bricks according to Vitruvius.=—As is well known, the
Romans were good brick-makers, and they were well aware that bricks
made from “ductile and cohesive” “red or white chalky” earth were far
preferable to those made of more gravelly or sandy clay. The Roman
bricks were both sun-dried and kiln-burned.
=60. Roman Timber.=—Timber was a material much used by the Romans, and
the greater part of that which they used probably was grown in Italy,
although considerable quantities were imported from other localities.
Vitruvius writes in considerable detail concerning the selection of
timber while standing, as well as in reference to its treatment before
being used in structures. Like every material used by the old Romans
in construction, the various kinds and qualities of timber received
careful study from them, and they were by no means novices in the art
of producing the best results from those kinds of timber with which
they were familiar.
=61. The Rules of Vitruvius for Harbors.=—In Chapter XII of his Fifth
Book Vitruvius lays down certain general rules for the selection and
formation of harbors, and it is known that the Romans were familiar
with elaborate and effective harbor construction, as is shown by
that at Ostia. He appreciates that a natural harbor is one which has
“rocks or long promontories jutting out, which from the shape of the
place form curves or angles,” and that in such places “nothing more
is necessary than to construct portices and arsenals around them, or
passages to the markets.” He then proceeds to state that if such a
natural formation is not to be found, and that if “on one side there
is a more proper shore than on the other, by means of building or of
heaps of stones, a projection is run out, and in this the enclosures of
harbors are formed.” He then proceeds to explain how “pozzuolana” and
lime, in the proportion of two of the former to one of the latter, are
used in subaqueous construction. He also prescribed a mode of building
a masonry wall up from the bottom of an excavation made within what we
should call a coffer-dam, formed, among other things, “of oaken piles
tied together with chain pieces.” The Romans knew well how to select
harbors and how to construct in an effective manner the artificial
works connected with them, although it appears that the effects of
tidal and river currents in estuaries were neither well understood in
themselves nor in their transporting power of the solid material which
those currents eroded.
=62. The Thrusts of Arches and Earth; Retaining-walls and
Pavements.=—Although the Romans possessed little or no knowledge
of analytical mechanics they attained to some good qualitative
mechanical conceptions. Among other things they understood fairly
well the general character of the thrust of an arch and the tendency
of the earth to overthrow a retaining-wall. They knew that a massive
abutment was needed to receive safely the thrust of an arch, and they
counterforted or buttressed retaining-walls in order to hold them
firmly in place. They also realized the danger of wet earth pressing
against a retaining-wall, and even made a series of offsets or teeth
on the inside of the wall on which the earth rested in order to aid in
holding the wall in place. Vitruvius recommends as a safeguard against
the pressure of earth wet by winter rains that “the thickness of the
wall must be proportioned to the weight of earth against it,” and that
counterforts or buttresses be employed “at a distance from each other
equal to the height of the foundations, and of the same width as the
foundations,” the projections at the bottom being equal in thickness to
that of the wall, and diminishing toward the top.
He gives in considerable detail instructions for the forming of
pavements and stucco work, so many examples of which are still existing
in Rome. These rules are in many respects precisely the same as would
govern the construction of similar work at the present time. There are
also described in a general way the methods of producing white and
red lead, as pigments of paints, and a considerable number of other
pigments of different colors.
=63. The Professional Spirit of Vitruvius.=—It is evident, from many
passages in the writings of this Roman architect-engineer, that the
ways of the professional men in old Rome were not always such as led to
his peace of mind. Vitruvius utters bitter complaints which show that
he did not consider purely professional knowledge and service to be
adequately recognized or appreciated by his countrymen. He writes that
in the city of Ephesus an ancient law provided that if the cost of a
given work completed under the plans and specifications of an architect
did not exceed the estimate, he was commended “with decrees and
honors,” but if the cost exceeded the estimate with 25 per cent added
thereto, he “was required to pay that excess out of his own pocket.”
Then he exclaims, “Would to God that such a law existed among the Roman
people, not only in respect to their public but also to their private
buildings, for then the unskilful could not commit their depredations
with impunity, and those who were the most skilful in the intricacies
of the art would follow the profession!”
=64. Mechanical Appliances of the Ancients.=—It is well known that the
ancients possessed at least some simple types of machines, for the
reason that they raised many great stones to a considerable height in
completed works after having transported them great distances from the
quarries whence they were taken. Undoubtedly these machines were of a
simple and crude character and were made effective largely by the power
of great numbers of men. We are not acquainted with all the details
of these machines, although the general types are fairly well known.
The elementary machines, including the lever, the inclined plane, the
pulley, and the screw, which is only an application of the inclined
plane, were all used not only by the Romans, but probably by every
civilized ancient nation. Vitruvius describes a considerable number of
these machines, and from his descriptions it is clear that they had
wide application in the structural works of the Romans. The block and
fall, as we term the pulley at the present time, was a common machine
in the plant of a Roman constructor, as were also various modifications
and applications of the lever, the roller, and the inclined plane.
=65. Unlimited Forces and Time.=—It is neither surprising nor very
remarkable that with the use of these simple machines, aided by a
practically unlimited number of men, the necessary raising or other
movement of heavy weights was accomplished by the Romans and other
ancient peoples. It is to be borne in mind that the element of time was
of far less consequence in those days than at present, and that the
rate of progress made in the construction of most if not all ancient
engineering works was what we should consider intolerably slow.
PART II.
_BRIDGES._