the length of a voyage from Western Europe to India. Compared with some
of the other great canals of the world, its construction was easy. The
total length between termini is about 101 statute miles, of which about
nine miles required no excavation; sixteen miles more required only
a slight excavation to make the channel of sufficient depth through
existing dry depressions, called “lakes;” and the remaining seventy-six
miles of excavation were cut chiefly through a soft alluvial soil. At
only one point did the excavation reach fifty or sixty feet in depth,
and here also was found the only instance of rock excavation. Even
this rock (gypsum) was so soft that part of it was excavated by the
steam shovels. About 80,000,000 cubic yards of material were removed.
If this material had been loaded on to cars carrying twenty-five cubic
yards per car, made up into trains of twenty cars per train, and
the trains were strung along at the rate of five per mile, it would
have required 32,000 miles of such trains to transport the material
that was excavated. Work was actually begun in 1800. The Viceroy of
Egypt originally agreed to furnish the laborers required, and at
one time about 30,000 laborers were thus employed. On a change of
administration in Egypt, the new Viceroy refused to furnish the native
labor, and it then became necessary to import labor from Europe, and to
supplement this insufficient and high-priced supply of labor by very
large dredging machines, or steam shovels, of which about sixty were
employed. The task of supplying water for the vast army of workmen
was an engineering feat of no mean character and cost, as the entire
route lies through an arid desert. A system of waterworks, having its
source at Cairo, on the Nile, and distributing the water throughout
the length of the canal, was therefore constructed. In the latter part
of 1869, the waters of the Red and Mediterranean seas were joined,
large arid depressions had been transformed into great lakes, and
ocean-going vessels were sailing through what had been a desert. The
canal is 26 feet deep, 72 feet wide at the bottom, the sides sloping
variably, according to the nature of the material, the resulting width
at the top varying from 190 to 328 feet. Although not deep enough for
the very largest vessels afloat, it will accommodate the great bulk
of ocean travel, including war vessels. The total cost of this work,
including the breakwaters, lighthouses, etc., at each terminus, was,
approximately, £20,000,000, or $100,000,000.
[Illustration: COMPLETE ROCK CUT. CHICAGO DRAINAGE CANAL.
(Depth 35 feet.)]
Unlike most canals, the Suez canal has no locks. The original plan
of the Panama canal did not include locks, but the revised plan
provided for them, in order to save excessive cutting. The Nicaragua
canal scheme necessarily includes locks. The water for the Suez
canal comes directly from the seas which are connected. A canal with
locks necessarily requires an ample water supply from some river or
fresh-water lake. If the Suez canal had been constructed at a higher
level than the Mediterranean and Red seas, had been supplied with water
from the Nile, and had, therefore, been constructed with suitable
locks at each end (as was actually recommended by some engineers), the
cost of construction, as well as the perpetual expense of maintenance,
would have been greatly in excess of its actual cost. And so the fact
that it was possible to construct the canal without locks, and without
providing for a supply of water, was a great advantage that facilitated
the promotion of the enterprise.
MANCHESTER CANAL.—This canal, having a total length of only
thirty-five and one half miles, has transformed the city of Manchester,
England, from an inland city to a seaport. Actual excavation was begun
in November, 1887, and just six years afterwards the whole canal was
filled with water. It has a depth of 26 feet, and a width at the
bottom of from 120 to 170 feet, thus giving a greater capacity than
the Suez canal or the proposed Panama canal. Some of the greatest
difficulties involved arose from the necessity of providing for the
existing canals and railroads with which that busy portion of England
is so crowded. Perhaps the most interesting feat of engineering was
the drawbridge carrying the Duke of Bridgewater’s canal at Barton.
This small canal, having originally a depth of only four and one half
feet, here crosses the River Irwell. It was justly considered a great
feat of engineering when James Brindley constructed the canal, during
the eighteenth century, so that it crossed the river on a viaduct. A
waterway crossing a waterway on a viaduct was then a new idea. But
this old canal was constructed considerably above the desired level of
the Manchester canal, and yet, of course, not so high that a masted
ship might pass under it. Therefore a draw became necessary. To add to
the complication, the water supply of the small canal being somewhat
limited, it was considered very undesirable to lose a troughful of
water (roughly, 200,000 gallons) each time the draw was opened. To
allow this water to flow into a tank and then pump it back would
consume too much time, to say nothing of the expense. Therefore the
bridge must swing with the trough full of water. That required gates
at each end of the draw, as well as at the ends of the canal on each
abutment. These gates were comparatively simple; but the difficult
problem was to ensure a water-tight joint between the ends of the draw
trough and the corresponding ends of the canal. Temperature changes, as
well as many other considerations, would preclude the possibility of
making even a fairly tight joint by swinging the draw to a close fit
with the abutments. The desired result was accomplished by placing at
each end of the draw a very short U-shaped structure, having the same
cross section as the cross section of the trough, and having beveled
ends fitting corresponding bevels on the ends of the trough. These
beveled ends are faced with rubber. To open the draw the gates are
closed, the water between the gates at each end (a comparatively small
amount) is drained off and wasted, the U-shaped wedges are raised,
and the draw is then free to turn. The wedges are operated by hydraulic
rams.
[Illustration: AN “ATLAS” POWDER BLAST UNDER A TRAVELING CABLEWAY.
CHICAGO DRAINAGE CANAL.]
CHICAGO DRAINAGE CANAL.—It will probably be a surprise to many people
to learn that this “drainage” canal has a greater cross section
throughout the “earth-work” sections than any ship canal in existence,
and is only exceeded through the rock sections by the Manchester canal.
The city of Chicago obtains its water supply from Lake Michigan. The
“intake” pipe was at first located comparatively near the shore. As the
population of the city grew and the volume of its sewage increased,
it was observed that the water supply was becoming contaminated. The
Chicago River, into which the sewage was emptied, became so foul that
the odor was intolerable. The very evident fact of this odor probably
had more to do with the promotion and accomplishment of the means of
relief adopted than the far less evident but very dangerous pollution
of the water supply. An extension of the intake pipe to a point several
miles from shore by means of a tunnel (which was in itself a notable
feat of engineering) only deferred the time when the water supply would
again be fatally contaminated if the sewage continued to flow into
the lake. It was accordingly determined to dispose of the sewage by
discharging it into an artificial channel where it might become diluted
with water from Lake Michigan, and thence pass from the watershed of
the Great Lakes to the watershed of the Mississippi. The level of Lake
Michigan is so high that there was no trouble about obtaining the
requisite grade, and the divide between the watersheds is so low that
the depth of the required cutting at the summit was not forbidding. But
why have such a large canal? It was required that the sewage should
be diluted, so as not to become offensive to the inhabitants of the
region through which the canal must pass. The law under which the work
was authorized required that the flow should be 600,000 cubic feet per
minute, and that the minimum width at the bottom of the channel must be
160 feet. According to the well-known laws of hydraulics, it was seen
that a deep canal would have a greater capacity per unit of excavation
than a very wide shallow canal. This is especially true through the
sections of deepest cut, since excavation _above_ the water line adds
nothing whatever to the capacity for flow. The sections adopted called
for a depth of water of 22 feet. The side walls in rock are practically
vertical, the width of channel being 160 feet at the bottom and 162
feet at the top. In earthwork the cross section is larger than in rock,
thus reducing the velocity of flow and danger of scouring the banks.
The width of channel at the bottom is 202 feet, the width at the water
surface being 290 feet, and the side slopes 2 horizontal to 1 vertical.
A very expensive feature of this great work was the necessity for
constructing a diversion channel for the Desplaines River throughout
that portion of the river valley occupied by the canal. Lack of space
forbids a further discussion of this feature. The canal drains into the
Desplaines River at a point where the slope of the river is so great
that there will never be danger that a strong west wind or an unusual
lowering of the level of Lake Michigan can possibly cause the current
to flow eastward.
Work on the canal was commenced only after many years of discussion,
planning, legislation, litigation, and bitter opposition by the varied
interests which considered themselves more or less injured. But the
work was actually commenced in July, 1892. The estimated excavation
was approximately 40,000,000 cubic yards—about one half that of the
Suez canal; but the length is only 29 miles, compared with 101 miles
for the Suez canal. The total cost was estimated at something over
$27,000,000. On August 22, 1900, the Congressional River and Harbor
Committee approved the work as far as completed.
V. GEODESY.
It may be that many, who have read of the incredulity of all Europe
when the voyages of navigators during the fifteenth and sixteenth
centuries first demonstrated the sphericity of the earth, will be
surprised to learn that this knowledge had been acquired almost
two thousand years before, and had since then been _forgotten_.
To Eratosthenes, a Grecian, belongs the honor of first making a
measurement (about the year 230 B. C.) of the size of the earth, which,
while very rude and inaccurate, used the same fundamental principle
as is now employed by geodesists. But the appliances of those ancient
Grecians and of the Arabians, who later carried on the work, were
exceedingly crude. Even during the sixteenth and seventeenth centuries,
when the French, English, and Dutch were working very hard on the
problem, and were gradually obtaining results which came closer and
closer to those now known to be correct, the appliances for measuring
angles were so rough and inaccurate that it was only possible to assert
that the earth is spherical, with a diameter of about 7900 miles. The
seventeenth century was nearly past when Picard first used spider lines
to determine the “line of collimation,” or the true line of sight, in a
telescope. This marked a new era in methods of work, but the eighteenth
century was about half gone when it was first authoritatively
proven that the earth is not a sphere, but is more truly an “oblate
spheroid,”—such a figure as would be obtained by flattening a sphere
at the poles. Some idea of the accuracy of the work done, even at this
stage, may be obtained by considering that the computed flattening is
so slight that if we had a perfect reproduction of the earth, reduced
to a diameter of 12 inches, the flattening would be less than 1/25 of
an inch—almost imperceptible even to a trained eye. The very highest
mountain would be considerably less than 1/100 of an inch in height on
such a sphere.
The present marvelous state of the science is due to the great
improvements which have been made in the construction and use of
angle-measuring instruments and of “base bars;” also to the development
of the mathematical theory and processes involved, notably that of
the “method of least squares.” As an illustration of the accuracy
attainable in the construction of theodolites, the writer recently
made an elaborate test of the error of the centering of one of these
angle-measuring instruments. Of course no _direct_ measurement is
possible. The result is based on a long series of observations, which,
when combined according to certain mathematical principles, will give
the desired result. The error was thus computed to be _forty-two
millionths_ of an inch. To realize what is meant when an angle is
measured with a “probable error” of a few hundredths of a second of
arc, it should be remembered that one second of arc on a circle 10
inches in diameter is less than 1/40000 of an inch. The accuracy which
has been attained in the measurement of base lines is not easily
realized by a layman. An engineer realizes the practical impossibility
of measuring a line twice and obtaining _precisely_ the same result
to the finest unit of measurement. The initiated are therefore able
to appreciate the achievement of measuring a base line having a
length of over nine miles, with a “probable error” of less than one
five-millionth of its length. The words “probable error,” as used
above, have a scientifically exact meaning, but they may be taken by
the uninitiated as representing a measure of the precision obtained.
At about the close of the last century the great mathematician,
Laplace, had declared that the results of the surveys which had then
been made were inconsistent with the theory that the form of the earth
is exactly that of an oblate spheroid. That form would require that
the equator and all parallels of latitude shall be true circles, and
that all meridian sections shall be equal ellipses. Laplace showed that
the discrepancies between the actual results obtained and the results
which the theory would call for are too great to be considered as mere
inaccuracies in the work done. With the extension, during this century,
of the great geodetic surveys, carried on by the various governments
of the world, more and more evidence has developed that the meridian
sections of the earth are not equal, which is equivalent to saying that
the equator is not a perfect circle. This has led to the next stage,
which has been to prove that the form of the earth may be more closely
represented by an “ellipsoid” than by a spheroid, that is, that _every_
section of the earth is an ellipse. Several calculations have been made
to determine the length and location of the principal axes of such a
figure. But these calculations are considered unsatisfactory, because
evidence has developed that the true form of the earth cannot be
represented even by an ellipsoid. This figure is symmetrical above and
below the equator. There are reasons for believing that the southern
hemisphere of the earth is slightly larger than the northern, and that
the form of the earth is more nearly that of an “ovaloid,”—a figure of
which the ordinary hen’s egg is an exaggerated example.
All the above forms, the sphere, spheroid, ellipsoid, and ovaloid
are geometrical forms which represent with more and more exactness
the true form of the earth, but even this increasing exactness will
not account for the discrepancies and irregularities which have been
found at various places, and which cannot be explained on the ground
of inaccurate work. Geodesists have been forced to the conclusion that
the true form of the earth is not a regular geometrical form, but is
a “geoid,” that is, like the earth and like nothing else, unless we
admit the exaggerated comparison that it is “like a potato.” It should
be understood that the words “form of the earth” do not refer to the
actual surface of mountain, valley, or ocean bottom, but to the actual
ocean surface, and to the surface which the free ocean would assume if
it could penetrate into the heart of the continents. The astounding
accuracy of the work done may be appreciated when we consider that the
differences between the “geoid” and the more accurate mathematical
forms are distances which should be measured in feet rather than in
miles. For many purposes, it is sufficiently exact to consider the
earth as a sphere. For some very precise work it is necessary to
consider it as a spheroid. The more exact forms have little or no
utilitarian value, and the vast amount of work that has been spent
on these researches has been due to man’s thirst for knowledge as
such,—due to the same enthusiasm which advances the sciences in fields
which only broaden man’s knowledge of the world in which we live.
VI. RAILROADS.
The achievements of engineering skill on the line of bridges, canals,
tunnels, etc., have been great, but their effect is insignificant
compared with the social revolution that was created by the invention
and development of railroads. The railroads of this country represent
a value of about $12,000,000,000—one sixth of the national wealth.
Their pay-rolls include about 850,000 employees—1/28 of the working
population. They support, directly or indirectly, about 5,000,000
people. They collect an annual revenue of about $1,200,000,000, which
is greater than the value of the combined products of gold, silver,
iron, coal, and other minerals, wheat, rye, oats, barley, potatoes,
and tobacco, produced by the entire nation. Such a stupendous social
institution requires special discussion, and it will be found treated
separately under the heading of “Evolution of the Railway.”
VII. TUNNELS.
Tunnels are of exceedingly ancient origin, if by tunnels we include
all artificial underground excavations. From prehistoric times natural
caves have been used as burial places, and, following this practice,
tunnels and artificial rock chambers have been cut out by kings and
rulers in Thebes, Nubia, and India during periods so ancient that we
call the study of their history archæology. Nor were the ancient
tunnels confined to tombs. The Babylonians constructed tunnels through
material so soft that a lining of brick masonry had to be used to
sustain the work. The Romans constructed a tunnel over three and one
half miles long to drain the waters of Lake Fucino. About 30,000
laborers were occupied on this work for eleven years. The nineteenth
century can hardly boast of works that represent a greater amount
of labor (measured in mere days of work) than some of these ancient
monuments of constructive skill, but the masterpieces of this century
are works which have been greatly aided and even rendered possible by
three modern inventions,—compressed-air drilling machines, modern
explosives, and the compressed-air process used in subaqueous work.
The advance in methods of tunnel surveying is as great and nearly
as important. Progress in excavating tunnels is necessarily slow,
because the working face is so small that only a few men can work
there at a time, and the rate of advance depends upon them. As an
illustration: although the Mont Cenis tunnel belongs to the latter
half of this century, the first blast being made in 1857, yet for
the first four years hand drilling was employed, when the average
progress was about nine inches per day. Then machine drilling with
compressed air was adopted, when the rate of advance was multiplied
five times. The invention of compressed-air drills simultaneously
solved two difficulties: (1) The compressed air furnishes an extremely
convenient and safe form of power, which enables holes to be drilled
much more rapidly than it is possible to drill them by hand. (2) The
compressed air, after doing its work, is exhausted into the tunnel,
and thus furnishes a continuous supply of fresh air. The necessity
for ventilation has often required the construction and operation of
expensive ventilating plants. Add to these improvements the lighting of
the tunnel, even during construction, by electric lights which consume
no oxygen, and the comparison between ancient and modern methods
becomes especially marked. Before the invention of explosives, hard
rock was sometimes broken by building wood fires next to the rock,
and then, when the rock had become very hot, cooling it suddenly with
water. The sudden contraction would split the rock. Ventilation was
attempted by waving fans at the tunnel entrances. With torches and
fires to consume the precious oxygen, and no effective ventilation, it
is a wonder how those earlier tunnels were constructed. The compressed
air methods for subaqueous work will be referred to under a special
case. The essential principles have already been described under
caissons.
[Illustration: AMERICAN PORTAL, ST. CLAIR TUNNEL. NORTH OF DETROIT,
MICH.]
TUNNEL SURVEYING.—The tunnel surveying developed during this century
is one of the marvels of surveying work. If a tunnel is to be several
miles in length, not only is the excavation commenced at each end, but
one or more intermediate shafts are frequently sunk to the level of the
tunnel, and excavation is extended in each direction from the shafts.
It is extremely important that these sections of the tunnel should
“meet” exactly. If they should fail to do so by any appreciable amount,
the necessary modifications are frequently costly and therefore justify
the most elaborate precautions in the surveying work, especially since
the surveying costs much less than the consequences of such a blunder.
The Hoosac tunnel is over 25,000 feet long. The heading from the east
end met the heading from the central shaft at a point 11,274 feet from
the east end and 1563 feet from the shaft. The error in alignment was
five sixteenths of an inch, that of levels “a few hundredths,” error
of distance “trifling.” The corrected alignment was then carried on
toward the heading from the west end, which it met at a point 10,138
feet (nearly two miles) from the west end and 2056 feet from the shaft.
Here the error of alignment was 9/16 of an inch and that of levels
about 1-5/8 inches. The surveying work of the spiral tunnels on the
St. Gothard Railway (to be described later) is another example of
marvelously accurate work under peculiarly unfavorable circumstances.
ST. GOTHARD TUNNEL.—To appreciate the magnitude of the problem
involved, of which this great tunnel is the crowning feature, some idea
should be obtained of the Alpine topography lying between Silenen, in
Switzerland, and Bodio, in Italy, less than forty miles apart. The
idea of connecting Switzerland and Italy by a railroad passing over or
through the Alps, by utilizing the St. Gothard Pass as far as possible,
dates back to 1850, or even earlier. An enterprise of such magnitude
could be consummated only after years of discussion, planning,
surveying, negotiations, and even international agreements. In 1871 a
treaty was finally ratified between Germany, Italy, and Switzerland, by
which the construction and financiering was duly authorized. On August
7, 1872, the contract for the construction was signed, with a proviso
that the work must be completed within eight years. On April 30, 1880,
the advance headings met, and soon thereafter the mails were regularly
carried through, although the tunnel was not actually completed in the
specified time.