the Panama Canal in 1894, considers that about one half of the total
excavation has already been done, and one half of the total length of
the canal almost finished, and remaining in comparatively good
condition. At both ends, including 15 miles on the Atlantic side, there
is water 18 to 24 feet deep. “Besides the work already done, the Canal
Company has on hand, distributed at both terminals, and at convenient
points along the canal route, an immense stock of machinery, tools,
dredges, barges, steamers, tug-boats, and materials for continued
construction. At Panama, La Boca, and Colon, as well as along the canal,
are numerous buildings—large and small—for offices, workshops,
storehouses, and warehouses, and for lodging and boarding the men who
were employed on the work. The finished work, as well as all the
machinery, tools, materials, buildings, etc., are well taken care of and
looked after. The Canal Company employs one hundred uniformed policemen,
besides numerous watchmen, machinists, and others, whose sole duty
consists in watching the canal and looking after needed repairs of plant
and care of materials. In fact, the work and the whole plant is in such
a condition, so far as I could ascertain, that renewed construction
could be taken up and carried to a finish at any time it is desired to
do so, after the Company’s finances will permit.”
An enormous amount of money has already been expended on the Panama
Canal, and much of it lavishly and unnecessarily. A reorganised company
may probably be able to form such estimates of the probable cost of
completing the work under careful and efficient management, that
financial confidence in it maybe restored. The canal not only already
possesses the requisite plant, but the route has the special advantages
of assistance in transport from the railway everywhere at but a short
distance from it, and fine commodious harbours for its ocean mouths. If
it were finished as originally designed, vessels could pass through it
with one tide, say in about six hours. It is understood that before the
Panama enterprise is again proceeded with, the Company think that a sum
of about £25,000 should be expended in a complete survey and re-study of
all the conditions, and the results submitted to the most eminent
engineers.
A rival scheme for carrying a ship canal across the isthmus that divides
the Atlantic and Pacific Oceans is that known as the Nicaragua Canal, as
the proposed route is to cross Lake Nicaragua, an extensive sheet of
water situated some 400 or 500 miles north-west of the Panama Canal. The
lake is 110 miles long and 45 miles broad, and is on its western side
separated from the Pacific by a strip of land only 12 miles wide, having
at one point an elevation not exceeding 154 feet, which is probably the
lowest on the isthmus. The lake drains into the Caribbean Sea on the
east, by the San Juan river, a fine wide stream, 120 miles in length,
which is navigable for river boats from the Caribbean Sea up to the
lake, except near its upper part, where some rapids at certain times
prevent the passage of the boats. This canal project first took definite
form in 1850, when a survey was made and routes reported on. The scheme
attracted some attention in the United States, and in 1872, and again in
1885, further surveys and estimates were made at the instance of the
States Government. The earlier schemes provided for the rise and fall
between sea and lake-–108 feet, a considerable number of locks—eleven on
each side, making the total length from sea to sea 181 miles. The report
of the latter advocated the canalization of the San Juan by a very bold
measure, namely, the construction of an immense dam, by which the waters
were to be retained in the valley for many miles at the level of the
lake. A company was formed to promote the project, and again in 1890
there were more surveys and estimates made. This company actually
expended a considerable sum of money in attempting to improve the
harbour at Greytown, which would have formed the eastern terminus, but
had become silted up. But it was found afterwards that it would be
better to recommend the formation of an artificial harbour at another
point, by constructing two long piers running out into the sea, although
this change would involve the abandonment of a few hundred yards of
canal already excavated by the company near Greytown. The company has
also laid down about 12 miles of railway along the proposed route, with
wooden and iron sheds as workshops, offices, etc., and, moreover, had
dredges and other appliances at work. At this stage it was proposed that
the United States Government should guarantee the bonds of the Nicaragua
Canal Company to the extent of more than twenty million pounds sterling.
By an Act of Congress passed in March, 1895, a commission of engineers
was appointed for the purpose of ascertaining the feasibility,
permanence, and cost of construction and completion of the Nicaragua
Canal by the route contemplated. The report of this commission is an
elaborate and exhaustive review of the whole scheme based upon a
personal examination of the route, and on the plans, surveys, and
estimates made for the company, whose records, however, are stated in
the report to be deficient in the supply of many important data. The
Canal Company’s project provided for the improvement of Greytown
harbour, as already stated, and from that place the canal was to proceed
westward at the sea-level to the range of high ground on the eastern
side of the isthmus, which elevation was to be ascended by three locks
of unusual depth, and a deep cut more than 3 miles in length, through
rock to a maximum depth of 324 feet. After passing this enormous cut,
the route provides for a series of deep basins, in which the water is
confined by numerous dams or embankments, the canal excavations being
confined to short sections through higher ground separating these
basins. The total length of these embankments will be about 6 miles, and
their heights will vary from a few feet to more than seventy. About 31
miles from Greytown the canal reaches the San Juan river, which,
however, by means of an enormous dam across the valley at a place called
Ochoa, 69 miles below the point at which it receives the waters of Lake
Nicaragua, is there practically converted into an arm of the lake. This
dam, which would raise the water of the river 60 feet above its present
level, and would, of course, flood the valley back to the lake, is the
most notable feature of the project. Its maximum height would be about
105 feet, and the weirs on its crest, to discharge the surplus water,
would require a total length of nearly a quarter of a mile. Twenty-three
smaller embankments would also be needed for retaining the waters; the
river would have to be deepened in the upper part, and a channel dredged
out in the soft mud of the lake for 14 miles beyond the river. The big
Ochoa dam is said to have no precedent in engineering construction, on
account of its great height and the enormous volume of the waters it is
intended to retain. No doubt its construction and safe maintenance are
within the range of engineering skill, when a thoroughly exhaustive
survey of the site has been made, and the necessary funds are
forthcoming. From the western shore of the lake its level would also be
extended by another great dam crossing the valleys of the Tola and the
Rio Grande, with a length of 2,000 feet and a height of 90 feet. The
canal would then be carried to the sea-level by a series of locks. The
length of the canal from sea to sea would be 170 miles, but of this only
40 miles of channel would require to be excavated. The total cost of the
work, as estimated by the Nicaragua Canal Company, would be about
fifteen million pounds sterling, but the State Commission of Engineers
thinks about double that amount would be a safer calculation, and taking
into account the imperfection of the data, even this might be exceeded
in certain contingencies. The Government of the United States has been
urged to expend a few thousand pounds on another engineering commission,
to make complete surveys, and consider all the practical problems
involved, including the final selection of a route.
[Illustration:
FIG. 138.—_Britannia Bridge, Menai Straits._
]
IRON BRIDGES.
The credit of having invented the arch is almost universally assigned to
the ancient Romans, though the period of its introduction and the date
of its first application to bridge building are unknown. That some
centuries before the Christian era, the timber bridges of Rome had not
been superseded by those of more permanent construction is implied in
the legend of the defence of the gate by Horatius Cocles—a tale which
has stirred the heart of many a schoolboy, and is known to everybody by
Macaulay’s spirited verses, in which
“Still is the story told,
How well Horatius kept the bridge,
In the brave days of old.”
Some of the arched bridges built by the Romans remain in use to this day
to attest the skill of their architects. The Ponte Molo at Rome, for
example, was erected 100 B.C.; and at various places in Italy and Spain
many of the ancient arches still exist, as at Narni, where an arch of
150 ft. span yet remains entire. Until the close of the last century the
stone or brick arch was the only mode of constructing substantial and
permanent bridges. And in the present century many fine bridges have
been built with stone arches. The London and Waterloo Bridges across the
Thames are well-known instances, each having several arches of wide
span, attaining in the respective cases 152 ft. and 120 ft. The widest
arch in England, and one probably unsurpassed anywhere in its
magnificent stride of 200 ft., is the bridge across the Dee at Chester,
built by Harrisson in 1820. At the end of last century _cast_ iron began
to be used for the construction of bridges, a notable example being the
bridge over the Wear at Sunderland, of which the span is 240 ft. But
with the subsequent introduction of _wrought_ iron into bridge building
a new era commenced, and some of the great results obtained by the use
of this material will be described in the present article. In order that
the reader may understand how the properties of wrought iron have been
taken advantage of in the construction of bridges, a few words of
explanation will be necessary regarding the strains to which the
materials of such structures are exposed.
Such strains may be first mentioned as act most directly on the
materials of any structure or machine, and these are two in number,
namely, extension and compression. When a rope is used to suspend a
weight, the force exerted by the latter tends to stretch the rope, and
if the weight be made sufficiently great, the rope will break by being
pulled asunder. The weight which just suffices to do this is the measure
of the _tenacity_ of the rope. Again, when a brick supports a weight
laid upon it, the force tends to compress the parts of the brick or to
push them closer together, and if the force were great enough, the brick
would yield to it by being crushed. Now, a brick offers so great a
resistance to a crushing pressure, that a single ordinary red brick may
be capable of supporting a weight of 18 tons, or 40,320 lbs.—that is,
about 1,000 lbs. on each square inch of its surface. Thus the bricks at
the base of a tall factory chimney are in no danger of being crushed by
the superincumbent weight, although that is often very great. The
_tenacity_ of the brick, however, presents the greatest possible
contrast to its strength in resisting pressure, for it would give way to
a pull of only a few pounds. Cast iron resembles a brick to a certain
extent in opposing great resistance to being crushed compared to that
which it offers to being pulled asunder, while wrought iron far excels
the cast metal in tenacity, but is inferior to it in resistance to
compression.
The following table expresses the forces in tons which must be applied
for each square inch in the section of the metals, in order that they
may be torn apart or crushed:
┌───────────────────────┬──────────────────────┬──────────────────────┐
│ │ Tenacity per square │Crushing pressure per │
│ │ inch, in tons. │square inch, in tons. │
├───────────────────────┼──────────────────────┼──────────────────────┤
│Cast iron │ 8 │ 50 │
│Wrought iron │ 30 │ 17 │
│Iron wire │ 40 │ ... │
└───────────────────────┴──────────────────────┴──────────────────────┘
Besides the direct strains which tend to simply elongate or compress the
materials of a structure or of a machine, there are modes of applying
forces which give rise to transverse strains, tending to twist or wrench
the pieces or to bend them, or rupture them by causing one part of a
solid to slide away from the rest. Strains of this kind no doubt come
into play in certain subordinate parts of bridges of any kind; but if we
divide bridges according to the nature of the strains to which the
essential parts of the structure are subject, we may place in a class
where the materials are exposed to crushing forces only, all bridges
formed with stone and brick arches; and in a second class, where the
material is subjected to extension only, we can range all suspension
bridges; while the third class is made up of bridges in which the
material has to resist both compression and extension. This last
includes all the various forms of girder bridges, whether trussed,
lattice, or tubular. The only remark that need be here made on arched
bridges is, that when cast iron was applied to the construction of
bridges, the chief strength of the material lying in its resistance to
pressure, the principle of construction adopted was mainly the same as
that which governs the formation of the arch; but as cast iron has also
some tenacity, this permitted certain modifications in the adjustment of
the equilibrium, which are quite out of the question in structures of
brick and stone.
[Illustration:
FIG. 139.
]
[Illustration:
FIG. 140.
]
[Illustration:
FIG. 141.
]
[Illustration:
FIG. 142.
]
The general principle of the construction of girder bridges is easily
explained by considering a simple case, which is almost within
everybody’s experience. Let us suppose we have a plank supported as in
Fig. 139. The plank will by its own weight sink down in the centre,
becoming curved in the manner shown; or if the curvature be not
sufficiently obvious, it may always be increased by placing weights on
the centre, as at _g_. If the length of the plank had been accurately
measured when it was extended flat upon the ground, it would have been
found that the upper or concave surface, _a b_, had become shorter, and
the lower or convex surface, _c d_, longer when the plank is supported
only at the ends—a result sufficiently obvious from the figure it
assumes. It is plain, then, that the parts of the wood near the upper
surface are squeezed together, while near the lower surface the wood is
stretched out. Thus, the portions in the vicinity of the upper and lower
surfaces are in opposite conditions of strain; for in the one the
tenacity of the material comes into play, and in the other its power of
resisting compression. There is an intermediate layer of wood, however,
which, being neither extended or compressed, receives no strain. The
position of this is indicated by the line _e f_, called the _neutral
line_. If the plank, instead of being laid flat, is put upon its edge,
as in Fig. 140, the deflection caused by its weight will hardly be
perceptible, and it will in this position support a weight which in its
former one would have broken it down. There is in this case a neutral
line, _e f_, as before; but as the part which is most compressed or
extended is now situated at a greater distance from the neutral line,
the resistance of the material acts, as it were, at a greater leverage.
Again the portions near the neutral line are under no strain; they do
not, therefore, add to the strength, although they increase the weight
to be supported, and they may, for that reason, be removed with
advantage, leaving only sufficient wood to connect the upper and lower
portions rigidly together. The form of cast iron beams, Fig. 141, which
were used for many purposes, depends upon these principles. The
sectional area of the lower flange, which is subjected to tension, is
six times that of the upper one, which has to resist compression,
because the strength of cast iron to resist pressure is about six times
greater than its power of resisting a pull. If the upper flange were
made thicker, the girder would be weaker, because the increased weight
would simply add to the tension of the lower one, where, therefore, the
girder would be more ready to give way than before. If we suppose the
vertical web divided into separate vertical portions, and disposed as at
Fig. 142, the strength of the girder, and the principle on which that
strength depends, will be in no way changed, and we at once obtain the
box girder, which on a large scale, and arranged so that the roadway
passes through it, forms the tubular bridge. It is only necessary that
the upper part should have strength enough to resist the compressing
force, and the lower the extending force, to which the girder may be
subject; and wrought iron, properly arranged, is found to have the
requisite strength in both ways, without undue weight. The various forms
of trussed girders, the trellis and the lattice girders, now so much
used for railway bridges, all depend upon the same general principles,
as does also the Warren girder, in which the iron bars are joined so as
to form a series of triangles, as in Fig. 143.
[Illustration:
FIG. 143.
]
Girders have been made of wrought iron up to 500 ft. in length, but the
cost of such very long girders is so great, that for spans of this width
other modes of construction are usually adopted.
_GIRDER BRIDGES._
[Illustration:
FIG. 144.—_Section of a Tube of the Britannia Bridge._
]