tudinal curvature from the ends of the bridge to the centre, and its own weight, give to the whole structure that degree of rigidity and stability which are its marked characteristics, as contrasted with suspension bridges usually.

Some of the principal dimensions of the means of suspension are given in the following statement:

Span of both cables between axis of piers, 8211 feet.
Versed sine of cables of lower platform, 64 feet.
Versed sine of cables of upper platform, 54 feet.
Diameter of each cable, 10 inches.

Area of cross-section of each cable, 60.4 square inches. Area of cross-section of upper links of anchor-chains, 372 square inches.

Ultimate strength of anchor-chains, 11,904 tons.
Number of wires in the four cables, 14,560.
Average strength of one wire, 1,648 lbs.

Ultimate strength of the four cables, 12,000 tons.

Permanent weight borne by the cables, 1,000 tons.

Length of anchor-chains, 66 feet.

Length of upper cables, 1,261 feet.

379

Length of lower cables, 1,193 feet.

Number of suspenders, 624.

Number of over-floor stays, 64.

Number of under-floor stays, 56.

Length of platforms between piers, 800 feet.

Height of railway track above middle stage of water, 245 feet.

678. East River Bridge. The East River Bridge, which is now in process of erection, will, when completed, be the longest span suspension bridge which has been erected up to this date. It will form a suspended highway connecting New York and Brooklyn cities. The terminus in New York city will be opposite City Hall, in Chatham street; and in Brooklyn in the square bounded by Fulton, Sands, Washington, and Prospect streets. Its total length will be 5,989 feet. The central span will cross the river without impeding navigation, in a single span of 1,595 feet 6 inches from centre to centre of tower.

On each side of the central opening on the land sides there will be spans supported by the land cables of 930 feet each. The remaining distances, which form the approaches, will be supported by iron girders and trusses, and will rest at short intervals upon small piers of masonry or iron columns, located within the blocks of buildings which will be crossed and occupied. These pillars will form part of the walls needed for the division of the occupied ground into stores, dwellings, or offices.

The grade from the New York terminus to the centre of the bridge will be three feet and three inches per hundred feet, and the same on the Brooklyn side from the centre of the bridge to the anchorage, but the grade of the Brooklyn approach will be two feet and nine inches per hundred feet.

The floor of the bridge will be 85 feet in width from out to out. The floor is divided into five spaces by six lines of iron trusses. The outer spaces will be in the clear eighteen feet each, and will accommodate each two lines of iron tramways for ordinary vehicle travel, as well as for street cars, drawn singly by horses, or in pairs by light dummies. The next two spaces will be thirteen feet two inches wide each, provided with an iron track for running of two passenger trains back and forward alternately. These trains will be attached to an endless wire rope, propelled by a stationary engine, which will be located on the Brooklyn side, underneath the floor, the two tracks being operated like an inclined plane,

with a speed of twenty miles per hour, the whole transit occupying only five minutes from terminus to terminus.

The central or fifth division of the bridge floor will form a promenade for foot travel, fifteen feet in width. It will be elevated five feet above the roadway, affording a view over both sides of the bridge.

The roadway will pass the towers at an elevation of 119 feet, and the centre of the main span will be 135 feet above mean high tide, or 140 feet above mean low water.

The width of the roadway, from outside to outside, will be 85 feet.

The bridge will be supported by four main cables, each 16 inches in diameter, composed of galvanized tempered caststeel wire, No. 6 gauge, having a strength of 160 pounds per square inch of section. There will also be 104 stays to aid

the cables.

The total weight of the structure, including the cables, is estimated to be 5,000 tons.

This grand structure was devised, and works superintended till his death, by the late John A. Roebling. It is now engineered by his son Col. W. A. Roebling.

VIII.

MOVABLE BRIDGES.

for

679. The term movable bridge is commonly applied to a platform supported by a framework of timber or of cast iron, by means of which a communication can be formed or interrupted at pleasure between any two points of a fixed bridge, or over any narrow water-way. These bridges are generally denominated draw-bridges, but this term is now, the most part, confined to those movable bridges which can be raised or lowered by means of a horizontal axis, placed either at one extremity of the platform, or at some intermediate point between the two ends, and a counterpoise which is so connected with the platform in either case, that the bridge can be easily manoeuvred by a small power acting through the intermedium of some suitable mechanism applied to the counterpoise. The term turning or swinging bridge is used when the bridge is arranged to turn horizontally around a vertical axis placed at a point between its two ends, so that the parts on each side of the axis balance each other; and the term rolling bridge is applied when the bridge,

resting upon rollers, can be shoved forward or backward horizontally, to open or interrupt the passage.

To the above may be added another class of movable bridges used for the same purpose, which consist of a platform supported by a boat, or other buoyant body, which can be placed in or withdrawn from the water-way as circumstances may require.

680. Draw-Bridges. When the horizontal axis of this description of bridge is placed at the extremity of the platform, the bridge is manoeuvred by attaching a chain to the other extremity, which is connected with a counterpoise and a suitable mechanism, by which the slight additional power required for raising the bridge can be applied.

Fig. 200-Shows the manner of manoeuvring a drawbridge either by a framed lever, or by a counterpoise suspended from a spiral eccentric.

A, abutment.

a, section of the platform. b, framed lever.

c, chain attached to the ends of the lever and the platform.

d, strut movable around its
lower end.

e, bar with an articulation
at each end that confines
the strut to the platform.
J, spiral eccentric connected
with the counterpoise g by
a chain passing over the
gorge of the eccentric.
h, chain for raising the
bridge, one end of which
is attached to the extremity
of the platform, and the
other to the axle of the
eccentric.

1, fixed pulley over which the
chain h is passed.

m, wheel fixed to the axle of the eccentric for the purpose of turning it by means of animal power applied to the endless chain n.

A number of ingenious contrivances have been put in practice for these purposes. They consist usually either of a counterpoise of invariable weight, connected with additional animal motive-power, which acts with constant intensity, but with a variable arm of lever; or of a counterpoise of variable weight, which is assisted by animal motive-power acting with an invariable arm of lever. In some cases the bridge is worked with a less complicated combination, by dispensing

with a counterpoise, and applying animal motive-power, of variable intensity, acting with a constant or a variable arm of lever.

Among the combinations of the first kind the most simple consists in placing a framed lever (Fig. 200) revolving on a horizontal axis above the platform. The anterior part of the frame is connected with the movable extremity of the platform by two chains. The posterior portion, which forms the counterpoise, has chains attached to it by which the lever can be worked by men.

When the locality does not admit of this arrangement, the chain attached to the movable end of the platform may be connected with a horizontal axle above the platform, to which is also attached a fixed eccentric of a spiral shape (Fig. 200), connected with a chain that passes over its gorge and sustains a counterpoise of invariable weight. Upon the same axle an ordinary wheel is hung, over the gorge of which passes an endless chain to manoeuvre the bridge by animal power.

Fig. 201-Shows the ar-
rangement of a draw-
bridge with a variable
counterpoise.

A and B, abutments.
g, variable counterpoise
formed of a chain with
flat links, one end of
which is attached to a
fixed point, and the
other to the chain c at-
tached to the movable
end of the platform.
i, fixed pulley over which
the chain c passes to
the small wheel & fixed
on a horizontal shaft,
to which is also attach-
ed the wheel m and
the endless chain 12
for manoeuvring the
bridge.

of a

Of the combinations of variable counterpoises the mechanism of M. Poncelet, which has been successfully applied in many instances in France for the draw-bridges of military works, is one of the most simple in its arrangement and construction. The movable end of the platform (Fig. 201) is connected by a common chain, that passes over the gorge wheel hung upon a horizontal shaft above the platform, with another chain of variable breadth, formed of flat bar links, which forms the counterpoise. The chain counterpoise is attached at its other extremity to a fixed point in such a way, that when the platform ascends a portion of the weight of the chain is borne by this fixed point; and thus the weight of