supported at its ends, although the ends were pinned to keep them from rising when only one part was loaded. 2d. One half was supposed to be loaded while the other end was held down by the pin; and 3d. The bridge was supposed to be loaded uniformly throughout.

The call for proposals specified that the rolling load should be 3,200 lbs. per lineal foot of the bridge, and that the wrought iron should not be strained in tension to exceed 12,000 lbs. per square inch, or in compression 8,000 lbs. per square inch. The following tables give the results of the original computations for the strains and the dimensions of the pieces used. The engineer, Charles McDonald, of New York City, states that a review of the computations after the structure was completed, confirmed the general results, although in some cases the actual strains exceed those previously determined by a small amount. Although the analysis shows (see Table II.), that there is compression on the fourth and fifth bay of the upper chord, yet there is no tendency to a strain on the counter-diagonals in those panels. The inclination of the upper chord acts as a brace and thus prevents any strain in the direction of the counter-tie in those panels.

TABLE NO. I.—Showing Total Strains on Parts when the Bridge is Open, but Unloaded.

(The sign plus is for compression and minus for tension.)

TABLE NO. II.-Showing Total Strains on Parts with Bridge Closed and one-half fully Loaded, the Unloaded end being Latched.

TABLE NO. III.-Showing Total Strains on Parts with Bridge closed and

683. Rolling-bridges. These bridges are placed upon fixed rollers, so that they can be moved forward or backward, to interrupt or open the communication across the waterway. The part of the bridge that rests upon the rollers, when the passage is closed, must form a counterpoise to the other. The mechanism usually employed for manoeuvring these bridges consists of tooth-work, and may be so arranged that it can be worked by one or more persons standing on the bridge. Instead of fixed rollers turning on axles, iron balls, resting in a grooved roller-way, may be used, a similar rollerway being affixed to the framework beneath.

684. Boat-bridge. A movable bridge of this kind may be made by placing a platform to form a roadway upon a boat, or a water-tight box of a suitable shape. This bridge is placed in, or withdrawn from the water-way, as circumstances may require, a suitable recess or mooring being arranged for it near the water-way when it is left open.

A bridge of this character cannot be conveniently used in tidal waters, except at certain stages of the water. It may be employed with advantage on canals in positions where a fixed bridge could not be placed.

IX.

AQUEDUCT BRIDGES.

685. In aqueducts and aqueduct-bridges of masonry, for supplying reservoirs for the wants of a city, or for any other purpose, the volume of water conveyed being, generally speaking, small, the structure will present no peculiar difficulties beyond affording a water-tight channel. This may be made either of masonry, or of cast-iron pipes, according to the quantity of water to be delivered. If formed of masonry, the sides and bottom of the channel should be laid in the most careful manner with hydraulic cement, and the surface in contact with the water should receive a coating of the same material, particularly if the stone or brick used be of a porous nature. This part of the structure should not be

commenced until the arches have been uncentred and the heavier parts of the structure have been carried up and have had time to settle. The interior spandrel-filling, to the level of the masonry which forms the bottom of the water-way, may either be formed of solid material, of good rubble laid in hydraulic cement, or of beton well settled in layers; or a system of interior walls, like those used in common bridges

for the support of the roadway, may be used in this case for the masonry of the water-way to rest on.

686. In canal aqueduct-bridges of masonry, as the volume of water required for the purposes of navigation is much greater than in the case of ordinary aqueducts, and as the structure has to be traversed by horses, every precaution should be taken to procure great solidity, and secure the work from accidents.

Segment arches of medium span will generally be found most suitable for works of this character. The section of the water-way is generally of a trapezoidal form, the bottom line being horizontal, and the two sides receiving a slight batir; its dimensions are usually restricted to allow the passage of a single boat at a time. On one side of the waterway a horse or tow-path is placed, and on the other a narrow footpath. The water-way should be faced with a hard cutstone masonry, well bonded to secure it from damage from the passage of the boats. The space between the facing of the water-way, termed the trunk of the aqueduct, and the head-walls, is filled in with solid material, either of rubble or of beton.

A parapet-wall of the ordinary form and dimensions surmounts the tow and foot paths.

The approach to an aqueduct-bridge from a canal is made by gradually increasing the width of the trunk between the wings, which, for this purpose, usually receives a curved shape, and narrowing the water-way of the canal so as to form a convenient access to the aqueduct. Great care should be taken to form a perfectly water-tight junction between the two works.

687. When cast iron or timber is used for the trunk of an aqueduct-bridge, the abutments and piers should be built of stone. The trunk, which, if of cast iron, is formed of plates with flanches to connect them, or, if of timber, consists of one or two thicknesses of plank supported on the outside by a framing of scantling, may be supported by a bridge-frame of cast iron, or of timber, or be suspended from chains or wire cables.

The tow-path may be placed either within the water-way, or, as is most usually done, without. It generally consists of a simple flooring of plank laid on cross-joists supported from beneath by suitably-arranged framework.