piers were founded directly upon the rock, the others might safely be put upon piles, the care taken to protect the pile foundations increasing with their nearness to the channel. This arrangement was believed to have the farther advantage of making it practicable to begin work on the sand-bar piers at an earlier date than would otherwise have been possible. It was accordingly determined that Piers Nos. 1, 2, and 3, should rest directly upon the rock, while the four more northerly piers should have pile foundations; the piles were to be driven in excavated pits, cut off in every instance at a considerable depth below the usual bed of the river, and further secured by an ample protection of riprap; those under Pier No. 4 were to be driven home to the rock, and cut off at least 25 feet below extreme low-water. The experience acquired during the progress of the works led to subsequent changes in the plan; Pier No. 4 was treated as a channel pier, and founded on the bed rock, and the piles under Pier No. 5 were driven home to the rock, thus reducing the number of pile foundations to three, only two of which depend upon the frictional surface of the piles for their support. Piers Nos. 5 and 6 are on the dry land of the sand bar for nine months in every year, and Pier No. 7, situated within the line of the wooded shore, is exposed to the action of the water only on rare occasions. During the progress of the works no current strong enough to produce scour was noticed about any one of these piers.

All the foundations were put in of a sufficient length for a double-track bridge.

Although the foundations may properly be grouped in the two classes of channel and sand-bar foundations, as above mentioned, the characteristics of several pier sites were so different that it became necessary to treat each by itself, and to prepare as many sets of plans as there were piers. The south abutments and the two small pillars on the bank were built on the rock which was found a few feet below the surface of the ground, their foundations presenting no greater difficulties than are common to every cellar wall. They were built in the fall and early winter of 1867-68, the work upon them being executed at such intervals as the masons were not employed upon the river piers. The other foundations were taken up in the order of convenience; the dates at which the work in the river was begun in each instance, and the dates

at which the several foundations were ready for the masonry, were as fol

These dates do not in themselves give a correct idea of the time actually consumed in putting in the substructure of the piers; the work on some of them was more than once suspended, either on account of the season or to accommodate other work which had momentarily become of greater importance. After the caisson had been built at the site of Pier No. 5, it was left standing for two months, until machinery could be spared to sink it. At Pier No. 4, the first set of works was entirely destroyed in the spring of 1868, in consequence of which the site of the pier had to be changed, a new plan of foundation prepared, and the first pile in the new false works was not driven till the 9th day of August, 1868; moreover, the date given above as that at which the masonry of this pier was begun, is in reality that at which the laying of masonry was resumed after the completion of the foundation and the removal of the upper false works; the bulk of the subaqueous masonry here was put in during the process of founding. In the cases of Piers 1 and 2, the caissons were built on shore, and the time occupied in the foundations, including this preparation, was therefore greater than the table indicates.

A correct idea of the time occupied in the different foundations, as well as of the characteristic features of the several plans, can only be given by separate narratives of the substructure operations at each of the seven pier sites.

PIER No. 1.

This pier is situated on the south side of the main steamboat channel, and at ordinary stages of water it stands about 100 feet from the shore line. The bed rock was found at an elevation of 84, or 13 feet below the extreme low-water

mark; at the pier site it was of irregular form, and though found to fall off rapidly a few feet farther north, it here presented a surface that was almost level, though inconveniently rough. This rock was almost bare, being seldom covered with more than a foot or two of deposit. The current was but little less rapid than in the middle of the channel, and was far too strong to allow of any extensive operations being carried on within it. It was therefore thought necessary first of all to obtain slack water about the pier site, after which the foundation works, more properly so called, could safely proceed.

A large timber caisson, designed to serve as a water deadener, or break. water, was built on the shore, about midway between the south end of the bridge line and the Company's machine shop. It was built of oak, with pointed ends; the entire floor and the sides to a height of 15 feet were solid, and of square timber; its outside measurements were 65 feet long from nose to nose, 18 feet wide, and 27 feet and three inches high; it was stiffened internally by rows of vertical truss bracing, and bound together by long iron rods built into the solid timber of the bottom and sides; the whole was thoroughly caulked, and valves were provided for admitting or excluding the water.* After it had served its purpose as a water deadener, it was raised and finally sunk below Pier No. 2, where it forms the foundation of the lower draw rest. On the 19th of May, 1869, this caisson was launched; it was kept anchored to the shore till the 7th of the following August, when the river was thought to be low enough to begin work; it was then towed to a point about 100 feet above the pier site, secured by four wire cables reaching to the shore, and sunk by admitting water through the valves and throwing in a ballast of broken stone. Being placed transversely with the current, it formed a complete water deadener, and quiet slack water was secured at the pier site.

A bottomless caisson, which should serve as an enclosure to build the pier in, was also constructed. It was built on a floor placed between four boats well braced together; it was a frame structure, entirely of oak and planked vertically; the ground plan was substantially the same as that of all other caissons built upon the work, the ends being formed of two short sides making a right angle together; its total length was 70 feet, and the width 19.5 feet. The first

* The plans of this caisson are given on Plate III.