Early in the spring quarries were opened at Roxbury, Conn., in the expectation that all the arch stones, except the voussoirs of the faces, would be furnished from them. But it became apparent by midsummer. that a sufficient quantity of material could not be obtained from them without delaying the work. Material for two arches could be supplied. A sub-contract was made for stone for one arch from a gneiss quarry near Sufferns, on the Erie Railroad. Granite from Maine was ordered for three others, and for the seventh, over Undercliff Avenue, red granite from Leetes Island, Conn., was adopted, the same as that used in the pedestal of the Statue of Liberty.

The contract for Sufferns stone was abandoned when half the material had been delivered and cut. Happily it was of good quality, and harmonized in color with the Maine granite, with which it was alternated in the arch.

SUBSTRUCTURE.

At the site of pier I., on the west side of the river, the solid rock was exposed, at or very near the surface of the ground, and no other preparation was required than to remove the surface-earth and shape the rock to receive the masonry. Some small holes under the face were filled with concrete, but generally the face-stones were fitted to the rock. All the piers to seat of metal arches are built of concrete, with a facing of granite in courses two feet high, not less than two feet wide, with headers five and six feet long, bonding the granite facing firmly to the concrete heart. The face-stones were set in mortar of Portland cement-the concrete is generally made with Rosendale cement-and the backs of the granite courses were plastered with cement-mortar before the concrete was placed against them, in order to secure a better union. The excavations for pier I. were commenced in August, and the masonry was begun on the 1st of October, 1886. It was ready for the pedestals in May, 1887, but they were not set until September.

At pier II., the solid gneiss rock, which is at or near the surface on the New York side, and again at the top of the hill on the eastern side, lies at a considerable depth between the bluffs of the river valley except at one point, where it rises to within seventeen feet of mean high tide in the river. This point, on the eastern side of the channel and close to the tracks

of the New York Central Railroad, was, of necessity, selected as the site of pier II. The surface of the rock, however, was very uneven, sloping in every direction from the high point, and it was covered with boulders and heavy gravel to within eight feet of low water, the remainder being mud.

As the question of time was of the first importance, a foundation upon a timber caisson, to be sunk by means of compressed air, was adopted as least exposed to the delays and contingencies which might arise from bad winter weather and from accidents. Plans submitted by Anderson

& Barr were approved on the 3d of September, 1886, and work upon it was immediately begun.

The caisson, which was built in place, is 104.8 feet long, 54.4 feet wide at base, and 13.3 high, with one foot batter in its height on all the sides. It is made of Florida pine timbers, twelve inches square, except the shoe or cutting edge, which is of oak. The walls of the chamber, three feet thick, are formed of superimposed horizontal timbers on the outside and inside, with an intermediate series of vertical timbers. Alternate verticals are cut off at the top of the working chamber; the intermediate timbers rise nearly to the top of the caisson. By this arrangement, half the transverse and longitudinal timbers of which the roof is composed, extend to the extreme ends and sides of the caisson, and hold in place the verticals, which thus serve as stiffeners to the sides and ends. The roof or deck of the caisson is composed of four courses of 12 x 12 inch timbers laid transversely, and two intermediate courses laid diagonally. The latter are stopped some ten feet from the end of the caisson, and the course is completed with short timbers placed longitudinally, to secure the end verticals and tie them to the remainder of the structure. Each course was well fastened with drift-bolts to the course below, the whole forming a solid mass of timber about six feet thick. The sides of the chamber and the first courses of the roof were poured, as the timbers were placed, with hot coal-tar; the remainder was grouted with cement. The inside of the working chamber was first lined with rough boards, then with a layer of asphalted paper, which was covered with tongued and grooved boards, 1‡ inches thick, the joints of which were put together with white lead.

The caisson was made with two longitudinal division walls, each two

VIEW FROM SOUTHWEST.

feet thick, extending from the roof to within two feet of the bottom. They were built of horizontal courses of 12 x 12 inch timbers, with openings 2 x 2 feet, for communication between the chambers formed by them; these were firmly secured by straps and bolts to the end walls. The side walls were braced and tied at nine points, with timber braces 12 x 12 inches, and iron tie-rods. There were two air-locks, as shown on the plans (Plate XXXIII.), one on top of the shaft, four feet in diameter, for general service; the other, five feet in diameter, was placed inside the caisson, as shown, and used for the extraction of material, which was loaded into buckets, placed in the air-lock, and hoisted out. There was also a shaft eighteen inches in diameter, through which the concrete for filling the chamber was introduced, and a number of blow-out pipes, four inches in diameter. The caisson was built in an enclosure formed by a temporary cofferdam. During its construction it settled two or three feet into the mud, and moved slightly out of position. It was secured to the bank with chains, and readily brought into place by means of turn-buckles on the chains.

The material to be excavated was first, mud to a depth of from eight to eleven feet, then heavy gravel with large boulders, and, finally, a very irregular rock, partly gneiss, partly marble, with veins and pockets of very hard quartz. A small portion of the mud was blown out by compressed air through the four-inch air-pipes, but the method did not work well, and it was soon abandoned. The rock on the northeast corner of the caisson rose to seventeen feet below mean high water, while at the southwest corner it was more than forty feet below.

The rock was drilled with a "6 Rand" drill, operated by air that was supplied to the drill at a pressure of eighty to one hundred pounds per square inch. Its exhaust was against a back pressure of about eighteen to twentytwo pounds, giving an effective pressure of sixty-two to eighty pounds per square inch. At first, dynamite ("forcite") was used as an explosive, but its gases were found to affect the workmen unfavorably, and it was replaced by "rack-a-rock," which proved entirely inoffensive, and was used until completion. The holes were usually four to six feet deep and three to four feet apart, 24 inches in diameter, charged with one, two, or sometimes