to correspond with the rods in the caisson. These screws were twenty-four feet long, ten of them three inches in diameter, and the other fourteen, which had been used in lowering the masonry of the Quincy Railroad bridge, two and a half inches. Ten additional screws of the same size as the latter were kept in

reserve.

On the morning of the 21st of October the caisson was successfully launched and towed to the false-works. Two or three weeks previously a large flat-boat loaded with sand, in attempting to shoot the works, had struck against one of the upper piles and sunk; the wreck had caused a sand deposit at the pier site, so that, though there was plenty of water to float the caisson, which drew only three feet and a half, it could not be brought under the trusses without removing the suspension rods; they were, accordingly, unscrewed, taken out, and the caisson brought into position, when they were replaced and easily screwed into the nuts, which were held by the square recesses cut in the triangular timber below the sill. This was accomplished in a day, but the want of deep water proved a more serious obstacle in the way of removing the false bottom. It had first been proposed to sink the bottom by throwing in sand, water being already admitted above it, make fast to it with the steamboat, and pull it out below; the depth of water proving insufficient for this, it had to be broken in pieces, and taken out in small parts, an operation which involved nearly two weeks' delay, and which, it was feared, would cause trouble by leaving unremoved fragments; an apprehension which fortunately proved groundless. A week later a sand bar, which had already been observed forming in front of the launching ways, had so much increased that it would have been impossible to launch the caisson, so that a tedious portage by land was narrowly escaped.

On the 11th of November, the work was begun of filling the spaces between the double walls of the caisson with beton, while the false-works were completed, and the machinery mounted as fast as could well be done.* The false-works were built with three floors; the lower one, intended for the use of carpenters and masons, was placed at an elevation of 108.7, and made a

* Full plans of these works and machinery are given on Plates. IV. and V.

continuous platform extending on all sides of the caisson; it was generally left open on all sides, but a small house was built at the south-east corner, in which a twenty-five horse-power engine and a donkey pump were placed; a room was also enclosed in the middle of the south side for the use of the divers, where the air-pump and submarine apparatus was kept. At the south-west corner a staircase led to the second floor, which was placed on a level with the lower chords of the trusses. This floor extended over the caisson, having four holes in it through which the dredges worked; it was completely housed in, was provided with work benches, warmed by stoves, and contained the lamp-room and superintendent's office. At either end a staircase led to the third floor, a narrow platform, resting upon the upper chords of the trusses, where stood the four hand-crabs used in handling the dredges.

The excavating machinery consisted of four large dredges of the endless chain pattern. They were mounted with vertical telescopic frames of wood, the lower tumbler being attached to a single frame, inclosed by a double frame which carried the upper tumbler; the boxes of the upper tumbler were set on adjustable blocks.* By this arrangement the dredges could be lengthened to suit the depth at which they were operating, the length being varied from 51 to 85 feet; this was done by removing the bolts which united the two frames, putting in an additional length of dredge chain, with the proper number of buckets, and raising the outer frame till the length of the added chain was taken up; the bolts were then replaced and such slack as might remain in the chain taken out with the adjusting screws. The entire frames were raised and lowered, independently of this change in their length, by chains which passed through sheaves on the sides of the double frame, and were worked by the crabs on the upper floor. Two of these dredges had originally been used on the Quincy bridge and were now rebuilt to adapt them to this work; a third was similar, and had been made from the same patterns, though designed in the first instance for use at Kansas City, on the old No. 4 foundation. These three dredges had square tumblers of cast-iron; the links of the chains measured 22 inches between centres, and the buckets were bolted on every fourth link

* For plans and details of dredges, see Plates V. and VI. The plan on Plate V. shows the frame of shorter length than that actually used.

through holes drilled for the purpose. The other dredge was constructed especially for use on this pier; the tumblers were of hexagonal form, made of oak and bound with wrought-iron; the chain links were only 12 inches long between. centres formed with upset ends; the buckets, whose form was novel, were placed on every sixth link and held by the same pins by which the links were coupled, an arrangement relieving the links of any transverse strain, the merit of which was proved by the fact that the chain of this dredge never broke; the hexagonal tumbler was also found to give a steadier motion than the square ones.*

A single line of shafting mounted on hangers attached to the trusses, and driven by the engine on the floor below, extended from the east end of the house till opposite the western dredge; on this shaft were placed four pulleys, each arranged with clutch and lever, by which it could be thrown out of gear independently of the others, and the power was carried to the dredges by belts driven by these pulleys. The new dredge was mounted at the west end of the pier, and the dredge at the east end was also provided with buckets of the new pattern. These two dredges were worked through bevel gearing, the power being transmitted at any elevation by a pinion sliding on a vertical shaft; the other two were driven more directly by the belts, which were kept tight under all elevations by a loaded tightener sliding in a vertical frame; the latter arrangement proved the better one. Each dredge was completely boxed in between the second and third floors, to confine the splash, thus keeping the machinery and works upon the second floor dry and in good working order. The dredges discharged towards the north, the sand falling on inclined troughs which led to the lower platform, from which it was carried off in wheelbarrows, on runways built for the purpose, and deposited a hundred feet north of the

works.

The machinery for handling stone was on the lower platform. It consisted of a railway and cars, the same which had been used at Pier No. 1, running along the west end of the works, and two travellers running lengthwise with the pier, between the wells and the sides of the caisson. A floating derrick was

* A patent for the improvements in these dredges was issued to the authors of this work, bearing date January 18, 1870.