44 ANNUAL OF SCIENTIFIC DISCOVERY. 



The dimensions of the caisson are rectangular; length 168 feet, 

 width 102 feet, height 9 feet 6 inches. Thickness of roof 5 feet. 

 The sides form a V, and are 9 feet thick where they join the roof, 

 sloping down to a round edge. The inner slope of the V has an 

 angle of 45 degrees. The lowest part of the slope is formed by a 

 semicircular section casting, protected by a sheet of boiler plate, 

 which extends up 3 feet each side. A heavy oak sill rests on the 

 casting, and it consists of a stick nearly 2 feet square. The 3 suc- 

 ceeding courses are laid lengthwise, after that the alternate 

 courses are heading courses. The whole mass is thoroughly 

 bolted together by drift bolts, screw bolts, and wood-screw bolts. 

 In addition there are heavy angle irons uniting the V to the roof. 

 At the corners the courses of timber are halved into each other, 

 and strapped together for further security. The roof is composed 

 of 5 courses of 12-inch square j'ellow pine sticks, laid close 

 together, bolted sidewaj's and vertically, and having a set of 

 heavy bolts running through the 5 courses. The outer edge of 

 the caisson has a batter inward of 1 in 10 to facilitate its descent 

 into the ground. 



To make the caisson air-tight, the seams were all thoroughly 

 caulked for a depth of 6 inches, inside and out. and in addition a 

 vast sheet of tin, unbroken throughout, extends over the whole 

 caisson, between the fourth and fifth course, and down the 4 

 sides to the shoe. The tin on the outside is further protected by 

 a sheeting of yellow pine. The space between the timbers was 

 filled with hot pitch. As air under pressure of 40 or 50 pounds 

 w r ill penetrate wood with ease, the inside of the air-chamber was 

 coated with an air-tight varnish, made of resin, minhaden oil, and 

 Spanish brown. The air-tightness up to the present time is quite 

 satisfactory, and only one-fifth of the air-pump on hand is suffi- 

 cient to keep the water out. 



The yellow pine timber was selected specially for the purpose. 

 It came principally from Georgia and Florida, and much of it was 

 so pitchy that the sticks would not float. The average specific 

 gravity of all the timber was 48 degrees per cubic foot. Every 

 bolt-hole is bored with a large drift to ensure the hold of the 

 bolts. As the construction of the caisson proceeded, the iron 

 work of the water-shafts, air-lock-shafts, and supply-shafts was 

 put in. 



The water-shafts, 2 in number, "are square shafts, three-eighths 

 boiler plate, properl} T stiffened by angle irons, and well secured to 

 the caisson. The} T are 7 feet by 6 feet 6 inches, and are open 

 above and below, the lower edge extending 20 inches below the 

 edge of the shoe. The water inside of them rises and falls with 

 the state of the tide outside. The material to be taken out is 

 shoved under the edge into the water-shaft by the laborers inside, 

 and is then taken out by the so-called clam-shell dredge of Morris 

 & Cumniings, of New York, the only known instrument which 

 possesses the precise action of the human hand in picking up 

 things. Any other arrangement for excavating in the shape of a 

 revolving dredge or a sand-pump was out of the question. The 

 air-shafts "are 3.0 feet in diameter, and extend simply through the 



