MECHANICS AND USEFUL ARTS. 45 



timber on top of which the air-locks are placed. The supply- 

 shafts are 2 inches' timber, 21 inches' diameter, and of indefinite 

 length, they have a door at the bottom and one on the top with 

 an equalizing pipe. They are filled full of made air, and the 

 whole contents fall into the air-chamber below. 



It was the original intention to have made the air-chamber 

 under the caisson one entire space without any divisions into 

 compartments, thus facilitating the excavation of the material. 

 Various considerations led to the abandonment of that view. 

 Since the caisson was to be launched like a ship, a certain num- 

 ber of launching-ways were required, and these required a stiff 

 frame from the launching-way up to the roof. Again, in the 

 boulder soil, only a few points of the edge would have rested and 

 supported the weight at any one time. But the chief point was 

 the rise and fall of the tides and their effect on the caisson. The 

 extreme rise and fall is 7 feet. If the inflated caisson is just 

 barely touching the ground at high water, it will press upon the 

 base with a force of 4,000 tons at low tide, all of which has to be 

 met by the strength of the shoe and the frames. And it is not 

 until the caisson is permanently righted down that the continuous 

 excavation can take place inside. The frames are proportioned 

 somewhat to the strains in launching, and form a heavy truss of 

 pine posts and stringers with 3-inch sheathing on each side, and 

 side-braces to the roof every C feet. The ends of the frames are 

 secured to the sides or the V by knees. 



It was concluded to limit the pressure of the caisson during the 

 launch to 2^ tons per square foot of launching surface. This 

 required 7 ways in all, 2 under the edges and 5 under the frames. 

 The total launching weight of the caisson was 3,000 tons, con- 

 taining 111,000 cubic feet of timber and 250 tons of iron. It was 

 launched sideways, that is, with the long face of 168 feet by 14 

 feet 6 inches high 'facing the water. The ground-ways were laid 

 at an angle of 1 inch per foot, the caisson standing 50 feet back 

 from the end of the ways. To buoy up the forward end of the 

 caisson as it entered the water, and thus prevent its entire im- 

 mersion, a temporary water-tight compartment of 2-inch plank 

 was put in, one-third the distance across. It served its purpose 

 admirably. A full complement of wheel-barrows, crabs, and 

 winches were likewise stowed away in it. The ground-ways 

 consisted of 2 timbers, of 11 inches square each, bolted together 

 sideways. They were grooved like the guide of a planer, and 

 the upper launching-way fitted their grooves correspondingly. 

 The great danger of launching so large a mass on 7 ways consists 

 in the liability of one end going faster than the other, and thus 

 wedging the caisson fast on the ways. Only the outer ways were 

 provided with ribbands. They, however, proved superfluous to 

 accelerate the motion of the caisson as it entered the water, and 

 thus overcome the increasing resistance. The ways were laid 

 crowning to the amount of 18 inches in their length. The 

 launching-ways were likewise continued 10 feet back of the 

 caisson, and provided with shoes against the sides ; it was desir- 

 able that the rear edge of the caisson should leave the end of the 



