4G ANNUAL OF SCIENTIFIC DISCOVERT. 



ground-ways uniformly, and not stick fast on one, a thing likely 

 to occur, since the ways stopped at the low-water line, and the 

 rear edge would iall at once into deep water. The above 

 arrangement answered the purpose. 



On the 19th of March, 1870, the launch took place; in, every 

 respect a success. As soon as the last block was split out, the 

 caisson commenced to move. The impetus it had acquired in the 

 first part of its course proved sufficient to overcome the immense 

 resistance offered by the water. The caisson has daily been ris- 

 ing with every high tide and resting on the ground again at low 

 water, requiring most of the work inside to be don 13 at low water, 

 where the caisson is comparatively free from water. As the edge 

 does not readily sink into the hard soil, it is expected that there 

 will always be some water. Since the edge of the shoe is round- 

 ing, it allows the air to blow off before the level of the water 

 has reached the lowest limit. This is caused by any trifling agita- 

 tion in the level of the water inside, which gives the escaping air 

 a chance to establish an outgoing current before the head of water 

 inside becomes sufficiently great to overcome it. 



By constantly building up on top the centre of gravity has been 

 raised considerablv, and the caisson is now in a condition of un- 



V ' 



stable equilibrium, that is, it does no longer rise uniformly with 

 the rise of the tide. One end will remain on the ground and the 

 other rises as much more in proportion, and the more it rises the 

 more surface it presents to the upward pressure of the air on that 

 side, the general level of the water inside being governed by the 

 level of the highest point of the shore. 



This rising of one end of the caisson is attended by another 

 phenomenon of imposing appearance. As the tide rises, and the 

 downward pressure of the caisson is about being overcome by the 

 increased tension of the air inside as well as the buoyancy of the 

 water outside, one end of the caisson will suddenly rise 6 inches 

 or more. The result is that for a few minutes the tension of the 

 air inside exceeds the head of water outside, and a tremendous 

 outward rush of air takes place under the shoe, carrying along a 

 column of water of hundreds of tons to a height of 60 feet at 

 times. This continues until a return wave inside of the caisson 

 checks it. These blow-offs are not felt to any extent by the men 

 inside, beyond the warning noise and momentary draft created. 



The magazine of force contained in 170,000 cubic feet of com- 

 pressed air is so large that the loss of a few hundred tons is a 

 trifle. A system of pipes is put in the air-chamber for the pur- 

 pose of illuminating the air-chamber with calcium lights, a trial 

 of which has resulted favorably ; with moderate pressures, candles 

 answer very well. The first course of stone is now being 

 laid. Its weight, together with the concrete on top of the timber, 

 will probably suffice to ground the caisson permanently, and thus 

 permit the erection of setting derricks on the caisson. The stone 

 setting will then keep uniform pace with the excavation, and by 

 the time the desired point is reached the masonry is far above the 

 water level. 



The stone used for these land courses, which will be perma- 



