656 



HISTORY OF THE SEA. 



with a stop-cock. As the orlop deck, with its gallery, fills 

 with water the machine gets heavier and sinks, while the hold 

 becomes at the same time filled with air. Though the air 

 thus forced into the hold would tend to float the hydros tat, 

 this tendency is counterbalanced by the rilling of the orlop 

 deck with water. When the hold is filled with air, the work 

 men in the between decks open the shaft and descend to the 

 bottom. A sufficient number remain in the between decks to 

 haul up and dispose of the material excavated, and to attend 

 to the pumps which maintain the supply of air for those in 

 the hold. When they want to rise again, the men ascend from 

 the hold by the shaft to the between decks, closing the shaft 

 again. The air is then let from the hold to the orlop deck and 

 gallery ; the hold fills with water, while the orlop deck and 

 gallery become rilled with air, and the hydrostat rises to the 

 surface ; the men open the hatch of the between decks and ob- 

 tain free communication with the outer world again. 



The dimensions of the hydrostat are as follows: The hold 

 is square, the sides measuring each 26 feet, and being 6 feet 6 

 inches high. The orlop deck is of the same size. The be- 

 tween decks have the same depth, but are only 16 feet in the 

 sides. The base of the hold therefore covers 676 square feet. 

 This ingenious machine has been already used with the most 

 perfect success in performing various work, such as cleaning 

 out and deepening harbors ; searching for lost treasure ; re- 

 moving obstructions in channels, and so on. 



One of the most important and interesting pieces of subma 

 rine engineering ever done in this country was that undertaker 

 for removing the rocky obstructions in Hell Gate, at the en- 

 trance, through Long Island Sound, of New York harbor. The 

 first attempt to remove these was by drilling and blasting, as 

 in an ordinary quarry. This work was, however, quite slow, 

 since the current is there so rapid that operations could be car- 

 ried o^ ^nly a fetf minutes each day at the turns of the tides. 



