120 GENERAL SCIENCE 



rise to surface or its descent into the depths greatly 

 accelerated. Small but powerful gas engines are used when 

 the submarine is at the surface. Electricity from dynamos 

 and storage batteries furnishes light for the boat. 



While provision can be made to keep up artificial respira- 

 tion for a considerable length of time, any accident to the 

 machinery of a submarine when submerged is likely to result 

 in its becoming the burial casket for its imprisoned crew. 

 So enormous is the pressure of the water outside at any 

 considerable depth 1 that attempts to escape through any 

 opening made in the side of the craft may be futile. 



Caissons (ka'-sonz) are usually large hollow steel cylinders 

 made up of short lengths riveted together. These sections 

 when taken to the place where a bridge pier is to be built are 

 lowered into the water one at a time, and on top of one 

 another, till the lowermost one reaches bottom. Each 

 section in turn is riveted to the upper edge of the section 

 last lowered into the water. While this is being done the 

 cylinder hangs suspended. It is weighted by masonry 

 built within it to keep it down in the water, and to prevent 

 its being swept away. The lowermost part of the cylinder 

 is an air-tight chamber, and compressed air forced into this 

 chamber from above keeps out the water. Excavation can 

 then go on inside the bottom of the cylinder, allowing it to 

 settle deeper and deeper by reason of its own weight. The 

 material excavated is hoisted out through shafts having 

 " air-locks" (partitions) that prevent escape of the com- 

 pressed air. In due time from the bedrock up the cylinder 

 is filled with solid masonry. 



When workmen pass into or out of the working chambers 



1 The pressure of water in pounds per square foot upon a submerged body 

 is found by multiplying 62.4 pounds (the weight of one cubic foot of water) 

 by the depth of the body below surface. In sea water the weight per cubic 

 foot is about 2^ per cent greater. 



