•i-ZO TFT? BOTTOM OF THE SKA. 



clothing of the air it contains, and thus make it 

 answer the purpose of a swimming-bladder. They 

 may manoeuvre in the most varied manner when 

 beneath the water by simply turning a tap. The 

 blocks of stone are thus built up with as much 

 regularity as on dry land, and are capable of oppos- 

 ing the greatest possible resistance to the incessant 

 attacks of the waves. 



7. Divmg Bells — Stationary Compressed-air Apparatus. 



Previous to the invention of the apparatus which 

 we have now described at some length, diving-bells 

 were employed in the construction of jetties, fortifica- 

 tions, lighthouses, docks — in a word, in all important 

 submarine work. This invention consists of a large 

 cast-iron bell, communicating, at its upper part, with 

 a force-pump. 



Invert a common drinking-glass in a basin of water ; 

 the air diminishes in volume as the glass is sunk 

 further in the water ; indeed, its bulk may be seen to 

 decrease by the gradual rise of the water in the 

 glass. The air collects in the upper part of the glass, 

 and becomes gradually more compressed — preventing 

 the water, however, from completely filling the vessel. 

 Make a communication between the bottom of the 

 glass and a reservoir of air compressed to the same 



