268 PRESSURE OF AIR ON FLOATING BODIES 



the enclosed air is still under a greater pressure than 

 originally, and the volume of the air does not recover 

 its original magnitude until the diver has again 

 ascended to the surface. It follows from this, that if 

 the apparatus is of considerable size, and the relative 

 quantities of air and water have been well adjusted, 

 the pressure of the column of water above it may be 

 sufficient to keep the diver at the bottom when it has 

 once descended. It must then be raised again to the 

 surface by some means, for example by a hook of wire, 

 and will float when raised; by pressure it will again 

 sink, and remain at the bottom, until lifted again, and 

 so on. 



A diver of this kind may be made of a medicine bottle, about 4 cm 

 wide and 8 cm high. The volume of the enclosed air must neces- 

 sarily be large, so that a small increase of pressure produces a 

 comparatively great diminution of volume. The bottle is there- 

 fore made to float nearly immersed by winding thick lead wire round 

 its neck, which also keeps it in an upright position, its mouth down- 

 wards. 



In fig 186 J., a body -c d ef floats in the liquid con- 

 tained in a vessel which is not much larger than the 



o 



body. The weight of the displaced liquid must hence 

 be equal to the weight of the body, or, in other words, 

 the pressure of the liquid upon the surface ef balances 

 the weight of the body. But this does not strictly 

 represent the whole of the circumstances of the case;, 

 for the atmosphere exerts also a definite amount of 

 pressure upon the floating body and upon the narrow 

 surface a b of the liquid which surrounds the body 

 a ring. The pressure of the atmosphere upon the 

 of the body acts in a contrary direction at every pair 

 of opposite points, and no effect is produced; but the 



