THE SEA 15 



The pressure in water increases rapidly as the 

 distance beneath the surface increases. At the sur- 

 face the atmosphere exerts a pressure of fifteen 

 pounds per square inch. Beneath the surface the 

 pressure increases fifteen pounds per square inch 

 for each ten meters (32.8 feet). Thus at 1000 meters 

 (3280 feet) the pressure is that of a hundred at- 

 mospheres or 1500 pounds per square inch, and at 

 the greatest depths the pressure is about six and a 

 half tons on each square inch. 



Solid objects or objects that are freely permeable 

 to water throughout, as animal bodies, remain prac- 

 tically unchanged when they sink to great depths. 

 But objects having internal cavities containing air 

 are strongly compressed and distorted when sub- 

 jected to the enormous pressure at the bottom of the 

 ocean. Wood used in deep-sea trawls is so much 

 compressed that the denser knots stand out from 

 the surface. Closed glass tubes dropped to great 

 depths are crushed to powder. Scientific investi- 

 gators on the famous Challenger expedition sent a 

 rabbit down on a line to a depth of 500 fathoms. 

 The body came up very little altered in appearance ; 

 the bones were all intact, and the lungs were the 

 only viscera that seemed to be affected by the pres- 

 sure. Deep-sea fishermen are aware that the eyes 

 of fish, brought rapidly to the surface, "pop out," 

 and the fish are killed by the disorganization of their 

 tissues caused by the sudden release from pressure. 

 If deep-sea fishes accidentally get above their accus- 

 tomed depth, the expansion of air in their swim- 

 bladders renders them so buoyant that they continue 



