102 THE WEALTH OF THE SEA 



hands. If he fails to notice signals on the life-line, 

 or stops pumping air for a minute or so, the diver 

 may lose his life. 



Only persons with strong constitutions can con- 

 tinue to follow the diving profession for many years 

 without permanently injuring their health. The ill 

 effects arising from injuries are a much greater 

 danger than sharks, or other fishes, or possible 

 fouling of the air or life lines. 



Divers attired in scaphanders can work comfort- 

 ably at as great a depth as 125 feet. Recently it has 

 been learned that divers can work at even greater 

 depth without great danger, provided they ascend to 

 the surface very slowly, thus permitting the air to 

 be released from the bl5od gradually. The danger 

 appears to lie in the sudden change in pressure 

 which the diver undergoes, and not in the great 

 pressure itself. 



The pearl oyster prefers tropical seas and is found 

 more or less abundantly on the shoals and reefs about 

 the shores of every land lying between thirty degrees 

 north and thirty degrees south of the equator. It 

 prefers coral reefs and limestone bottoms. The 

 largest and finest pearl shells are nearly all found 

 in the Pacific Ocean within a range of twenty degrees 

 south of the equator. The best white mother-of-pearl 

 shells are found on the northern and western coasts 

 of Australia and on the Aru Islands. The abalone is 

 usually found in cooler waters, being abundant on 

 the coast of California and Japan. 



Among the fisheries which are especially well 



