1 8 PRESSURE OF WATER AT GREAT DEPTHS. 



Some idea of what that represents may be 

 gathered from the description given of it by the same 

 authority : 



" The conditions of pressure at great depths are extra- 

 ordinary. Pressure increases at the rate of one ton on the 

 square inch for each 1000 fathoms of increasing depth ; so 

 that the inhabitants on the floor of the ocean, at its aver- 

 age depth of about 2500 fathoms, sustain a pressure of two 

 and a half tons on each square inch of surface, compared 

 with 14 Ibs. of atmospheric pressure sustained by the inhabi- 

 tants of the upper earth." * 



Therefore the inhabitants of. the mighty deep 

 continually sustain a pressure 400 times greater than 

 we do. 



Some curious results caused by the pressure of the 

 water upon the wooden portions of the trawl used on 

 board the Challenger for bringing up specimens from 

 great depths, are worth recording. It seems they 

 found the trawl gave better results than the dredge, in 

 very deep water, where there was a smooth bottom 

 free from rocks; and the report states that: 



" In very extreme depths, say between 3000 and 4000 

 fathoms, the enormous pressure affects the wood of the beam. 

 On one occasion when a pine beam was used, the wood was 

 compressed till the knots stood out one tenth of an inch 

 beyond the general surface, and on another the beam was 

 crushed as if it had been passed between two rollers." f 



Nevertheless, as we see, various specimens of living 

 animals are so organized as to find these peculiar 

 conditions not only supportable, but even necessary to 

 their very existence. 



* Report of H.M.S. Challenger's Scientific Expedition, 1880, 

 Vol. i., page 41 of Introduction. 



j* Ibid., pp. 9 and IO of Introduction. 



