ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



41 



at the surface and lowest at about 500 metres, where some of the oxygen 

 is consumed by decaying organic matter. Lower down, the southern 

 water rich in oxygen makes itself felt. 



The curve T in the main figure shows, on a larger scale, the temperature 

 from 3,000 metres down to the bottom. Just as the salinity is virtually 

 constant from about 4,000 metres down to the bottom, so in effect is the 

 temperature, despite the observed rise of one degree. Coming originally 

 from the south at depths round about 4,000 metres, and slowly sinking in 

 the Philippine Trench, the water has been slightly compressed by the 

 increased pressure. Compression causes a rise in the temperature of water 

 as of air; but should the water subsequently rise, the temperature will 

 fall by as much as it previously rose. The term "potential temperature" 

 denotes the temperature of the water if it were raised to the surface. The 

 curve for this temperature (0) is vertical; in short, the water column is in 

 neutral equilibrium, and does not exchange heat with its surroundings. 



Only close to the bottom, where the Snellius Expedition had found a 

 somewhat lower temperature, did we find a rise of 0.03° C in the poten- 

 tial temperature. Probably there is a slight increase due to heat from 

 the interior of the earth. 



Bay of Biscay, 46° 28 N. lat., 8° 01' W. long. 



Temperature 



2,000 metres 

 2,500 — 

 3,000 — 

 3,500 — 

 4,000 — 

 4,500 — 



+0.08' 

 +0.08' 



+ 0.10' 

 +0.13' 

 + 0.13' 

 — 0.03' 



Temperature readings in the Bay of Biscay: Dana 1922, Dana 1930, and Galathea 

 1950. Whereas the upper layers of water are subject to irregular fluctuations, partly 

 due to the influence of supplies from the Mediterranean, the temperature in the abyss 

 is very constant. A slight rise of about 0.1° is observable since 1922, except at the 

 very bottom. 



Salinitv 



2,000 metres 

 2,500 — 

 3,000 — 

 3,500 — 

 4,000 — • 

 4,500 — 



35.02 "/oo 



3498 - 



34-97 - 



3491 - 



3491 - 



3489 - 



3504 

 3500 

 3496 

 3496 

 34-93 

 34-92 



ooo/o 

 99 - 

 99 - 

 97 - 

 95 - 

 90 - 



Salinity is determined with exactly the same accuracy as temperature, and this also 

 siiows great constancy in the abyss. 



