148 FOUNDERS OP OCEANOGRAPHY 



in the history of oceanography — some have a detachable 

 weight which is left at the sea bottom to avoid delay in 

 winding in the wire; in some the wire runs out over a 

 measuring wheel connected with a dial from which the depth 

 (said to be correct to 1 fathom in great depths) can be read 

 off as the weight touches bottom. 



In some cases the sounding machine brings up in a tube or 

 other receptacle a small sample of the bottom deposit, which 

 may be sufficient to show the nature of the bottom for chart- 

 ing. The distribution of the submarine deposits on the 

 floor of the ocean in relation to depth will be considered 

 further on (Chapter X). 



The floor of the deep sea is icy- cold, receives no light from 

 the sun, and is under a pressure of several tons to the square 

 inch — over a ton for each thousand fathoms of depth. 



Tempeeatuee. 



Quite apart from seasonal variations in temperature 

 (which are only of large amount in the temperate zones), 

 some parts of the ocean are naturally much warmer than 

 others. The surface of the sea in the tropics may be over 

 80° P. (the highest record is 96° P. in the Persian Gulf), and 

 in the polar regions is at or below freezing-point (the lowest 

 known being 26° P. — making the extreme recorded range 

 70° P.). The freezing-point of sea-water is 28° P. (-2-22° C). 



The range of seasonal variation in the year in the surface 

 temperature of the sea is least in Arctic and Antarctic waters 

 and in the tropics, where it (the range) is less than 10° P. 

 In the southern temperate zone the range is from 10° to 30°P., 

 and in the northern temperate zone from 10° to 50° P. The 

 range is seen at its greatest about latitude 40° in both north 

 and south hemispheres. 



These surface temperatures are determined primarily by 

 the latitude, and secondarily modified by cold and warm 

 currents and other influences. The surface isotherms, then, 

 are rarely found running with much regularity east and 



