80 THE OCEAN 



indicate that even in deep water there may 

 be a slight seasonal or other variation, and 

 this is supported to some extent by tempera- 

 ture observations taken by the ** Michael 

 Sars" in 1910 in the same positions where 

 observations had been taken by the 

 "Challenger" in 1873. 



To show the gradual decrease of temperature 

 with increase of depth in the open ocean we 

 give here a table of mean temperatures for the 

 whole ocean, as calculated from all the observa- 

 tions in all latitudes at the specified depths, 

 from the " Challenger " Report on Oceanic 

 Circulation : — 



At a depth of 100 fathoms between lat. 30° N. 

 and 30° S., the temperature in the western 

 portions of the ocean basins is very much 

 higher than in the eastern portions. This 

 is brought about by the ocean currents setting 

 in to the west, originated and maintained by 

 the north-east and south-east trade-winds, 

 which drive the warm surface waters before 



