28 Temperature of the Ocean. 



draws the whole system of terrestrial, including atmospheric, 

 temperatures after him from one hemisphere to the other ; but 

 the water of the ocean, absorbing- heat much more slowly and 

 retaining it much longer than the atmosphere, is less subject 

 than the latter to those extremes of temperature caused by the 

 change of the season^ A range of 5° C. or 10° F., reduced to 

 a few degrees between the tropics and within the polar circles, 

 measures the difference between the mean temperature of the 

 surface-layer of the sea in the warmest and in the coldest months 

 of the year. 



The alteration of the seasons, if it does not cause a wide 

 difference between the winter and summer temperatures of the 

 open ocean, considerably affects the distribution of oceanic 

 temperature not only in the surface-layer, but also in the lower 

 strata. Both equatorial and polar currents present considerable 

 variations in volume, velocity, and direction according to the 

 season of the year ; the warm currents acquiring a preponde- 

 rance towards the end of summer, the cold currents towards the 

 end of winter. There is thus a perpetual contest between polar 

 and equatorial currents, especially perceptible in the latitudes 

 where they meet, and which, on that account, may be called the 

 critical latitudes, of which more hereafter. 



The total range of oceanic temperatures embraces about 36 

 degrees of the Centigrade scale, commencing at — 3°.67 C, the 

 freezing-point of salt water, and rising above 32° C. The 

 highest observed surface-temperatures do not belong to the 

 Equator, but are found only in a chain of more or less land- 

 locked seas ranged along the parallel of lat. 20° N. They are 

 the Carribean Sea and the Gulf of Mexico in the Atlantic, the 

 Arabian Sea, with the Red Sea and the Persian Gulf, and the 

 Bay of Bengal in the Indian Ocean, the China Sea and the 

 numerous smaller seas extending down to Torres Strait, and 

 the large basin between the Philippine and Ladrone Islands in 



