CHAPTER II. 



TEMPERATURE OF THE OCEAN. 



Surface Temperature— Deep-Sea Temperature— The Miller- Casella Thermometer — 

 Serial Soundings and Temperature Curves— Deductions from the Curve. 



Surface Temperature. — The temperature of the ocean 

 depends mainly on three conditions — latitude, currents, and the 

 season of the year. Owing to the unequal exposure of the 

 different portions of the spherical surface of our planet to the 

 rays of the sun, the amount of solar heat received gradually 

 diminishes from a maximum between the tropics to a minimum 

 in the polar regions. We find, in consequence, that the tem- 

 perature of the surface-layer of the ocean decreases as we 

 proceed from the Equator towards the Poles, slowly at first 

 between the tropics, more rapidly in the temperate zones, until 

 it falls to zero and even below zero before we reach the Arctic 

 and Antarctic Circles (Plate 4). 



In the absence of any other disturbing cause, the 

 isotherms, or lines of equal temperature, would therefore form 

 a system of lines running parallel with the Equator. This, 

 however, is found not to be the case. Warm currents flowing 

 from the tropical towards the polar regions, and cold currents 

 issuing from the ice-bound confines of the Arctic and Antarctic, 

 and penetrating into warmer latitudes, considerably interfere 

 with the uniform decrease of temperature between the Equator 

 and the Poles, although this decrease continues to be the 

 leading feature in the distribution of oceanic temperature ; but 

 the isotherms, as a general rule, assume an oblique direction, 

 as shown in the annexed diagram (Fig. 11). 



The sun, in his apparent progress from tropic to tropic, 



