296 PHYSICAL GEOGRAPHY. 



Thouars, was found at the temperature of 2.8 and 2.5 Cent, 

 (37 and 36.5 Mi.), and even lower. The almost icy tern-, 

 perature found at great sea depths within the tropics first led 

 to the knowledge of a submarine polar current flowing from 

 either pole towards the equator ; as without this current the 

 depths of the tropical seas could only have a degree of cold 

 equal to the lowest temperature of their surface water. The 

 apparent anomaly offered in the Mediterranean, whose waters 

 are found of a higher temperature than is usual at great 

 depths, has been explained by Arago, and results from the 

 fact that, at the Straits of Gibraltar, where the surface water 

 of the Atlantic flows in as a westerly current, a counter current 

 prevails beneath, and prevents the influx from the ocean of 

 the cold current from the pole. 



The Ocean is the great moderator and equalizer of ter- 

 restrial climates ; at a distance from coasts, and where its 

 surface is not disturbed by currents of cooled or heated 

 water, it maintains throughout the tropics (and especially in 

 the equatorial region from 10 N. to 10 S.) a wonderful 

 uniformity and constancy of temperature ( 363 ) over spaces of 

 many thousand square miles. It has, therefore, been well 

 said, p 64 ) that an exact and long-continued investigation of 

 the thermic relations of the tropical seas might afford, in the 

 simplest manner, a solution of the great and long-contested 

 problem of the invariability of climates and of the tempera- 

 ture of the globe. Great changes of solar heat, of considerable 

 duration, would be reflected in the altered mean temperature 

 of the sea, with even greater certainty than in the mean 

 temperature of the land. 



Those zones in which the waters of the ocean attain in the 

 one case their maximum of temperature, and in the other their 



