336 



PHYSICAL GEOGRAPHY. 



pests mingling the inferior strata of the water with the upper, for there is great uniformity 

 everywhere in the maximum of heat in the equinoctial seas. Within the tropics, the 

 temperature is never higher than 85 or 86 of Fahrenheit ; the mean is about 80 ; and 

 the general range from 77 to 84. If it were not for the action of the currents, 

 mingling together the waters of different depths and regions, the temperature of the 

 tropical ocean would be much higher than it is, because the surface of the water reflects 

 infinitely fewer of those rays which approach the perpendicular, than of those which fall 

 in a more oblique direction. The annexed Table is one of Humboldt's, compiled from 

 his own observations, and several nautical journals : 



TEMPERATURE OF THE ATLANTIC OCEAN IN DIFFERENT DEGREES OF LONGITUDE. 



A considerable number of experiments have yielded the following results, upon which 

 general dependence may be placed : 



1. In the torrid zone, the temperature of the ocean is found to diminish with its depth ; 

 in the polar seas it increases with the depth ; and about 70 of latitude it is nearly 

 the same at all depths. 



2. At noon, the open sea is colder than the atmosphere noticed in the shade, and at 

 midnight warmer ; an assertion first made by M. Peron, and confirmed by a large 

 number of observations. 



3. Morning and evening, the temperature of the sea and of the atmosphere usually 

 correspond. 



4. Observations taken at six in the morning, at midday, at six in the evening, and at 

 midnight, of the temperature of the ocean at the surface, and of the atmosphere, 

 show the mean to be higher with reference to the sea in every latitude. The ocean 

 is thus in general warmer than the atmosphere with which it is immediately in 

 contact. 



5. Banks diminish the temperature of the sea, so that it is always colder over them 

 than where it is deeper ; and the difference is greater, the greater the shallows. 



The preceding tables and remarks refer chiefly to the ocean north of the equator ; but 

 there is no sensible difference between the general temperature of its tropical waters, 

 north or south. Beyond the southern tropic also, as far as 35 or 40, the temperature 

 corresponds with that of similar extra-tropical northern latitudes ; yet it is generally 

 supposed that towards the poles the cold of the seas is greater in the southern than in 

 the northern hemisphere, though the difference between the temperature of these high 



