330 ^f ^^^^ Propagation of Heat 



Ocean is no other than one of these currents, — that at 

 the surface, which moves from the equator towards the 

 north pole, modified by the trade winds and by the 

 form of the continent of North America ; and the pro- 

 gress of the lower current may be considered as proved 

 directly by the cold which has been found to exist in the 

 sea at great depths in warm latitudes, — a degree of 

 temperature much below the mean annual temperature 

 of the earth in the latitudes where it has been found, 

 and which of course must have been brought from colder 

 latitudes. 



The mean annual temperature in the latitude of 67° 

 has been determined by Mr. Kirwan, in his excellent 

 treatise on the temperature of different latitudes, to be 

 39° ; but Lord Mulgrave found on the 20th of June, when 

 the temperature of the air was 48|-°, that the temperature 

 of the sea at the depth of 4680 feet was 6 degrees below 

 freezingr, or 26° of Fahrenheit's thermometer. 



On the 31st of August, in the latitude of 69°, where 

 the annual temperature is about 38°, the temperature of 

 the sea at the depth of 4038 feet was 32° ; the temper- 

 ature of the atmosphere (and probably that of the water 

 at the surface of the sea) being at the same time at ^()^ . 



But a still more striking, and I might, I believe, say 

 an incontrovertible proof of the existence of currents 

 of cold water at the bottom of the sea, setting from the 

 poles towards the equator, is the very remarkable differ- 

 ence that has been found to subsist between the temper- 

 ature of the sea at the surface and at great depths, at the 

 tropic ; though the temperature of the atmosphere there 

 is so constant that the greatest change produced in it by 

 the seasons seldom amounts to more than five or six de- 

 grees, yet the difference between the Heat of the water 



