82 



THE OCEAN. 



from the polar regions to the equatorial zone, the water becomes 

 warmer with an almost regular gradation, and, from the freezing- 

 point under the Arctic circle, the temperature rises to 68^ and IV 

 Fahr. under the tropics, and to 86° and even above 90° Fahr. in 

 the Pacific, the Eed Sea, and the Indian Ocean.* With regard to 

 the increase or decrease of heat in a vertical direction, we had till 

 recently only the vaguest notions, in consequence of the want of 

 exact soundings. It is in fact very difficult to lower to a depth of 

 several hundred, and even several thousand fathoms, thermometrical 

 apparatus strong enough to resist the enormous pressure of one 

 atmosphere for every 33 feet. 



Sir James Ross was one of the first who attempted to apply the 

 resources of modern science to a systematic exploration of the 

 temperature in the depths of the sea ; but he seems to have com- 

 mitted the error of generalizing too hastily from the incomplete 

 results which he had obtained ; and in his eagerness, he believed he 

 had discovered a law, which the subsequent researches of navigators 

 have not confirmed. He thought that he could establish the fact 

 that under the equator the temperature of the water diminishes 

 gradually to 1200 fathoms, whore it is only 39-2 degrees Fahr. 

 On each side of the equator the upper waters gradually cool, and 

 the limit of four degrees is progressively raised towards the sur- 

 face ; it is at the fiftieth degree of latitude, in the southern hemi- 



Fig. 11. Sheet of Water presumed to be at a temperature of 39-2 Fahr. 



sphere, that it finally reaches the level of the sea. Farther in 

 the direction of the pole the superficial water continues to grow 

 colder, while the line of four degrees sinks gradually to the depth of 

 765 fathoms. Thus, as the accompanjdng figure shows, the line of 

 • Fitzroy, Weather- Book, p. 84. 



