CANARY ISLANDS. 29 



America, exhibits very different degrees of temperature, 

 but very slow in its changes. In passing from Sandy 

 Hook to this place the increase of temperature has been 

 only 3-5°. The maximum observed during the voyage 

 was 82°, the minimum 73°, with a mean of 76-1°. Hum- 

 boldt, in travelling from Spain to South America, in July, 

 1799, observed a gradual increase from 50° to 77°, the 

 maximum being only 799°. In comparing the observa- 

 tions of several navigators, I find that the thermometer has 

 not been known to rise, in the open air at sea, above 93° 

 in any place between the tropics of either hemisphere ; 

 while in corresponding latitudes of the continents of Asia, 

 Africa and America, it attains a temperature of 120°, 130°, 

 and even 140°. 



The gradual increase of temperature during a voyage 

 from the temperate to the torrid zone, is highly conducive 

 to the health of voyagers, as it prepares them, by degrees, 

 for the intense heat which they have to encounter. This 

 change is attributed, in a measure, to the evaporation of 

 the water, increased by the wind and the waves, together 

 with the property possessed by transparent liquids of ab- 

 sorbing very little light at their surface. 



Temperature of the Sea. — From New York to this 

 place, the temperature of the ocean has been, in general, 

 uniform in its increase, although inequalities have occur- 

 red, caused by the Gulf Stream. The mean temperature 

 has been 74-4^ ; maximum 80°, and minimum 660^. In 

 the inter-tropical seas, there is everywhere a great uni- 

 formity in the maximum of heat, which varies, according 

 to Humboldt, from 82° to 84-5°. This proves that the 

 ocean is in general warmer than the atmosphere in direct 

 contact with it, the mean temperature of which, near the 

 equator, is from 78*8° to 80'6°. As sea-water is a bad 

 conductor of heat, its temperature changes less suddenly 

 and less easily than that of the atmosphere ; hence the 

 cause of its uniformity. Besides, the visible solar rays 

 cannot heat the bottom of the sea, as they penetrate only 

 to the depth of about 700 feet. Beyond that limit, the sea 

 receives no more light. The temperature of the depth of 

 the ocean would appear, then, to follow that of the tempe- 

 *3 



