50 



first part of the voyage, a difference of a degree 

 of temperature corresponded to 1° 48' of lati- 

 tude ; in the second part, we must traverse 2° 

 30' of latitude to see the thermometer rise one 

 degree. The maximum of the heat, which the 

 air gradually attains two hours after the passage 

 of the sun over the meridian, did not exceed, 

 during this voyage, 26° 6' (21*3° Reaumur) ; ne- 

 vertheless we were in the month of July, and 

 ten degrees to the south of the tropic of Cancer. 

 The evaporation of the water, augmented by the 

 motion of the air and of the waves, and the pro- 

 perty which transparent liquids have* of absorb- 

 ing very little light at their surface, contribute 

 equally to moderate the heat in the part of the 

 atmosphere that surrounds the equinoctial seas. 

 It is well known, that as long as the breeze blows 

 under the torrid zone, navigators are never ex- 

 posed to violent heats. 



If we compare^ the numerous observations 

 made in the South Sea and the Atlantic Ocean 

 during the voyages of Cook, Dixon, d'Entrecas- 

 teaux, and Krusenstern, we find, that, between 

 the tropics, the mean temperature of the air at 



* The rays of light penetrate the water to considerable 

 depths ; and the first strata, by freely transmitting them, are 

 not heated like the earth and rocks. 



+ See an excellent memoir by Messrs. Horner and Langs- 

 dorf in the memoirs of the Academy of Petersburgh, vol. i, 

 p. 467. 



i 



