58 TEMPERATURE DURING THE VOYAGE. 



transparent liquids of absorbing very little light at 

 their surface. On comparing the numerous obser- 

 vations made by navigators, we are surprised to see 

 that in the torrid zone, in either hemisphere, they 

 have not found the thermometer to rise in the open 

 sea above 93 ; while in corresponding latitudes 

 on the continents of Asia and Africa, it attains a 

 much greater elevation. The difference between 

 the temperature of the day and night is also less 

 than on land. 



Temperature of the Sea. From Corunna to the 

 mouth of the Tagus, the temperature of the sea 

 varied little (between 59 and 60-8), but from lat. 

 39 to 10 N., the increase was rapid and generally 

 uniform (from 59 to 78 '4), although inequalities 

 occurred, probably caused by currents. It is very 

 remarkable that there is a great uniformity in the 

 maximum of heat everywhere in the equinoctial 

 waters. This maximum, which varies from 82 to 

 84'2, proves that the ocean is in general warmer 

 than the atmosphere in direct contact with it, and 

 of which the mean temperature near the equator is 

 from 78-8 to 80'6. 



Hygrometrical State of the Air. During the whole 

 of the voyage, the apparent humidity of the atmo- 

 sphere indicated by the hygrometer underwent a sen- 

 sible increase. In July, in lat. 13 and 14 N., 

 Saussure's hygrometer marked at sea from 88 to 

 92, in perfectly clear weather, the thermometer 

 being at 75*2. On the banks of the Lake of Ge- 

 neva the mean humidity of the same month is only 

 80, the average heat being 66 '2. On reducing 

 these observations to a uniform temperature, we find 

 that the real humidity in the equinoctial basin of the 

 Atlantic Ocean is to that of the summer months at 

 Geneva as 12 to 7. This astonishing degree of 

 moisture in the air accounts to a great extent for the 

 vigorous vegetation which presents itself on the 



