TEMPERATURE. 57 



bits a very slow increase. From Corunna to the cmvp. iv 

 Canary Islands, the thermometer, observed at noon, and 

 in the shade, gradually rose from 50° to 64°, and from 

 TenerifFe to Cumana from 64° to 77°. The maximum 

 of heat observed during the voyage did not exceed 79*9°. 



The extreme slowness with which the temperature Oradnai 

 increases during a voyage from Spain to South America ^ '""^es. 

 is highly favourable to the health of Europeans, as it 

 gradually prepares them for the intense heat which 

 they have to experience. It is in a great measure 

 attributable to the evaporation of the water, augmented 

 by the motion of the air and waves, together with the 

 property possessed by transparent liquids of absorbing 

 very little light at their surface. On comparing the 

 numerous observations made by navigators, we are 

 surprised to see that in the torrid zone, in eitlier hemi- 

 sphere, 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. Tlie difference between the tempera- 

 ture of the day and night is also less than on land. 



Temperature of the Sea. — From Corunna to the mouth Tempera- 

 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 every 

 where 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 HyCTo- 

 the voyage, the apparent humidity of the atmosphere ^'^['^of .ij. 

 indicated by the h3'grometer underwent a sensible in- 

 crease. In July, in lat. 13° and 14° N., Saussure's 

 hygrometer marked at sea from 88° to 92°, in perfectly 

 clear weather, tlie thermometer being at 75-2°. On the 



