517 



The temperature of the atmosphere varied 

 on the summit of the Silla from eleven to four- 

 teen degrees, according as the weather was 

 calm or windy. Every one knows how diffi- 

 cult it is to verify on the summit of a moun- 

 tain the temperature, that must be employed 

 in the barometric calculation. The wind was 

 east, which seems to prove, that the breeze, or 

 the trade-winds, extends in this latitude much 

 higher than fifteen hundred toises. Mr. Von 

 Buch had observed, that, at the Peak of Tene- 

 riffe, near the northern limit of the trade- winds 

 there exists generally at one thousand nine 

 hundred toises of elevation a contrary current, 

 or wind from the west. The academy of sci- 

 ences had engaged the natural philosophers 

 who accompanied the unfortunate La Perouse, 

 to employ little air balloons in order to exa- 

 mine the extent of the trade-winds under the 

 tropics at sea. Such experiments are very dif- 

 ficult, unless the observer quits the surface of 

 the Earth. The little balloons do not in gene- 

 ral reach the height of the Silla ; and the light 

 clouds, that are sometimes perceived at an ele- 

 vation of three or four thousand toises, for in- 

 stance^ the fleecy clouds *, remain almost fixed, 

 or have such a slow motion, that it is impossible 

 to judge of its direction. 



* What the French call moutons. 



