116 



notwithstanding it's pyramidical form, is a great 

 part of the year enveloped in vapours, and is 

 sometimes, during several weeks, invisible from 

 the road of Santa Cruz. It's position to the west 

 of an immense continent, and it's insulated situ- 

 ation in the midst of the seas, are no doubt the 

 causes of this phenomenon. Navigators are 

 well apprised, that the smallest islets, those 

 which are without mountains, collect and har- 

 bour the clouds. The decrement of heat is also 

 different above the plains of Africa, and above 

 the surface of the ocean* ; and the strata of air, 

 brought by the trade winds, cool in proportion 

 as they advance towards the west. If the air 

 has been extremely dry above the burning sands 

 x)f the desert ; it is very quickly saturated wheu 

 it has entered into contact with the surface of 

 the sea, or with the air that lies on this surface. 

 It is easy to conceive, therefore, why the vapours 

 become visible in the atmospherical strata, which, 

 at a distance from the continent, have no longer 

 the same temperature as when they began to be 

 saturated with water. The considerable mass 

 of a mountain, which rises in the midst of the 

 Atlantic, is also an obstacle to the clouds, which 

 are driven out to sea by the winds. 



We waited long and impatiently the permis- 

 sion of the governor of the place to land. I em- 

 ployed this time in making the necessary ob- 



* Obs. Ast. t. i, p. 126, 



