Effect of Woods on Climates. 11 



importance whether we ascribe these clouds, capable as they 

 are of completely obstructing the solar rays, to the burning 

 of forests and savannahs, or to emanations from the earth. 

 Their formation, and their arrival in a given place, will remain 

 equally beyond the predictions of science ; the variations of 

 temperature, and meteors of every kind which may be caused 

 by these clouds, will never be pointed out beforehand in our 

 meteorological almanacs. 



Tlie accidental darkening of the air, in 1783, embraced so 

 extensive a space (from Lapland to Africa), that it was as- 

 cribed to the matter belonging to the tail of a comet, which, 

 it was alleged, had mingled with our atmosphere. It is out 

 of the question to maintain that an accidental state of the 

 atmosphere, which enabled us, for a period of nearly two 

 months, to look at the sun at mid-day with the naked eye, 

 was without influence on terrestrial temperatures. 



Forests cannot fail to exercise a sensible influence on the 

 temperature of the surrounding regions ; because, for ex- 

 ample, snow remains there for a much longer time than in 

 the open country. The destruction of forests, therefore, 

 ought to produce a modification in our climates. 



In given instances, what is the precise influence of forests, 

 estimated by the centigrade thermometer \ The question is 

 very complicated, and has not hitherto been solved. 



In all very mountainous regions, the valleys are traversed 

 by periodical diurnal breezes, particularly sensible in May, 

 June, July, August, and September. These breezes ascend 

 the valleys, from seven or eight o'clock in the morning to 

 three or four in the afternoon, the time when they reach their 

 greatest force, and from four o'clock to six or seven in the 

 evening. For the most part they blow with the force of a 

 decided wind, and sometimes with that of a violent wind ; 

 they must, therefore, exert a sensible influence on the 

 climates of the countries which lie around these valleys. 



What is the cause of these breezes \ Every thing concurs 

 to shew that the cause is to be found in the manner in 

 which the solar rays warm the central mass whence these 



