ANNOTATIONS AND ADDITIONS. 83 



the Samothracian tradition of a great flood which had 

 changed the form of that district." 



( 9 ) p. 12. u Prevents precipitation taking place from 

 clouds." 



The vertically-ascending current of the atmosphere is a 

 principal cause of many most important meteorological 

 phenomena. "When a desert or a sandy plain partly or 

 entirely destitute of plants is bounded by a chain of high 

 mountains, we see the sea breeze drive the dense clouds 

 over the desert without any precipitation taking place before 

 they have reached the mountain-ridge. This phenomenon 

 was formerly explained in a very inappropriate manner 

 by a supposed superior attraction exercised by the mountains 

 on the clouds. The true reason of the phenomenon appears 

 to consist in the ascending column of warm air which rises 

 from the sandy plain, and prevents the vesicles of vapour 

 from being dissolved. The more complete the absence "of 

 vegetation, and the more the sand is heated, the greater is 

 the height of the clouds, and the less can any fall of rain take 

 place. When the clouds reach the mountains these causes 

 cease to operate ; the play of the vertically-ascending atmo- 

 spheric current is feebler, the clouds sink lower, and dissolve 

 in rain in a cooler stratum of air. Thus, in deserts, the 

 want of rain, and the absence > of * vegetation, act and react 

 upon each other. It does not rain, because the naked sandy 

 surface having no vegetable covering, becomes more power- 

 fully heated by the solar rays, and thus radiates more heat ; 

 and the absence of rain forbids the desert being converted 



