100 TERRESTRIAL ADAPTATIONS. 



in the opposite direction ; and prevents or much 

 retards that continual flow of hot vapour into 

 colder regions, by which a constant precipitation 

 would take place in the latter situations. 



If, in this state of things, the flow of the current 

 of air, which blows from any colder place into a 

 warmer region, be retarded or stopped, the aqueous 

 vapours will now be able to make their way to 

 the colder point, where they will be precipitated 

 in clouds or showers. 



Thus, in the lower part of the atmosphere, 

 there are tendencies to a current of air in one 

 direction, and a current of vapour in the opposite ; 

 and these tendencies exist in the average weather 

 of places situated at a moderate distance from the 

 equator. The air tends from the colder to the 

 warmer parts, the vapour from the warmer to the 

 colder. 



The various distribution of land and sea, and 

 many other causes, make these currents far from 

 simple. But in general the air current pre- 

 dominates, and keeps the skies clear and the 

 moisture dissolved. Occasional and irregular oc- 

 currences disturb this predominance ; the mois- 

 ture is then precipitated, the skies are clouded, 

 and the clouds may descend in copious rains. 



These alternations of fair weather and showers 

 appear to be much more favourable to vegetable 

 and animal life than any uniform course of 

 weather could have been. To produce this 



