ACTION OF THE UPPER AIR. 219 



lower part of the atmosphere may resist it, and 

 dissolve it again and again, if the cause continues 

 to act in the upper part of the atmosphere, the 

 earth and the lower part must, in the end, give 

 way, and rain must be the consequence. The 

 cloud, too, or the " gum" as it is sometimes called 

 when it is merely a tinge of colour without any 

 definite and limited shape, intercepts part of the 

 light and heat of the sun, and thus lessens the 

 resisting power of the lower atmosphere and the 

 earth, and that hastens the coming of the rain. 

 The gummy appearance is probably even more 

 suspicious than the curl-cloud, because it shows 

 that a region higher, and therefore more sensible, 

 is affected, and it also shows that the cause is 

 more widely extended. 



The quantity of water which the air can sustain 

 in a state of vapour, supposing the air to be of the 

 same density, diminishes more rapidly than the 

 temperature ; and thus when two currents of air 

 of different densities meet, a certain degree of 

 percepitation of moisture always takes place ; and 

 if the difference of temperature be considerable, 

 and the currents, or any one of them, rapid, instant 

 rain may be the consequence, and continued rain 

 may be the consequence of their continuance. 

 Spring and summer showers come on far more sud- 

 denly than the rains of autumn and winter ; and 

 the wind always shifts before the continued rains 

 begin to fall. The upper current may be consi- 

 dered as the one which more immediately produces 

 the rain; although clouds may be borne across 

 the horizon by the under current, long before any 

 rain actually begins to fall. 



But the currents of the air do not always blow 

 u 2 



