212 ACTION OF THE UPPER AIR. 



sphere 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 resist- 

 ing 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 precipi- 

 tation of moisture always takes place ; and if the dif- 

 ference 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 sum- 

 mer showers come on far more suddenly than the 

 rains of autumn and winter ; and the wind always 

 shifts before the continued rains begin to fall. The 

 upper current may be considered as the one which 

 more immediately produces the rain; although clouds 

 may be borne across the horizon by the under cur- 

 rent long before any rain actually begins to fall. 



But the currents of the air do not always blow 

 the one above the other, or the one in opposition to 

 the other. Air moves with equal ease in all direc- 

 tions, whatever they may be, if the impelling force 

 tends that way. So that there are often many cur- 

 rents, moving in different directions and with differ- 

 ent velocities within a very small space. These 

 give rise to innumerable compound motions, the 



