TRAN^SACtlONS OF THE SECTIONS. 565 



^vhich at any time happens to exist in the air, collected into 

 drops by the capillary attraction of the elementary particles, 

 which now appear as spherules of water ; the spaces around 

 them becoming drier than before, and the whole space occupied 

 by the cloud having its elastic tension reduced in proportion 

 to the quantity of vapour converted into water. In conse- 

 quence of this circumstance, a diminution of sensible tempera- 

 ture will be occasioned, and a secondary formation of clouds 

 may take place, notwithstanding some addition, on the other 

 hand, from the development of the heat latent in the vapour. 



The atmospheric equilibrium being thus disturbed, wind will 

 blow from all points towai'd the cloud, and if this was previously 

 in motion, there will be a comparative calm before the cloud, 

 and a strong wind following it. Hence the appearance of the 

 edges of driving clouds varies ; small portions detach themselves 

 from the ragged posterior part, and float away, while the an- 

 terior part is of smoother outline, and suffers little change. 



Another consequence of the conversion of the vapour into 

 drops of water is an increase of electrical intensity in the 

 cloudy space. 



Clouds frequently divide into portions which have opposite 

 electrical states, when they come into contact with a hill, in 

 consequence of the effect of the ordinary laws of induction. 



2. Origin of Rain. — When two oppositely electrified clouds 

 rush together, and the spherules unite into drops, these descend 

 lower in the atmosphere, or fall in the form of rain, which is 

 more or less heavy, according to the densities of the original 

 clouds or the degree of their electrical intensity. On the 

 principles of electrical induction may also be explained the 

 cessations and renewals of rain, and the intermitting peals of 

 thunder. 



As the rain descends, a void space is occasioned in the place 

 lately occupied by the clouds, and a depression of temperature 

 in the superior regions, by the expansion of the air. An in- 

 crease of temperature, to a smaller extent, happens below, from 

 the condensation of the air. 



3. Origin of Wind. — Breezes and gales are produced by the 

 secondary formation of clouds, particularly when the clouds are 

 formed from amass that has, in appearance, attached itself to a hill. 



Squalls are gusts of wind caused by heavy showers passing 

 over the country in vast and distinct patches. In the front of 

 the shower the wind is driven out by the rain most violently 

 in the direction in which the general current was previously 

 moving. Towards the close of the shower, however, the wind 

 becomes moderate, or even reversed, the chief rush of the air 



