141 



most violent hail storms are caused by a sudden transference of 

 a body of warm air up beyond the term of perpetual congelation, 

 where the drops of rain are frozen into hail stones. This is some- 

 times accomplished by means of whirlwinds, by which the hail- 

 stones being sustained for some time, are occasionally accumula- 

 ted to a very large size. By referring to our figure of the curve 

 of perpetual congelation, page 126, it will be understood why 

 hail storms seldom occur in the equatorial regions, and most fre- 

 quently in the temperate zones, and seldom or never in the polar 

 regions. The term of congelation is too high at the equator for 

 the hot air raised by a whirlwind to pass beyond, or up to it, and 

 at the polar regions, the air seldom becomes as hot as is required 

 to form hail storms. Different names have been given to the va- 

 rious forms of clouds, derived from their appearance and charac- 

 ter, we will briefly notice them. First, we have, occurring at the 

 greatest elevation, the Cirrus, or curl-cloud, which is a thin fleecy 

 vapor, with a waving and striated appearance as shown in the 

 engraving below. This cloud is frequently called the curl-cloud 



from its flexuons form, and ix not unlike a bunch of wool pulled 

 out into fine pointed ends. After a continuance, of fine weather 

 the cirrus is often observed at groat heights like a Fine white line 

 stretching across the r:!--y. The peculiar' form of cirrus shown in 

 r.* 



