IV.] SNOW AND ICE. 65 



perfect spheres, as though the drops of rain had rapidly 

 congealed while falling. When broken open, a hailstone 

 occasionally exhibits crystals shooting out from the centre 

 in all directions towards the surface ; but it is more usual 

 to find a number of layers of ice, some clear and some 

 opaque, coating a white snowy central mass, around which 

 they appear to have been frozen in definite succession. As 

 a rule, hail falls in summer rather than in winter, and in the 

 day rather than in the night. The origin of hail is still 

 obscure, but it is probably formed by an intensely cold 

 current of air passing into a region of warm moist air, and re- 

 ducing the temperature of the whole below the freezing point. 



There is yet another form of atmospheric precipitate 

 that needs a passing notice. If the temperature after dew- 

 fall should sink below the freezing-point, the moisture 

 which would, under ordinary conditions be deposited as dew, 

 takes a solid form, and is then known as hoar-frost. Blades of 

 grass, and other objects cooled by freely throwing off their 

 heat into space, thus become coated with delicate icy crystals 

 instead of dew. The hoar frost is, in fact, nothing but dew 

 which has been frozen as it was formed. 



In one or other of the forms described in this and the 

 preceding chapter, all atmospheric moisture must be pre- 

 cipitated. It is not however always easy, nor is it by any 

 means necessary, to distinguish between these several 

 forms, and they are therefore practically massed together 

 under the general head of " rainfall." If then it is said that 

 the basin of the Thames is fed by a rainfall of twenty-six 

 inches, what is meant is that the total quantity of atmospheric 

 moisture precipitated within this area— adding together the 

 rain and the snow, the hail and the dew — amounts in the 

 course of an average year to a depth of six-and-twenty inches 

 spread uniformly over the surface of the basin. 



