310 



GEOLOGY. 



mine the law of growth it was found that when the temperature of th^ 

 atmosphere was above the melting-point the growth was appreciably^ 

 more rapid than when the air was colder, but there was, on the average^ 

 an increase under all conditions of temperature. A portion of this aver^ 

 age increase of the larger granules appears to come from the diminution 

 and destruction of the smaller ones, for the total number of granules 

 steadily diminishes. A portion of the gro^vth doubtless comes from 

 the moisture of the atmosphere which penetrates the snow and another 



Fig. 286. 



Fig. 287. 



Fig. 288. 



Fig. 289. Fig. 290. Fig. 291. 



Figs. 286-91.— Snowflakes. (Photographed by W. A. Bentle3^) 



portion from the moisture derived from surface melting; but beneath 

 the surface of a large body of snow the growth of the large granules is 

 probably chiefly at the expense of the small ones. To follow the process 

 it should be noted that the free surface of every granule is constantly 

 throwing off particles of water- vapor (evaporation); that the rate at 

 which the particles are thrown off is dependent, among other things, on 

 the curvature of the surface, being greater the sharper the curve; that 

 the surfaces of the granules are at the same time liable to receive and 



