THE WORK OF SNOW AND ICE 277 



center. These are first steps toward the pronounced granulation 

 so conspicuous in snow which has lain long on the ground. Meas- 

 ured from day to day, the larger granules beneath the surface of 

 coarse-grained snow are found to be growing. When the temper- 

 ature of the atmosphere is above the melting-point, the growth 

 is faster than when the air is colder, but there is an increase in the 

 average size of the granules, and a decrease in their number, under all 

 conditions of temperature. A part of the increase of the larger gran- 

 ules appears to come from the diminution and destruction of the 

 smaller ones; another part doubtless comes from the moisture of the 

 atmosphere which penetrates the snow and is condensed there, and 

 still another part from the descent of moisture derived from surface 

 melting. 



Deep beneath the surface of a large body of snow, the larger 

 part of the growth of the large granules is probably at the expense 

 of the small ones. To understand how this takes place, it should 

 be noted that the free surface of every granule is constantly throwing 

 off particles of water-vapor (i. e., evaporating); that the rate of 

 evaporation increases with the sharpness of the curve of the surface, 

 and that the smaller the particles, the sharper the curve; that the 

 surface of a granule is liable to receive and retain molecules evap- 

 orated from other granules, and that, other things being equal, the 

 retention of particles is more common on the surfaces which are 

 least curved. It follows that the larger granules of less curvature 

 will lose less and gain more, on the average, than the smaller gran- 

 ules of greater curvature. The larger granules therefore grow at 

 the expense of the smaller. Furthermore, small granules melt 

 more readily than large ones, and where the temperature is nicely 

 adjusted between melting and freezing, the smaller may lose while 

 the larger gain. 



Another factor that affects the growth of the granules is pressure 

 and tension. The granules are compressed at the points of contact 

 and put under tension elsewhere, and pressure and tension are, on 

 the average, likely to be relatively greatest for the smallest granules. 

 Tension increases the tendency to evaporation, and the capillary 

 spaces adjacent to the points of contact probably favor condensa- 

 tion. Pressure reduces the melting-point, while tension raises it. 



