58 ANSWERS TO PRACTICAL QUESTIONS 



stone. We frequently find frost on the nagging early in the 

 fall, but frost on the window is a sign of very severe winter 

 weather. 



68. IFhy will not snow "pack" into balls except in 

 mild weather ? 



The snow must be very near the melting-point for the press- 

 ure of the hand to be sufficient to melt enough of it to pro- 

 duce the phenomena of regelation. (Physics, p. 271, 1st note ; 

 also Tait's Recent Advances in Physical Science, p. 129, and 

 Tyndall's Forms of Water, p. 163.) This principle involves the 

 theory of Glaciers. " The masses of snow can not rest on the 

 steep slopes of Alpine summits. The pressure upon the under 

 layers is too great to allow them to remain upon their sloping 

 beds, and they are forced to descend. This descent is accom- 

 plished in two forms : that of an avalanche, one of the most 

 awful and imposing spectacles to witness ; or of a glacier, 

 which is really an avalanche of ice of extremely slow motion. 

 But the glacier differs from the ordinary avalanche not only in 

 that its motion is so slow, but in that it consists of ice, thick, 

 firm, and hard. The principles involved in this transition of 

 the loose, flaky snow which first falls upon the mountain-top 

 into the solid ice of the glacier, are very well illustrated, as 

 Helmholtz has remarked, in the manufacture of the school- 

 boy's snow-ball or snow-man. Very cold snow is always light 

 and flaky, and can not be made by the pressure of the hands 

 into a cohesive mass ; in order to succeed in that operation, 

 snow is always employed which is already at the melting-point, 

 or only so far below this temperature that the warmth of the 

 hand suffices to bring it to the required temperature, and then, 

 by dint of pressure and molding, an icy ball may be easily 

 produced. So with the formation of the glacier ice. A process 

 of almost simultaneous melting and freezing goes on among 

 the under layers of snow, and under an immense and ever- 

 constant pressure from the weight of the snow above ; thus 

 solid ice is formed. That this ice conforms itself to the various 

 windings, constrictions, and dilatations of its rocky channel 

 during its downward march, is a fact not less familiar than 

 wonderful." 



