88 MODERN FOREST ECONOMY. 



or under the breath of the warm south winds, the upper 

 layer of the field of snow begins to melt. The water thus 

 produced sinks by infiltration to the ground, warms it, thaws 

 it, and detaches thus the lower bed of ice which was 

 intimately bound with it. From the time that the equili- 

 brium is disturbed at this point the snow mass sinks down, 

 and slides under the action of its weight. The shock 

 propagates itself to surrounding masses, the impulsive 

 force extends to the inferior fields of snow. And the 

 avalanche, enlarged by the enormous mass of earth and 

 rocks which it tears from the sides of the mountain, 

 precipitates itself to the foot of the valley amongst 

 rumbling sounds sounds and claps resembling those of 

 thunder, scattering in its passage ruin, desolation, and 

 death. 



' There suffice often in the season of debacles the 

 slightest rise of temperature, the least change of weather, 

 or the vibrations communicated to the atmosphere by a 

 rifle-shot, to determine the setting off of an avalanche. 

 Sometimes, on the other hand, the least change in slope, 

 or a slight elevation in the ground, suffices to arrest a 

 sliding snow mass, or to alter its direction. A forest, 

 especially if it be situated at but a short distance from the 

 point where an avalanche originates, acts in this way. 

 But if the commencement of the avalanche be at too 

 great an elevation above the wood, the impulsive force 

 may have become too great, and the acquired speed too 

 giddying for the obstacle to resist it ; and then the forest 

 may be violently swept off and hurried along by these 

 impetuous torrents of snow, or be crushed under their 

 mass. Thus it is not so much in stopping the sliding down of 

 the avalanches, when once the movement has commenced, 

 as in preventing the formation and setting out of these 

 that woods and masses are so eminently useful. Every 

 bush in a forest, every trunk of a tree, or stem of a shrub, 

 by creating a division in the field of snow at the first, 

 diminishes the mass, and the weight of the avalanche, 

 which is the primary and essential condition of its slipping. 



