448 AVALANCHES. 



large quantities of earth, gravel and stones. These 

 " ground avalanches " usually occur in spring, when 

 the melting of the snows has set in, and are then of 

 course of a much more formidable nature than avalanches 

 of newly fallen snow. 



One of the most remarkable phenomena accompany- 

 ing avalanches, is the blast of air which they carry 

 with them : this rush of air, like that which accompanies 

 explosions of various kinds, may at times prove very 

 destructive, and extend its influence for a considerable 

 distance on each side of the track of the avalanche; it 

 is said that even forest trees growing near are thus 

 sometimes uprooted and laid prostrate, without having 

 been touched by the avalanche. Thus in 1819 the 

 village of Randa in the Visp had many houses destroyed 

 by the blast accompanying a falling glacier, * and the 

 east spire of the convent of Disentis was thus thrown 

 down by the wind of an avalanche which fell more 

 than a quarter of a mile off. f 



The explanation of this phenomenon is not far to 

 seek ; the vast bulk and momentum of the falling mass is 

 sufficient to create extensive displacement of the 

 surrounding air, which is impelled with a sudden and 

 violent rush on either side, and in advance of the track 

 of the avalanche, which thus has within certain limits 

 all the force of a violent tornado. Air, we must always 

 remember, though a very elastic gas, is subject to 

 considerable pressure at the earth's surface, which at 

 mean sea-level is estimated to amount to 14.7304 Ibs 

 to the square inch: and thus, as all experience shows, 

 when in rapid motion it represents a force whose 



* See Murray's Handbook for Switzerland, I5th Ed. 1891 part i., p. 69. 

 f Ibid., part i., p. 69. 



