245 



was necessary agaiusfc reckless advisers to keep it iutact, until now the 

 beneficial effect is fully rec()i> nized, and reforestations have been begun 

 everywhere under the protecting walls or other safety- works Avherever 

 the danger of avalanches exists. 



Yet even timber forest is not always absolute protection against 

 avalanches 5 since the place of incipient formation of the slide may lie 

 above timber-line. Such avalanches, formed ^above the forest line, if 

 small, first break a wedge into the forest below^ them, but by repeated 

 action the wedge is enlarged and gradually a road broken through the 

 forest. Large avalanches break, even in their first attempt, through the 

 best rooted full grown forest of entire mountain sides, hurling earth, 

 rocl^, and timber into the valley below. 



Water, the great mover of the earth, is an active agent in the forma- 

 tion of avalrinches. Not only the sirring and seepage water, but also rain 

 and melting snow-water exert their influence. Ground-slides (in the 

 Alps) occur mostly in warm Aveather in the spring, when the snow melts. 

 Then the snow settles and becomes more compact, has a greater specific 

 weight, and therefore more tendency to slide; the snow water pene- 

 trates through to the soil, and if the soil is not frozen, saturates it and 

 then seeks to flow oi£ between soil and snow, by which the hold of the 

 snow on the soil is loosened, the latter made slippery, and the sliding 

 facilitated. How soon this influence of water becomes active depends 

 on how soon the soil is filled up with water, and this again on the kind 

 of soil and subsoil. 



Clay soils soon fill up in their upper strata and the snow-water sooner 

 begins a superficial flow ; a penetrable soil on the contrary with pene- 

 trable subsoil does not attain saturation at all and the danger is avoided. 

 If the soil is frozen, the water can not penetrate at all and the sliding 

 takes place the sooner. Eain of course adds to the water which loosens 

 the snow. 



Configuration, as has been said, may prevent formation of avalanches 

 b^' presenting a number of points of support. Yet where there are 

 sink-holes or troughs in which the snow accumulates, the melting snow- 

 water collects below the snow and loosens the masses, which may thunder 

 into the valley, following the course of the ravine. 



Lastl}^, even an exterior pressure or disturbance may loosen the masses. 



A stone or an icicle falling, or snow dropping from the branches of 

 a tree, when accomj)amed by strong wind, is liable to start the snow. 



So can one avalanche, by the concussion of the air which it produces, 

 start others in its neighborhood. It has also been frequently observed 

 that a sound, as of a gun, of church bells, of an explosion from u)ines, 

 etc., may start the snow. This has given rise to the proposal to start 

 the slides by shotguns before they are likely to become dangerous. 



On the other hand, sometimes in the stillest weather the danger may 

 be greatest, as the snow falling during such weather accumulates to 

 large amount be^fo re it breaks loose; while in stormy weather smaller 



