Effects of a Thunder Sturm in the Alps. 23 



of a river or torrent below any cascade or rapid, we find that, 

 so soon as the channel resumes its usual degree of inclination, 

 the stream resumes its ordinary velocity. 



The principle being admitted, that, in the same channel, the 

 force of running water, exerted in carrying detritus, increases in 

 proportion to the depth of the water, let us now investigate 

 those causes, which, by producing variations in the volume or 

 mass of water, produce corresponding variations in its depth. 

 These are, 



1*^, Long continued rains, which fall upon extensive districts 

 of modei'ate elevation. They produce widely-spreading inun- 

 dations in the lower grounds, and, as we know from the expe- 

 rience of our own island, often wash away the alluvial banks of 

 rivers, and in various ways enlarge and deepen their bed. The 

 muddiness of the water indicates the vast quantities of earth 

 which they convey towards the sea. All tropical countries have 

 their rahiij season, when they are deluged by such inundations. 



2dlj^, Sudden heavy showers, especially thunder-storms. These 

 occur in summer, and produce as striking effects in elevated 

 regions as long-continued rains in low countries. A shower of 

 this description in the Alps, often swells the torrents in a few 

 minutes, so that they rush with irresistible fury, burst their or- 

 dinary limits, and carry down with them immense quantities of 

 sand and gravel, with large fragments of rocks. They subside 

 as quickly, leaving their channels almost dry. Being at Cha- 

 monix on the 16th of last July, I had the opportunity of wit- 

 nessinof some of the effects of a thunder-storm among; mountains. 

 It commenced after sunset, and lasted about three hours. The 

 noise of a torrent on the opposite side of the river was like con- 

 tinued thunder, being produced not only by the vast quantity 

 of water falling almost perpendicularly, but by great boulders 

 tossed over the precipices. Next morning the whole atmosphere 

 was clear, the Arve and its tributaries at their usual height. 

 But all along the northern and western flanks of Mont Blanc, 

 the effects of the brief shower were visible in cottages deserted, 

 pastures and corn fields destroyed, and roads wfished away. 

 The obtuse cones, and the levels surrounding them, were strewed 

 with fresli depositcs of boulders, pebbles, and sand, amounting 

 to the depth sometimes of several feet, and exhibiting various 



