THUNDER. 291 



the hissing sound in his alpenstock. The tips of rocks and grass points 

 were all hissing too. The party were in the midst of an electric cloud. 

 When the Professor turned the point of his alpenstock upwards, a vivid 

 flash was emitted, but no thunder followed. They descended as quickly 

 as possible from such a dangerous neighbourhood. 



It is observable that the properties of lightning and of the electric 

 spark are identical the faint crackle of the latter being magnified into 

 the loud rolling of the thunder. The disturbance of the atmosphere is the 

 cause of the loud reverberations, and echoes produced from clouds tend to 

 intensify and prolong the peal. The sound rises and falls, and varies 

 accordingly as the cloud is near or far. A smart sharp report or rattle 

 denotes the nearness of the lightning, while the gradual swelling and 

 subsidence, followed, mayhap, by an increasing volume of sound, which in 



Fig. 291. St. Elmo's fire. 



its turn dies away, tells us that the danger is not imminent. The cause of 

 this loud rolling, unless it proceeds from echoes from different clouds, has 

 not been satisfactorily explained. Sound travels less quickly than light, 

 and therefore we only hear the thunder some seconds after we have 

 perceived the flash. It is therefore conceivable that we may hear the last 

 reverberations and its echoes first, and the sound of the first disturbance 

 with its echoes last of all. Thus there will be distinct sounds. Firstly, 

 the actual noise we call thunder from the air strata nearest to us ; secondly, 

 the echoes of that disturbance from the clouds, of course fainter ; then we 

 hear the sound caused by the tearing asunder of the air particles farthest 

 off, and again the echoes of that disturbance. This theory will, we think, 

 account for the swelling peals of thunder, and the successive loud and 

 fainter reverberations. At any rate, in the absence of any other theory, we 

 submit it for consideration. The sound of thunder is seldom or never 

 heard at a distance greater than fifteen miles. 



