r 



(. 50^ THUNDER-STORMS. 



any aqueous vapor which might have been ejected from the submarine crater 

 must, have been condensed before the column reached the surface of the sea, 

 and the smoke which rose into the atmosphere must have, therefore, been free 

 from vapor ; yet this smoke or cloud of vjlcanic dust was intersected by light- 



OF THE HEIGHT OF STORMY CLOUDS. 



The distance of the clouds from which lightning proceeds is estimated by 

 obse'rving the interval of time which elapses between the moment at which the 

 flash is seen and that at which the thunder is heard. It has been demonstrated 

 by certain astronomical observations, that light is propagated through space at 

 tht- rate of about two hundred thousand miles in a second of time. This space 

 being greater in avast proportion than the greatest distance at -which any thun- 

 der cloud can be placed from the observer, it may be assumed that the moment 

 at which the lightning is seen is practically coincident with the moment at 

 which it emanates from the cloud. It has, however, been also proved that 

 sound is propagated through the air at about eleven hundred feet per second. 

 This rate is subject to some small variations, depending on the temperature of 

 the air, but for our present purpose it may be taken at its mean value. If, then, 

 the number of seconds be observed, which elapse between the moment a flash 

 of lightning is seen and the moment the thunder consequent upon it is heard, 

 and eleven hundred feet be allowed for each second in that interval, the dis- 

 tance of the place whence the lightning issues from the observer will be de- 

 termined. Thus, if five seconds elapse, the distance will be five thousand 

 five hundred feet ; for six seconds, it will be six thousand six hundred feet, and 

 so on. 



If the cloud be vertically over the observer, this distance will be equal to its 

 actual height above the level of the observer. If it be not vertical, then its an- 

 gular elevation must be observed, and the height above the level of the observer 

 will be obtained by multiplying the computed distance by the trigonometrical 

 sine of the angular elevation. 



' The height of thunder-clouds is also attempted to be determined, by observ- 

 ing the effects produced upon objects in elevated situations stricken by the 

 lightning which issues from them. If it be admitted that lightning always de- 

 scends from the clouds toward the earth, then it may be inferred that the place 

 where such effects are manifested must be lower than the position of the cioud 

 from which the lightning proceeds ; but, if it shall appear that lightnings some- 

 times dart upward, nothing respecting the height of the cloud can be inferred 

 from such effects. Among those effects which lightning produces when it 

 strikes the earth is the superficial vitrification of rocks. Such effects have 

 been observed on the summits of some of the highest mountains of South 

 America by Humboldt, on the summit of Mont-Blanc by Saussure, and on the 

 Pyrenees by Ramond. 



In cases where no means have been taken by those who witnessed thunder- 

 storms to determine the height of the clouds from which they proceed, the sit- 

 uations of the observers themselves afford a minor limit of the value of that 

 height. Bouguer and La Condamine were assailed by a thunder-storm on one 

 of the summits of the Cordilleras, in Peru. Saussure and his son encountered 

 violent storms on the Col du Geant and Mont-Bljw. MM. Peytier and lios- 

 sard witnessed thunder-storms on the Pic de Troumouse, the Pic tie Bal> 

 and the Tuc de Maitpas, in the Pyrenees. 



Such are the principal observations collected by M. Arago, made in mount 

 ainous localities. The comparison of the results of these with the heights u! 



