THUNDER-STORMS. 



507 1 1 



thunder clouds, computed from observations made in flat countries and at 

 would supply means of determining whether the development of storms H af- 

 fected by the density of the air in which the clouds float, or by their proximity 

 to the surface of the earth. Thus, if it should appear that, in rlouds at the 

 same height above the level of the sea, storms are developed more frequently 

 when these clouds are in the neighborhood of mountains, and therefore at *a 

 comparatively small distance from the surface of the earth, it would follow, with 

 a probability proportionate to the number and character of the facts observed, 

 that the earth exerts an influence on clouds charged with electricity independ- 

 ently of the atmosphere in which these clouds float. 



The height of thunder-clouds observed in a flat country, or at sea, are ob- 

 tained by the method first mentioned, that is, by observing the interval between 

 the flash and the thunder, and measuring or estimating the angular elevation 

 of the cloud. Unfortunately, the latter element of the computation has been very 

 frequently neglected by observers, the sole object having been apparently to 

 determine the distance of the cloud from their station, and not its vertical height. 

 In some cases it appears, incidentally, that the cloud from which lightning is- 

 sued was in the neighborhood of the zenith, and consequently the distance may 

 be taken as equivalent to the height. In some few the angular elevation has 

 been observed and recorded, and consequently the vertical height of the cloud 

 may be computed. 



The following results of the labors of various observers have been collected 

 by M. Arago : 



The height of thunder-clouds determined by other data being in some cases 

 greater than the heights of rocks vitrified by lightning, there is nothing in the 

 comparison of the results exhibited in the preceding table, to justify the suppo- 

 sition that the vitrifications observed by Humboldt, Saussure, and Ramon, .Ud 

 not proceed from lightning which issued from clouds at a greater elevation. 

 But, on the other hand, facts are not wanting to show that this inference can- 

 not be certainly made. There is a church in Styria erected on a summit of i 

 lofty peak called Mount Saint Ursula. Jean-Baptiste Werloschnigg,* medica 

 practitioner, who happened to visit this church on the first of May, 1700. ot 

 served a stratum of dense black clouds to be formed below him at about half he 

 elevation of the place where he stood. These clouds soon became the seat of 



