522 THUNDER-STORMS. 



cerfiined that at the moment this occurred a storm broke over Geneva the most 

 terrific that the people of that country ever witnessed. 



On ihe 21st of July, 1813, Mr. LUKE HOWARD, observed at Tottenham, near 

 London, in a clear sky, lightning, such as is called heat-lightning, appear tow- 

 ard the southeast. It was afterward ascertained that a violent storm at tha 

 moment raged in France, which extended from Calms to Dunkirk. This light- 

 ning, above fifty leagues distant, was visible in the atmosphere of London. 



It must then be admitted as proved, that silent lightnings may be and yo 

 tunes urr produced by the reflection in the atmosphere of lightning of which the 

 thunder is too distant to be heard. But it does not therefore follow that sucl 

 appearances must be and always are produced by that cause. On the contrary 

 heat-lightnings frequently present appearances, to explain which it would be 

 almost impossible to admit the hypothesis of distant storms. Thus it frequently 

 happens that when the whole visible firmament is unclouded, these lightnings 

 will play for entire nights on every side of the horizon, and will extend even to 

 the zenith. If distant storms were admitted to explain such phenomena, i 

 would be necessary to suppose that portion of the atmosphere visible from a 

 single place clear and serene, yet surrounded on every side by a ring of clouds 

 throughout which storms rage. The improbability of such an hypothesis is 

 apparent. 



M. Arago proposed for the decision of this question, the same expedien 

 which he suggested a few years ago, in his essay on comets, to determine 

 whether their tails were self-luminous, or derived their light from the sun. 

 There are certain crystals endowed with optical properties, in virtue of which, 

 objects viewed through them are seen under different appearances according 

 as those objects are self-luminous or illuminated by light derived from other 

 objects. He proposes that the silent lightnings shall be observed through such 

 crystals, and the question whether they be actual lightnings, unattended by 

 thunder, or only reflections of distant lightnings, be thus decided. 



Thunder unaccompanied by lightning, is explained by M. Arago, by sup- 

 posing two strata of clouds at different heights, of which the superior stratum 

 is the seat of the thunder-storm, and of which the inferior stratum is sufficiently 

 dense to be impervious to the light which precedes the thunder. Nevertheless, 

 the density of the inferior cloud will not at all impede the transmission of sound 

 through it, and the thundej will consequently be heard while the lightning is 

 invisible. 



The method of computing the distance of stormy clouds by observing the 

 interval which elapses between the flash and the thunder, is based upon the 

 assumption that the sound is produced in the cloud. It has been however main- 

 tained by some persons, that when the electric discharge takes place between 

 a cloud and the earth, the lightning issues from the earth to the cloud. Ac- 

 cording to the hypothesis of a single electric fluid, this would always be the 

 case when the cloud is negatively electrified. As a test of this, M. Arago pro- 

 poses to observe the interval between the appearance of the lightning and the 

 perception of the thunder under circumstances in which the distance of the 

 cloud is known by othrr means within a given limit. If the distance obtained 

 by computation from observing the interval between the light and the sound be 

 manifestly less than the known minor limit of the distance of the cloud, it must 

 then follow that the seat of the sound is not the cloud, but is some place in the 

 atmosphere less distant, which would necessarily be the case if the lightning 

 issued upward from the earth. This method of observation might be practised 

 in the neighborhood of any lofty tower or steeple, or near a hill, or by means 

 of a small balloon confined by a cord to a given height. If the cloud were ob- 

 served to be considerably above any such objects and yet the computed distance 



