Distance at which Thunder is heard. 97 



By means of the results we have just obtained concerning the 

 extreme distances which the noise of thunder reaches, we shall 

 be able to determine the important question, whether we must 

 regard the thunders occurring in serene and beautiful days, as 

 the murmurs of common thunder which has been elaborated in 

 the midst of clouds which are situated below the horizon, or 

 whether we are to consider them as thunders which are pro- 

 duced, and have broken forth, in the midst of the purest at- 

 mosphere. The following are the links by which these two 

 kinds of truths may be connected. 



If the horizon is quite clear, a man whose eye is elevated about 

 five feet (1™.6) from the ground, may see an object on tlie earth at 

 the distance of 1 league (of 4000 metres), about 2| miles Engl. 



If the object is elevated 82 ft., he may see it at the distance of 64 leagues. 



1640 do. do. 21 ... 



3280 do. of more than 29 



Let US now consider an observation of Volney's, whose accu- 

 racy is well known. On one occasion, when at Pontchartrain, 

 he very distinctly heard four or five peals of thunder. He 

 looked carefully around him, but could perceive no cloud, either 

 in the firmament, or near the earth. If these five peals did not 

 originate in the diaphanous atmosphere which covered the visible 

 horizon ; — if their focus, or cause, must be sought for in clouds 

 which were situated beyond the limitsof this horizon, these clouds 

 could not be at a greater distance than six leagues (15 miles) ; for, 

 were this the case, the detonation could not have been heard : 

 moreover, clouds that were invisible at the distance of six leagues 

 could not have possessed an elevation of more than about 98 feet. 



night than during the day. Is it equally certain that the difference depends, 

 as my illustrious friend ingeniously supposes, upon the currents of hot air 

 which rise from the soil to the upper regions of the atmosphere, during the 

 daytime ? It is an opinion very generally entertained, that the wind, when 

 it blows in a direction contrary to that in which the sound is proceedmg, con- 

 siderably diminishes its intensity. Facts, on this point, confirm tlie general 

 belief. This, however, is not true, of another opinion which is not less gene- 

 rally prevalent, viz. tliat those winds which are blowing in the stune direc- 

 tion with any aiven sound, aid its transport, and maintain its strength. The 

 observations of M. F. Delaroche seem to establish, that if there are winds, 

 which, so far as intensity is concerned, oppose its progress, there exist none 

 which favour it. 



VOL. XXVI. NO. LI. JANUABY 1839- ^ 



