i 02 On Peals of Thunder. 



In order to prove that peals of thunder do not always and 

 solely result from reflected sounds, we adduce an observation on 

 which such an opinion might be supported. On this occasion, 

 the sky is covered uniformly with clouds; a flash of lightning 

 issues from the zenith ; in less than two seconds the thunder 

 breaks forth, and a long peal follows ; a little while afterwards, 

 another flash cleaves the cloud in the same zenithal region^ the 

 thunder follows, but this time, the clap, though exceedingly 

 loud, is sharp and of no continuance. How are these great dif- 

 ferences to be explained, when we make the rolling peal of 

 thunder only a simple phenomenon of echoes ? 



Dr Robert Hooke, one of the most productive and ingenious 

 authors of whom England can boast, was, I believe, the first 

 who, in his explanation of peals of thunder, introduced an im- 

 portant consideration, unfortunately too much neglected by the 

 majority of modern philosophers. I allude to the essential 

 distinction which he has established (in his Posthumous Works, 

 printed in 1705, p. 424), between simple lightning and com- 

 pound or multiplied lightning. Every example of the former, 

 remarks the author, occupies but a point in space, and pro- 

 duces only a short and instantaneous noise. The sound of the 

 latter kind, on the contrary, is a prolonged peal, because the 

 different parts of the long lines to which these flashes extend^ 

 being in general at different distances, the sounds which they 

 produce, whether successively, or in the same physical instant, 

 must occupy spaces of time gradually unequal in reaching and 

 striking the ear of the observer. 



This ingenious view of Dr Hooke was again introduced to 

 notice by Professor Robison, fifty years ago, in the Encyclo- 

 paedia Britannica. As its adoption by such an individual 

 ought to recommend it to the attention of meteorologists, I shall 

 here introduce the lines which the celebrated Professor of 

 Edinburgh devoted to the subject. " On one occasion I no- 

 ticed a flash of lightning parallel to the horizon, which might 

 be about three miles in length. It appeared to me co-existent, 

 and no one could say at which end it commenced. The thun- 

 der was composed at first of a very sharp clap, and then of an 

 irregular peal, which continued for about fifteen seconds. I 

 imagine that the detonations happened simultaneously in the 



