1 00 On Peals of Thunder. 



duces the noise of tluinder. Their successors likewise have 

 made use of the atmosphere in the explication of the phenome- 

 non, but nearly in the i-everse way. They believe that the 

 lightning in its passage produces everywhere a vacuum. The 

 noise then, according to them, is the consequence of the rushing 

 in of the air, and is similar to that produced in experiments 

 with bladders. The sudden entry of air into a vacuum, will 

 unquestionably produce noise ; and if the lightning produces 

 a vacuum in traversing the atmosphere, thunder will be the con- 

 sequence. But here we have to inquire, by what physical cause 

 is it that the lightning produces a vacuum ? and to this ques- 

 tion no answer has hitherto been returned. Hence it follows 

 that an explanation of thunder has still to be discovered ; for 

 all that has yet been done has only been to set aside one diffi- 

 culty, and to substitute a greater. 



But, whatever may be the physical cause of the detonations 

 of thunder, we have still to inquire what is the origin of those 

 long rolling peals {roiilements) which every one has remarked, 

 and of those sudden changes in its intensity, which are so often 

 repeated, and which are known under the name claps of thun- 

 der {eclats). 



For a long time peals of thunder were regarded merely as 

 products of so many echoes. This explanation has since been 

 abandoned in the same hasty way in which it was adopted. 

 Let us now inquire, therefore, what place a serious discussion 

 ivill allow us to assign to it. 



All those who have witnessed a thunder-storm in a valley 

 surrounded with high mountains, know well how many local cir- 

 cumstances may give a resounding, intense, and continuous 

 character to the various peals. We have not then to inquire 

 whether sometimes echoes play a part in these phenomena; 

 the question is, xvhether they are always the cause of the peals 

 which occur. 



I have noticed many instances in which the thunder-peal has 

 continued for thirty-six, forty-one, and even forty-five seconds. 

 Has it then been proved, that echoes can produce sounds for 

 so long a period ? What occurs to me at this moment, as the 

 longest instance of any echo I have ever heard of, is an observa- 

 tion of my friend the Rev. Mr Scoresby. When near the lake of 



