Distance at which Thwider is heard. 95 



experiments, provided they were made in two places at the same 

 time, wherewithal to determine in which current the detonation 

 is produced. Finally, would it not confer a high degree of pro- 

 bability on their system, if, for example, the focus of these de- 

 tonations were discovered to be between the clouds and the 

 earth? 



Before leaving the numerical data upon which we have been 

 dwelling, we shall endeavour also to determine the greatest dis- 

 tances to which it is ascertained the sound of thunder has ever 

 extended. We have already had occasion to allude to the fact, 

 that De VIsle once reckoned 7^ seconds between the hs^ht- 

 ning and the thunder. This number, the most considerable 

 mentioned in the annals of meteorology, multiplied by 368, 

 gives for the distance of the cloud whence the lightning ema- 

 nated, 26,496 yards, or about 15 miles English. After tliis soli- 

 tary result of 72 seconds, the next longest period which I have 

 met is 49 seconds. This number, multiplied by 368, gives 

 18,032 yards, or about 10 miles. According to these data, the 

 greatest distance at which tiiunder has ever been heard, is lit- 

 tle more than 15 miles, and the greatest ordinary distances do 

 not amount to more than about 10 miles.* 



The small extent of the distance, according to these results, 

 cannot fail to surprise most people, especially when they r^em- 

 ber how much further the report of cannon extends. Thus, for 

 example, I find that the discharge of the cannon at Florence 

 sometimes extends from the old castle of Monte- Kotondo nearly to 

 Leghorn, a distance, in a straight line, of 20^ leagues (upwards 

 of 50 English miles). Also, when they fire the cannon at Leg- 

 horn, the report is sometimes heard at Porio-Ferraio, at a dis- 



* It maybe gratifying to supply here some limits of distances which have 

 been determined by direct methods. On the 25th of January 1757, a thun- 

 derbolt fell with iiji-ight/nl crash upon the steeple o{ Lesttritliid, in Cornwall 

 and nearly entirely destroyed it. The celebrated Smeaton was then about 

 30 miles distant, Avhence he saw the lightning, but did not hear the slightest 

 noise. Muschenbroek mentions that it frequently thundei"S very violently 

 at the Hague, without being at all heard at Leyden, a distance of 4 leagues, 

 or at Rotterdam, a distance of 5^. There are also instances noted of very 

 violent thunder-storms having occurred at Amsterdam, whilst no intimation 

 of it could be perceived, so far as sound was concerned, at Leyden, a distance 

 of 9 leagues. 



