96 Distance at which Thunder is heard. 



tance of 20^ French leagues. Once more, at the time that the 

 French were conducting the siege of Genoa, the noise of their 

 artillery was heard at Leghorn, a distance of 36| leagues (up- 

 wards of 90 English miles). 



The shortness of the distance which suffices completely to 

 subdue the noise of the loudest thunder has excited astonish- 

 ment in all countries. Thus, I find in the Memoires des Mis- 

 siannaires de la Chine, t. iv., that the Emperor Kang-hi, who 

 made learned researches concerning the phenomena of thunder, 

 regarded 10 leagues as the greatest extent to which the detona- 

 tions could reach ; whilst he satisfied himself, on the other 

 hand, that he had perceived the noise of artillery to the distance 

 of 80 leagues. The object of our researches should now be to 

 discover whether the great cause of the obscuring of the sound, 

 on which we have so long been dwelling, is not exclusively the 

 partial reflections to which it is subjected in obliquely encoun- 

 tering the different substances which separate the atmospheric 

 strata of different densities.* 



* We know, generally, exceedingly little concerning the different canses 

 which exerl an inflvience npon the intensity of sound, and concerning their 

 mode of action. Derham alleges that sounds extend farther, and are more 

 distinct in -winter, and especially during frost, than in summer. This opi- 

 nion lil^s been confirmed by Captain, now Sir Edward Parry. I read in his 

 first voyage — " The distance to which sounds extended in the open air, so 

 long as the intense cold continued, was extremely great, and invariably 

 excited our surprise, in spite of the frequent opportunities we had of mak- 

 ing the remark. For example, we often heard men who Avere convers- 

 ing at their usual tone, at the distance of a mile. On the lltli of February, 

 1 heard, even at a greater distance, a man who was singing to himself as lie 

 walkcxl along the beach." Derham thought likewise, that he had noticed 

 that new fallen snow is an agent in moderating sound, much more power- 

 ful, tliau that which has lain for a time on the ground, and whose surface 

 has become united by a crust of ice. He also says, that mists very consi- 

 derably destroy sonorous undulations. It is probable that those mists, which 

 are uniformly distributed, probably produce the effect announced by tlie 

 JEnglish philosopher ; but in other circumstances they produce quite a con- 

 trary effect. Thus, in November 1812, the atmosphere being covered to a 

 slight elevation, with a thick and continuous stratum of mist, Mr Howard 

 distinctly heard the noise of the carriages upon the stones of London, thougli 

 he was then at a mean distance from the town of not less than five miles. 



The observations of M. de Humboldt, made upon the banks of Oronocco, 

 cave completely established that sound propagates itself further during the 



