of the seat of the sound considerably below them, the conclasion jut stated 

 would he justified. 



From the observations which have been recorded of the lime between, the 

 flash and the thunder, it appears that although in one instance this interval 

 amounted to seventy-two seconds, it usually does not exceed forty-fi^lr 

 ouds. It follows, then, that the greatest distance from which the atmospheric 

 explosions which produce thunder are heard at about ten miles. If the single 

 Accorded observation of an interval of seventy-two seconds can be relied on, 

 it would follow that in that particular case thunder was heard at the distance 

 of fifteen miles. 



Evidence still more direct and convincing can be adduced that beyond the 

 distance of eight or ten miles thunder is inaudible. 



When the steeple of Lestwithiel in Cornwall was struck by lightning, on the 

 25th of January, 1757, and almost entirely destroyed, the thunder was terrihc ; 

 yet Smeaton the engineer, who was then within thirty miles of the place, 

 heard no thunder. Muschenbroeck states that thunder at the Hague is inaudi- 

 ble at Leyden and at Rotterdam, the distance of the former being ten and the 

 latter twelve miles. There are also examples of violent storms breaking over 

 Amsterdam which were inaudible at Leyden, the distance being about twenty 

 miles. 



To deduce right conclusions from these facts it will be necessary to con- 

 sider the distances at which other sounds, generally much less intense than 

 thunder, are heard. Cannon discharged at Florence are heard at Leghorn, 

 a distance of fifty miles ; at Leghorn, are heard at Porto Ferraio, the suine 

 distance. The cannonade at the siege, was audible at Leghorn, a distance of 

 about ninety miles. It may be added that the great bell of St. Paul's cathe- 

 dral in London, is said to be audible at Windsor, a distance of about twenty- 

 four miles. 



The conditions of the atmosphere, which affect the transmission of sound, 

 are imperfectly understood, and it is therefore the more necessary to accumulate 

 well-ascertained facts, to form a safe basis for general reasoning. It is generally 

 believed that sounds are heard more distinctly and at greater distances in win- 

 ter, especially in frost, than in summer. This popular impression has been 

 corroborated in the narrative of those who have made voyages to the polar re- 

 gions. Parry states that he frequently heard distinctly at the distance ot a 

 mile, men conversing in their ordinary voice. On the 1 1th of February, iti20, 

 he heard a man singing to himself (and therefore probably in rather a low 

 tone), at more than a mile distant. 



Durham observes that new-fallen snow impedes the transmission ot sound, 

 and that fogs also deaden its force. This latter effect, however, is not inva- 

 riable. In a November fog, in 1812, Mr. Howard heard distinctly at live miles 

 from London, the noise of the carriages rolling over the streets. 



Humboldt has proved that sounds are audible at greater distances by night 

 than by day ; and from the circumstances under which his observations were 

 made, it would appear that the silence of night could not be assumed as an ex- 

 planation of this. 



It seems to be established that an adverse wind is an impediment to the 

 transmission of sound ; but according to the observations of M. F. Delaroohe, 

 a favorable wind does not assist it. 



Volney, at Pontchartrain, heard four or five claps of thunder. Looking care- 

 fully round him, he could see no clouds either in the heavens or near the earth. 

 Now since thunder has never been heard at a greater distance than tlitecn 

 miles, and since an object to be invisible at that distance with a well-defined 

 horizon must have an elevation less than about one hundred feet, it follows 



