78 Mr Ch. Martins on the Intensity of Sound, §*c. 



On the grand plateau M. Bravais and I also remarked a 

 multiple echo, which repeated the human voice many times, 

 and which was extinguished only after a duration of seven 

 seconds. In the night of the 7th to the 8th of August we 

 were exposed in the same place to a storm which almost car- 

 ried away our tent. We were astonished at the slight noise 

 made by the thunder claps ; the short interval between the 

 lightning and the thunder proving to us that the concussion 

 was not more than 1000 metres from us. Is this an effect 

 of the rarefaction of the air, or must we seek elsewhere for 

 the causes of this singularity ? The same night we repeated 

 also the observation that we had already made at the rocks 

 of the Grand s-Mulets, 860 metres below the plateau, on the 

 night of the 28-29th July, and on the grand plateau itself on 

 the night of the 29th and 30th. During these two nights we 

 were assailed by a strong gale from the south-west. Like 

 Saussure on the Col du Geaut (3430™),* we were struck with 

 the terrible noise of the squalls, and with the dead calm of 

 the short intervals of repose which separated them. These 

 squalls succeeding the moments of profound silence pro- 

 duced a startling acoustic contrast, and Saussure has not 

 exaggerated when he compares the noise of the blast to that 

 of a discharge of artillery. 



I will not enter into an examination of the other causes 

 which in the mountains may favour the hearing at great dis- 

 tances. Many of them will depend on local circumstances, 

 such as the configuration and nature of the soil, the hygro- 

 metric state of the air, the absence or the presence of aerial 

 currents. But all these causes, the influence of which have 

 never been studied, appear to me secondary to that which I 

 have already signalized. Unfortunately the silence, more or 

 less perfect, which reigns in a place cannot be expressed 

 numerically. If that were possible, I doubt not that the in- 

 tensity of the sound would be in the direct ratio of the den- 

 sity of the air multiplied by a quantity that we may call the 

 coefficient of silence. 



* Voyages dans les Alpes, §§ 2031 and 2073. 



