152 ANNUAL OP SCIENTIFIC DISCOVERY. 



equilibrium with the ambient air. The experiments of M. Martins 

 were made after several days' sojourn at the elevations cited, and when 

 the senses, so to speak, were habituated to the aerial medium. So the 

 inhabitants of Paz and Quito, in America, do not suffer from the effects 

 of the rarefaction of the air, because they live habitually at a very 

 great elevation above the level of the sea. There are causes, however, 

 observes M. Martins, which favor the hearing of sound on high moun- 

 tains, that more than compensate the rarefaction of the air. In the 

 experiments made, a sound was heard at a greater distance, when the 

 density was not more than 0' 72, on a mountain, than at the level of 

 the sea ; and even when the density was only 0' 64, as on Mt. Blanc, 

 a sound was heard at greater distance than on a plain. Among these 

 causes, silence should hold the first rank. On the grand plateau of Mt. 

 Blanc there is a repose only broken by the noise of the wind or of thunder. 

 In calm weather the silence is so profound that sounds are heard at a 

 great distance, although their intensity be much less than in the low 

 country. The fall of avalanches, so common in these high regions, is ac- 

 companied with a noise which lias no relation to the masses of snow 

 or ice precipitated from the neighboring rocks ; nevertheless it is always 

 heard, because the least sound is perceived by the ear. There are 

 various other causes, which in the mountains favor the hearing at great 

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

 metric state of the air, the absence or presence of aerial currents. But 

 all these causes, the influences of which have never been studied, appeal- 

 secondary to that already noticed. Unfortunately, the silence, more 

 or less perfect, which reigns in a place, cannot be expressed numeri- 

 cally. If that were possible, the intensity of the sound would probably 

 be in the direct ratio of the density of the air, multiplied by a quantity 

 which we may call the coefficient of silence. 



TELOCITY OF SOUND. 



MM. WERTHEINE and Breguet have presented to the French Acad- 

 emy a paper touching the velocity with which sound is communicated 

 by means of iron wire. The experiments were made upon the wires of 

 the electric telegraph established along the Versailles railroad on the 

 right bank of the Seine. The length of wires between the operators 

 was 4,067-jSy metres, (13,344 feet.) The distance taken would have 

 been greater, but the intervention of a tunnel through which the road 

 passes was found to present insuperable obstacles. The wire was iso- 

 lated and prevented from touching the wall of the tunnel at any point, 

 but all to no purpose ; the sound could not be made to traverse the tun- 

 nel. From the distinctness with which the sound, produced by a blow 

 upon the posts supporting the wire, was heard, the operators judged 

 that, but for the tunnel, the sound would have propagated to a much 

 greater distance. The most perfect chronometers were employed, 

 and a variety of ingenious expedients to ensure accuracy in the experi- 

 ments ; and the result reported to the Academy is, that sound is prop- 

 agated over wire at the rate of 3,485 metres (11,434 feet) per second. 



