in the Rarified Air of High Mountains. 75 



ment, the afternoon of the 31st August, the sky was clear, 

 the air perfectly calm, the temperature — 3°-5 centigrade, the 

 tension of the vapour of water was 0™™-06, and the baro- 

 meter showed 477""" '88 For both of us the limit of hear- 

 ing was at 337 metres. 



These experiments, in which we have heard during the 

 day the diapason at greater distances upon the mountains 

 than in the plain, do not contradict the observations of 

 travellers who have been struck with the weakening of 

 sound at great altitudes. In fact, these travellers having 

 ascended suddenly from the plain upon the mountain, their 

 organs, and particularly that of hearing, have not had time 

 to put themselves in equilibrium with the ambiant air. We, 

 however, made our experiments after several days' sojourn 

 at the Faulhorn, and upon the grand plateau of Mont 

 Blanc, consequently our senses were, so to speak, habituated 

 to the aerial medium. So the inhabitants of Paz and of 

 Quito in America do not suffer from the effects of the rari- 

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

 great elevation above the level of the sea. 



To render the hearing distances obtained on the plains 

 and on the mountains comparable with each other, I have 

 reduced these distances to what they would have been in the 

 air at zero centigrade, and under the barometric pressure of 

 760 millimetres. In other words, I have deduced from the 

 experiments made at different temperatures and under differ- 

 ent pressures the limit of audibility when the air at zero has 

 a pressure of 760 millimetres, the density of which air I 

 designate by 1. Poisson, in his mechanics, shows what all 

 physicists agree in admitting ; 1st, that the intensity of sound 

 is proportional to the density of the medium on which it is 

 produced ; 2d, that at a great distance from the origin this 

 intensity decreases in the inverse ratio of the square of this 

 distance. If then we make 



r, the limit of audibility in air of the density d, as at the 

 time of observation, 



R, the limit in air of the density 1 (air at zero and 760"^™), 



I, the intensity of disturbance of the tympanum corres- 

 ponding to the limit of audibility in air of density 1, 



