NATURAL PHILOSOPHY. 151 



produced 2117 metres lower. To equalize the two sounds, it was nec- 

 essary to charge the lower mortar with 75 grammes of powder, and 

 the upper one with 90 grammes. The accounts of the ascents of the 

 highest mountains contain some observations upon the weakening of 

 the sound, but they often contradict each other. According to Saussure 

 and others, the sounds on the top of Mt. Blanc were remarkably weak. 

 A pistol shot made no more noise than an ordinary Chinese cracker, 

 and the popping of a bottle of champagne was scarcely audible. M. 

 Martins, in the same situation, was not able to verify these conclu- 

 sions. He states that he was able to distinguish the voices of the guides 

 in conversation at a distance of 400 metres, and to hear the tapping 

 of a lead pencil upon a metallic surface at a distance of from 15 to 20 

 paces. 



Desirous to acquire some ideas more positive upon this subject, the 

 means of obtaining a continuous sound, of a constant intensity, produ- 

 cible at will, was sought for. This M. [Martins effected by means of a 

 diapason, properly mounted and constructed, and so arranged as to give 

 512 vibrations of sound in a second. With a sound having always the 

 same intensity in an air of equal density, it is evident that the variable 

 distance at which it ceases to be perceptible, in mediums of different 

 densities, should give us the measure of the variations of this intensity. 

 The agitation of the air complicates these experiments. It was ascer- 

 tained by M. Holdat and De la Roche, in 1814, that the limit of dis- 

 tance for hearing is increased or diminished for a person having the 

 wind from or to the origin of the sound. Both these observers agree 

 in affirming that the sound is heard at the greatest possible distance in 

 air at rest, the noise from the wind, come from what part it may, in- 

 terfering with the perception of the sound. 



In experiments made with the diapason, on a desert plain in 

 a cahn day, barometer 744 mm 3, thermometer 24 C., the limit of 

 sound was found to be 254 metres. The same experiment, repeated at 

 11 o'clock in the evening, in the same place, and under nearly the 

 same circumstances, gave 379 metres as the limit of sound ; thus show- 

 ing a difference of 125 metres in the distance at which sound ceased 

 to be perceptible in the day and in the night. This result confirms 

 the observations of other experimenters, as well as a fact noticed by 

 Humboldt upon the banks of the Orinoco, where the cataracts were 

 heard much better during the night than during the day, although 

 the buzzing of insects was more intense, and the cries of wild animals 

 were louder than during the day. 



Experiments made 2620 metres above the level of the sea, 

 barometer 558 mm 4, gave 550 metres as the limit of sound ; at 3910 

 metres above the sea level, or 900 metres below the summit of 

 Mt. Blanc, barometer 477 mm S, the limit obtained was 337 metres. 

 These experiments, in which sounds were heard during the day at 

 greater distances upon the mountains than in the plain, do not con- 

 tradict 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 those of hearing, have not had time to put themselves in 



