3() INFLUENCE OF LIGHT ON THE PRGPAGATION OF SOUND. 



The diiTerence between the propugation of sound that took 

 pliice during the night from thai in the day comes out two 

 decrees of my scale, answering to about \6 lines. Whenever 

 I made any experiments on this subject I always acted vvith 

 the greatest caution, taking care to guard against the 

 Slightest inaccuracy. The subject has always appeared to 

 me difficult and delicate; and without venturing to assert 

 too much, I confine myself to an account of my thoughts 

 ^^ and inquiries, happy to have removed a doubt, that con- 

 cerns one of tl:e most important branches of our know- 

 ledge. 



Facts should After having called the attention of natural philosophers 

 be compaied. ,. . i i i r i 



to a discussion, which may serve ^o render the nature ot lu- 

 minous bodies better known, as well as to elucidate the na- 

 ture of sounds, and to discover the mutual action of the im- 

 perceptible substances that surround us, it seems to me not 

 improper, to compare together the facts, that appear to re- 

 late to this subject, and which may give vise to discussions 

 calculated to explain the phenomena. 

 Night sup- Hitherto night has been considered as more favourable 



▼oiuableTothe *^^" ^^Y ^^ ^he propagation of sound. That this is the case 

 propagation of with respect to our ears cannot be doubted : but this argues 

 soiind, but , . . . . _--. , n 1 1 -1 



jnerc'i' from nothing against my opmion. We hear farther by night on 

 the abitnce of account of the silenee; and this silence contribu-tes to it, 

 "°'''^' while, according to the celebrated Euler, the noise of a 



wind favourable to the propagation of a sound may prevent 

 The ear hears the sound from being heard*. I have reason to think, that 

 best by day: ^^^^ ^.^^ ^^^ more aptitude for hearing sounds by day than 

 by night, and this from the stimulant action, that light ex- 

 erts oh the nervous system : but this will not account for the 

 * History of the Academy of Sciences at Paris, 1738. 

 D's'inctionhe- Sound and noise, which are the same thing with respect to the object 

 vwen sound of my inquiries, exhibit essential differences when considered with regard 

 and noise. j^j our ears. By sound is to be understood that peculiar resonance, 



vfefhich proceeds from a sonorous body, and the tone of which we know. 

 Noise I conceive to bean assemblage of several sounds. 



When any sound predi)niinat<?s our ea-s caii distinguish it from other 

 -sounds in harmonic pro;;ortion with it. In noise the harmonic sounds 

 are confounded and lost. The celebra-ed Condillac, speaking of the na- 

 ture of sounds in his treatise on Sensations, has distinguished the two by 

 Ijie definition of appreciable sounds and inappreciable sounds. 



phenomenon 



