136 INTENSITY OF SOUND. SECT. XVI. 



periment. A tall glass half full of champagne cannot be made 

 to ring as long as the effervescence lasts. In order to produce a 

 musical note, the glass together with the liquid it contains must 

 vibrate in unison as a system, which it cannot do in consequence 

 of the fixed air rising through the wine and disturbing its homo- 

 geneity, because, the vibrations of the gas being much slower 

 than those of the liquid, the velocity of the sound is perpetually 

 interrupted. For the same reason the transmission of sound as 

 well as light is impeded in passing through an atmosphere of 

 variable density. Sir John Herschel, in his admirable Treatise 

 on Sound, thus explains the phenomenon : " It is obvious," he 

 says, " that sound as well as light must be obstructed, stifled, 

 and dissipated from its original direction by the mixture of air 

 of different temperatures, and consequently elasticities ; and 

 thus the same cause which produces that extreme transparency 

 of the air at night, which astronomers alone fully appreciate, 

 renders it also more favourable to sound. There is no doubt, 

 however, that the universal and dead silence generally preva- 

 lent at night renders our auditory nerves sensible to impressions 

 which would otherwise escape notice. The analogy between 

 sound and light is perfect in this as in so many other respects. 

 In the general light of day the stars disappear. In the con- 

 tinual hum of voices, which is always going on by day, and 

 which reach us from all quarters, and never leave the ear time 

 to attain complete tranquillity, those feeble sounds which catch 

 our attention at night make no impression. The ear, like the 

 eye, requires long and perfect repose to attain its utmost sen- 

 sibility." 



Many instances may be brought in proof of the strength and 

 clearness with which sound passes over the surface of water or 

 ice. Lieutenant Forster was able to carry on a conversation 

 across Port Bowen Harbour, when frozen, a distance of a mile 

 and a half. 



The intensity of sound depends upon the extent of the excur- 

 sions of the fluid molecules, on the energy of the transient con- 

 densations and dilatations, and on the greater or less number of 

 particles which experience these effects. We estimate that in- 

 tensity by the impetus of these fluid molecules on our organs, 

 which is consequently as the square of the velocity, and not by 

 their inertia, which is as the simple velocity. Were the latter 



