130 TRANSMISSION OF SOUND. SECT. XVI. 



of the Oronoco, which seemed to be three times as loud 

 by night as by day. This he illustrated by experiment. 

 A tall glass half full of champaigne cannot be made to 

 ring as long as the effervescence lasts. In order to pro- 

 duce 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 homogeneity, because the 

 vibrations of the gas being much slower than those of 

 the liquid the velocity of the sound is perpetually inter- 

 rupted. For the same reason the transmission of sound 

 as well as light is impeded in passing through an atmos- 

 phere of variable density. Sir John Herschel, in his 

 admirable Treatise on Sound, thus explains the phe- 

 nomenon : "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 differ- 

 ent 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 favorable to 

 sound. There is no doubt, however, that the universal 

 and dead silence, generally prevalent 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 

 continual 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 sensibility." 



Many instances maybe brought in proof of the strength 

 and clearness with which sound passes over the surface 

 of water or ice. Lieutenant Foster was able to carry 

 on a conversation across Fort Bowen harbor, when fro- 

 zen, a distance of a mile and a half. 



The intensity of sound depends upon the extent of 

 the excursions of the fluid molecules, on the energy of 

 the transient condensations and dilatations, and on the 

 greater or less number of particles which experience 



