SKCT. xvi.] TRANSMISSION OF SOUND. 15 1 



are then better heard than during the day, when its density is 

 perpetually changing from partial variations of temperature. 

 His attention was called to this subject on the plain sur- 

 rounding the Mission of the Apures by the rushing noise of 

 the great cataracts of the Oronoco, which seemed to be three 

 times as loud by night as by day. This he illustrated by ex- 

 periment. A tall glass half full of champagne cannot be 

 made to ring as long 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 homogeneity, because, the vibrations of 

 the gas being much slower than those of the liquid, the ve- 

 locity 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 astro- 

 nomers alone fully appreciate, renders it also more favour- 

 able to sound. There is no doubt, however, that the uni- 

 versal and dead silence, generally prevalent at night, ren- 

 ders 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 ut- 

 most sensibility." 



