with Reference to Sound. 223 
distant object over a fire or flame of a candle. The air is, therefore, 
during the day, a mixed medium, in which the sounds are reflected and 
scattered in passing through streams and strata of different densities, 
as in the experiment of mixing atmospheric air and hydrogen. At 
midnight, on the contrary, when the air is transparent and of a uniform - 
nsity, as may be seen by the brilliancy and number of the stars, the 
slightest sound reaches the ear without interruption.” 
In this greater distinctness of sounds by night, doubtless, some- 
thing must be attributed to the-absence of the usual noises of t 
day, and the consequent greater sensibility of the auditory appa- 
tats to impressions ; but the reasoning above given is philosoph- 
ically correct, 
the human voice—whatever be the quantity, pitch or quality 
of the original impulse, the sonorous wave reaches the ear in equal 
intervals of time. It would need but the slightest infringement 
of this law to change our highest enjoyment into the intensest 
suffering, 
A general idea of the divergence and decay of sound is obtain- 
ed from the illustration before given of dropping a pebble into an 
untufiled pool ; if not interrupted by the surface of a wall or other 
Stacie, the wave thus produced spreads from its common centre, 
diminishing gradually in height till, at length, it sinks into the 
general level. So sounds in empty space, as ordinarily produced, 
diverge in all directions from the sonorous centre till their energy 
IS lost in the distance. ‘The intensit y of sound decays in receding 
‘om its origin as the square of the distance increases. : 
he sympathy of sound and motion is exceedingly curious. 
Every fundamental note has its complimentary or harmonic ad- 
duced 
“If two cords of the same material and equal tension be taken, the 
phe being only one-third the length of the other, and the shorter string 
be sounded, the vibrations will be communicated to the other by the 
‘atervention of the air, which latter will vibrate in three parts each 
dual to the shorter string and each performing the same number of 
Vibrations ina given time.”’ 
This tendency of one vibrating body to throw another into the 
ime state of vibration, is well illustrated in the motion of two 
Fees fixed to the same support. For a long time it has 
