en nena ih 
with Reference to Sound. | 219 
Of the woods, fir appears to be among the best conductors, 
sound being conveyed through it at the rate of 17,300 feet per 
on 
The intensity, also, of sound differs in media of different 
chemical and mechanical natures. It varies in atmospheric air 
with its density or specific gravity. Hawksbee, in his experi 
ments detailed in the London Philosophical Transactions, with 
an atmosphere in the usual state, heard a bell at the distance of 
30 yards. With a force of two atmospheres at 60 yards. With 
a force of three atmospheres at 90 yards. But he did not notice 
4 corresponding increase of sound with greater densities. 
Priestly ascertained by experiment that the sound of a bell in 
air. 
in atmospheric air at the distance of fifty-six feet, ceased to be 
heard in oxygen at sixty-three feet, in carbonic acid gas at forty- 
eight feet, and in hydrogen at eleven feet. Chladni also found 
that the sound from h ydrogen gas in an organ pipe was feeble and 
difficult to distinguish, while that of oxygen was stronger than 
that of common air. If hydrogen gas be breathed for a few mo- 
meuts, the effect upon the voice is precisely the same as that no- 
heed by: travellers in ascending very high mountains; the vocal 
tones, in both instances, becoming enfeebled and raised in pitch. 
In certain states of the atmosphere sounds are conveyed over 
Water or a surface of frozen snow or ice with remarkable cioag 
ess, 1 to 
authenticated, in which, under these circumstances, and in the 
.°ar still air of a winter’s morning, a conversation has been car- 
“unrise, the voice, and occasionally the laugh of the sailors on 
b een heard at a place at Portsmouth, distant two and a half miles 
a ig shay line. On the authority of Derham,* the human voice 
a been 
miles. 'The sound of a military band, at the hour of roll-call, 
casa heard at a distance of twenty-one miles from Edinburg 
tle, 
effect of sound propagated through mixed media is ex- 
The 
ceedingly curious and instructive, and, in connection here, de- 
= careful consideration. We have already seen the fa- 
ment les imperfectly mixed, or is interrupted by empty 
* Philosophical Transactions, 1708. 
