PROPAGATION OF SOUND. 
679 
Sumatra and Borneo, and generally occur in localities into 
which commerce and civilisation have as yet but little pene- 
trated. Notwithstanding the continued destruction of the 
trees, for the sake of procuring the camphor, no means are 
taken for the future preservation of the species. This camphor 
is seldom seen in this country, except in museums. The 
Chinese eagerly buy it in preference to the ordinary camphor 
— their own produce — which they send in such large quantities 
into the European markets . — Society of Arts. 
PROPAGATION OF SOUND. 
Professor Tyndall, in his first lecture on “ Sound/’ 
recently delivered, began by ascribing all our sensations to a 
kind of motion conveyed by the nerves to the brain ; thus, 
the effect of an explosion upon the ear is propagated through 
it as a wave or pulse which strikes on the drum of the ear, 
causing it to shiver; its tremors are transmitted to the audi- 
tory nerve, and hence to the brain, giving the sensation of 
sound. Illustrations were then given of the propagation of 
sound in waves, analogous to those of water; and a sonorous 
wave was described as consisting of two parts, in one of 
vrhich the air was condensed, and in the other rarified, the 
rate of propagation depending upon the elasticity of the me- 
dium. It was shown, by means of an air-pump, that sound 
cannot pass through a vacuum, and that it becomes very 
indistinct in an attenuated gas like hydrogen. The distant 
effects of great explosions, such as those of gunpowder-mills, 
were referred to in illustration of the enormous increase of 
the elasticity produced by the violent evolution of heated 
gas; and the formation and propagation of the waves of 
sound were strikingly illustrated by means of diagrams, by 
the wave-machine, and by tuning-forks, to which small mir- 
rors were attached — the optical effects being thrown upon a 
screen. In reference to the velocity of the passage of sound, 
Dr. Tyndall stated that at the freezing temperature the rate 
is 1090 feet a second, and that it increases about tw T o feet for 
every degree of the centigrade scale added to the temperature 
of the air. Newton, who did not consider the change of 
temperature, gave the theoretic velocity at 9 1 (3 feet a second. 
The velocity of sound in w r ater is more than four times its 
velocity in air; in iron, seventeen times; and in pine-wood, 
ten times. Among many other interesting illustrations was 
an example of the “ telephonic concert ” of Sir Charles 
Wheatstone. The notes of a musical-box in a room two 
