72 PHYSICS. 
b. Transmission of Sound through the Air. 
By the vibration of a body a wave motion is communicated to the 
surrounding air, and this it is which brings the tone, arising in the vibration, 
to our ears. Not air alone, however, but every elastic medium, can propa- 
gate sound; in a vacuum this propagation does not take place. Of this 
fact we may be convinced by placing a small bell, moved by spring clock- 
work, and isolated by being set on a woollen mat, in the receiver of an air- 
pump. Cause the hammer of the bell to commence striking, and with an 
increasing rarefaction of the air, the sound will become fainter and fainter, 
until it disappears almost entirely. Re-admit the air, and the sound will be 
again audible, becoming more and more distinct. Saussure found, that on 
the summit of Mont Blanc, a pistol-shot made only an inconsiderable 
sound ; and Gay Lussac noticed, that when at a height of about 3000 feet 
in a balloon, his voice became less powerful. The loudest sound does not 
pass beyond the atmosphere, and terrible explosions might take place on the 
moon without our hearing anything of them. Water transmits sound very 
well, since divers hear at the bottom of the water, the voices of persons 
speaking on the shore. 
The manner in which the vibrations of sound are propagated through the 
air, may be best understood by supposing an open tube, bdtt’ (fig. 49°, pl. 
19), in which, from ¢’b, a piston may be moved quickly backwards and 
forwards. Suppose the length of the tube to be divided into a number of 
parts, equal to the length of the play of the piston, about in s, a, b, c; then 
when the piston is forced into a’, the air between a’p will fall into a 
vibratory motion, and this motion will be transmitted to the layer ps, when 
the piston has reached p, and will pass over into the second half to 6, when 
the piston has finished its advance and commenced its return. This 
motion cannot, however, be uniform, for previously mentioned reasons, and 
we obtain the velocity in the individual parts by describing a semicircle 
above sa, the length of the play of the piston, dividing this semicircle, as 
at z’ and y’, into equal parts, and letting fall the perpendiculars, xa’ and yy’. 
The motion must, from the elasticity of the air, be transmitted successively 
to all the strata, while, if the air were inelastic, the piston would drive out 
all the air before it. From these considerations we may readily understand, 
that during the ingress of the piston, the air in bs becomes compressed 
before the motion is transmitted to sa. When the piston begins its return, 
the compression is propagated to sa; the strata between s and b, however, 
enter upon a retrograde motion, and when the piston has reached b again, 
occupy their old position. With a new action of the piston, the first 
vibration passes over to ab; while the layers between a and s are making 
their retrograde motion, those, however, between s and b are compressed, 
&c. Sound waves are consequently formed, each of which has the 
duration of a forward and backward motion of the piston, and consists of a 
rarefied and a condensed part, which then corresponds to the wave valley 
and wave elevation. 
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