ACOUSTICS. 293 
The ears of animals are admirably adapted to receive impressions from the 
vibrations of the air caused by so-called sounding bodies. The external 
‘ear collects part of the sound-wave, and turns it into the passage which 
conducts it to the membrane from which the vibrations are conveyed to 
the auditory nerves. The car-trwmpet, which is a trumpet-shaped instru- 
ment, having a wide mouth at the end of a tube, is simply a contrivance 
by which a greater volume of the sound-wave is collected and conveyed 
into the ear. 
Musical Notes—We have hitherto spoken only of a single shock 
transmitted through the atmosphere as producing a single sound. The 
sensation conveyed to the ear when blows are struck with a hammer 
is a succession of distinct sounds. When sounds are produced in such 
quick succession that the successive pulses of the air cannot be dis- 
tinguished by the ear, a musical note is produced. For example, if a 
toothed-wheel, with a plate of some thin elastic substance resting on the 
teeth, be turned slowly, the shock produced by the plate falling off each 
tooth is conveyed separately to the ear, and separate sounds are heard; 
but when it is turned more rapidly, the shocks all blend into one, and the 
sound heard is a continued ‘whir;’ and the more rapidly the wheel is 
turned the more sharp and shrill the ‘ whir’ becomes. It is on this prin-* 
ciple that notes are produced from the strings of musical instruments. 
When a string stretched between two objects is pulled aside in the middle, 
its elasticity causes it to return to its proper position ; but the momentum 
‘it has meantime acquired carries it past to about the same distance 
on the other side. Its elasticity here comes into play again, and it 
is again carried past the middle straight position almost as far as it was 
pulled aside at first: the same thing is repeated; and this goes on, the 
vibrations on each side gradually becoming less and less, till the string 
at last comes to rest. When this is done with a slack string, the vibra- 
tions are slow; but when the string is tight, the vibrations are so 
quick that, as in the case of the toothed-wheel when turned rapidly, 
the separate pulses are blended into a continuous note; and the tighter 
the string is pulled, the sharper is the sound produced by its vibrations. 
A good illustration of successive vibrations blending to cause a continu- 
ous sound, is the ‘buzz’ of a fly, which is supposed to be not the voice 
of the insect, but the sound produced by the rapid vibrations of its 
wings. It is on this rapidity of vibration that the difference of sounds 
depends. We observed that the ‘whir’ of a toothed-wheel striking an 
elastic plate becomes more shrill the quicker the wheel is turned, and 
that the sound produced by a vibrating string is sharper the quicker the 
vibrations are made. Now, a short string will evidently vibrate more 
quickly than a long one; therefore, every degree of sharpness of sound 
can be produced from strings by making them of different lengths and of 
