number of the fork. If instead of a test-tube a bottle had been taken,
the result would have been the same. Every kind of a vessel can respond
to some tone of a definite wave-length, and a sphere has been found to
give the best results. These are made with a hole on one side for the
sound-wave to enter, and a projection on the opposite side, through
which a hole about the one-eighth of an inch is made, this to be placed
in the ear. Any sound that is made in front of the large orifice will
not meet any response, unless it be that particular one which the globe
can naturally re-enforce, when it will be plainly heard. Suppose, then,
one has a series of twenty or more of these, graduated to the proper
size for re-enforcing sounds in the ratio of one, two, three, four, and
so on. Take any instrument, say a flute: have one to blow it upon the
proper pitch to respond to the largest sphere, then take each of the
spheres in their order, applying them to the ear while the flute is
being sounded. When the overtones are present they will be heard
plainly and distinct from the fundamental sound. In like manner any or
all other sounds may be studied.

But Helmholtz did not stop after analyzing sounds of so many kinds: he
invented a method of synthesis, by which the sounds of any kind of an
instrument could be imitated. A tuning-fork, when made to vibrate by an
electric current, gives out a tone without harmonics or overtones. So if
a series of forks with vibration periods equal to the numbers of the
series of overtones given on p. 86 be so arranged that any of them may
be made to vibrate at will, it is evident that the resulting compound
tone would be comparable with that from an instrument having such
overtones. Thus, if with a tuning-fork giving a fundamental C, other
forks giving two, three, and four times the number of the fundamental
were associated, each one giving a simple tone, we should have for a
resultant the tone of a flute, as shown on p. 91. If one, three, five,
seven, and nine, were all sounded, the resulting tone would be that of
the clarionet, and so on. This he actually accomplished, and now makers
of physical apparatus advertise just such instruments.

Helmholtz also contrived a set of tuning-forks, which, when bowed, will
give out the vowel sounds like the voice.

It was remarked upon p. 89 that it has generally been considered that
age has a mellowing effect upon the sound of a violin. Once in
possession of the