Strength and pitch were the first two distinctions of different
tones. The third is the timbre. When we hear one and the same
tone sounded successively upon a violin, trumpet, clarionet,
oboe, upon a piano, or by a human voice, &c., although it is of
the same strength and of the same pitch, yet the tone of all
these instruments is different, and we very easily distinguish
the instrument from which it comes. The changes of the timbre
seem to be infinitely manifold; for, not to mention the fact
that we have a multitude of different musical instruments, all
which can give the same tone, letting alone also that different
instruments of the same kind as well as different voices show
certain differences of timbre, the very same tone can be given
upon one and the same instrument, or by one and the same voice,
with manifold differences of timbre.[10]
As now the strength of the tone is determined by the breadth of the
vibrations, and the pitch by their number, so the varieties of timbre
are ascribed to the different forms of the waves of vibration. For as
the surface of the water is stirred differently by the falling into
it of a stone, by the blowing over it of the wind, or the passing
through it of a ship, &c., so the movements of the air take different
shapes from sounding bodies. The movement proceeding from the string
of a violin over which the bow is drawn, is different from those
movements caused by the hammer of a piano or by a clarionet.