60 On the Variety of Voices. 
single vibrating body being determined by the force 
of the pulses of air, and the celerity with which they 
follow each other, the only modifications that can 
be inferred from any conjunction of these proper- 
ties are the two specified above. But every man’s 
experience will convince him how inadequate such 
a combination is to elucidate the subject of the pre- 
sent essay; for an acquaintance is easily recognized 
by his speech, whether he speak vehemently or softly, 
in a high or low key; and the voice of two singers 
may be made to sound in unison, though they be in 
other respects very dissimilar; on which account it 
is certain that some circumstances, not comprised in 
the definition of sound, enables men to identify per- 
sons by the ear, without the assistance of the eye. 
The diversity of sound so remarkable in the human 
voice and vocal organs of animals, prevails also in 
sonorous bodies of almost every description; for a 
musician can single out, from a number of instru- 
ments of the same kind, one that is familiar to him 
merely by hearing them separately; and a flute will 
play in concert with a violin, yet their notes, consi- 
dered apart, are as distinct to sense as any two things 
can be. The effect appearing (from the preceding 
appeals to common sense) to be general, must be re- 
ferred to a general cause; and, as it has been already 
proved that the diversity in question is not produc- 
ed by any modification of a simple sound, it follows 
that some combination of sounds must constitute 
