On the Variety of Voices. 59 
ignorant of its effects and extent. But the cause 
of the great difference in the tone of the vocal or- 
gans, is but badly understood; or, to speak more 
properly, has perhaps never been examined in a phi- 
losophical manner:. and, as it is the intention of this 
essay to enquire into the subject with more care and 
strictness, it will not be improper to begin by re- 
viewing the commonly received notion of the nature 
of sound. Sound is defined* to be a sensation excit- 
ed in the ear bya quick succession of aerial pulses 
corresponding to the vibrations of an elastic sub- 
stance; for a body of this kind, upon receiving a 
tremulous motion, immediately communicates it to 
the portion of air in contact with itself; and it is, in 
like manner, successively propagated through the 
whole of the air extending from the vibrating surface 
to the auditory organs, by which means men acquire 
a notion of sound, together with the whole class of 
ideas depending on the sense of hearing. 
The preceding definition is sufficient to account for 
all the phenomena of the musical scale, but it may be 
easily proved to be too simple to explain the present 
difficulty; for, were the sensations produced in the ear 
only modified by the cause assigned above, it is evi- 
dent that the variety of voices could consist in no- 
thing besides comparative loudness and acuteness ; 
because the effect produced on this organ by a ° 
* Smith’s Harmonics, sect, fT. 
