664 Theory of Compound\Sounds.- 
parts. His theory therefore differs from mine in 
this particular only : he pronounces the voice to 
be a compound by coalescence ; I deny the pos- 
sibility of such a compound, and call it a mixture 
of imperfect unisons, This mixture appears to 
be‘a single, sound, because it has but one direc- 
tion; for the proximity of the various parts con- 
tributing to the formation of it, disqualifies the’ 
ear, so that it cannot perceive their relative posi- 
tions, and compels it to refer them all to one 
place, by Corollary 2 to proposition V. 
A certain class of sounds, which, for the sake 
of brevity, were not noticed in my paper on the 
voice, deserve a place in the present communi- 
cation. If a finely-toothed file pass slowly over 
a smooth elastic substance, such as a piece of 
horn, it makes a grating noise; but if the velo- 
city of the instrument be sufficiently increased, a 
continued sound is produced, which becomes 
more or less acute, by giving a quicker or slower 
motion to the file. The grating noise is oc- 
casioned by a succession of short interrupted 
sounds, resulting from the united vibrations of 
the file and the body it scratches ; but the quick 
succession of these sounds, caused by an increase 
of velocity, gives rise to a secondary sound re- 
sembling the harmonical notes, being produced 
by alike cause. Now this sound becomes a pri- 
mary object with the-ear, in all probability be- 
