658 Theory of Compound Sounds. 
had no conception of two cotemporary sounds ; 
because the ear being in that case incapable of 
conveying the complex sensation, the idea of 
such an existence would have transgressed the 
sphere of human knowledge. The preceding 
arguments are drawn, for the most part, from 
common experience; and they shew, that the © 
free passage of cotemporary sounds through the 
air may be safely admitted as an axiom in har- 
monies. I shall therefore proceed to prove th 
_ same proposition to be consistent with the doc- 
trine of forces. | 
The propagation of sound through the atmos- 
phere, and the nature of aéreal pulses are com- 
monly explained in elementary books of natural 
philosophy ; I shall, for this reason, enumerate 
only a few particulars, the recollection of which 
will be found useful. 
Prorosir1on I, Two contiguous particles of 
air which are agitated by a vibrating body, either 
directly or by the intervention of an elastic me- 
dium, receive two motions from each impulse ; 
forst, an absolute motion carries them to a greater 
distance from the sounding body, and afterwards 
brings them towards it again, both the progress 
and regress being performed in the time of a 
Single.‘vibration: . second, a relative motion re- 
sulting:from the former, compels the two parti- 
cles to approach ‘and recede alternately, which 
