Theory of Compound Sounds. 661 
force to be combined with time, which combi+ 
nation is wanting in the present instance; where~ 
fore the coalescence of sounds is impossible. 
ProrosiTion W. It may be demonstrated 
from mechanical principles, that a number of 
distinct cotemporary sounds cannot do other- 
wise than produce distinct sensations. 
In order to make the necessary diagram as 
simple as possible, let the directions of two co- 
temporary sets of pulses be represented by thé 
right lines SM and TN, lying in the same hori- 
zontal plane, and intersecting in the point A; 
also, let BCD be the horizontal section of the 
hearer’s head, made by the same plane; and sup- 
_ pose the centre of the axis of hearing to be at 
O; draw OM, ON perpendicular to SM, TN. 
Now I have shewn in the preceding paper, that 
if a set of pulses move im either of the right 
lines SM, TN, it will excite a sensation in that 
part of the head which is cut off by a vertical 
plane, passing through one of the perpendicu- 
lars OM, ON. It also appears from the last 
proposition, that the impulses of the vibrating 
bodies, acting in the lines SA, TA, do not com- 
pel the particle A to move in any given interme- 
diate direction, as LA. But, according to the 
second proposition, the position of the particle 
A, is fixed in respect of the planes MO, NO; 
that is, though the corpuscle actually changes 
