662 Theory of Compound Sounds. 
place, in respect of the geometrical point A, it is 
always found in the intersection of the physical 
right lines SM, TN. Now the two vibrating 
bodies continue to act in the directions of these 
right lines, consequently the particle A is con- 
stantly urged in these lines by two forces, whichy 
though variable in magnitude, are combined with — 
time; which circumstance enables the corpuscle 
to transmit the impulses of one body to M, and 
those of the other to N. What has been demon- 
strated of the particle A, may be affirmed of any 
other particle, which is the intersection of two 
right lines parallel to SM, TN ; in other words, it 
may be affirmed of two sets of pulses; and the same 
demonstration may be extended to three sets, &c, 
Corollary 1. The substance of this and the 
preceding proposition will apply to all elastic 
mediums; hence it happens, that a plate of glass 
&c. in a state of vibration, will conduct a foreign 
sound, whilst it produces one of its own; for 
the same reason, if light be considered as a vi- 
brating medium, one particle of the luminous 
fluid may assist in transmitting two sensations. 
Corollary 2. When the inclination of the 
planes MO, NO, is less than a given angle, the 
ear cannot distinguish the relative positions of 
the sounding bodies; it therefore refers them to 
the same place, 
