654 Theory of Compound Sounds. 
barometer and its nonius vary, none on the slider 
coinciding with those on the fixed plate excepting 
the highest and lowest. I have chosen this fa« 
miliar instrument to illustrate Dr. Smith’s me- 
thod of explaining the physical cause of com- 
pound sounds, because it affords a visible exam- 
ple of a cycle of pulses, according to his notion 
of the subject. 
The sketch which I have exhibited of Dr. 
Smith’s hypothesis shews, that he allowed that 
a number of simple sounds might exist in con- 
cert, and strike the ear in a distinct manner, with- 
out suffering any interruption in their motions 
from the interference of their pulses. Buta late 
writer on sound rejects this axiom in Harmonies 
as a mathematical inconsistency ; and substitutes 
the following theory of compound sounds in the 
room of it. If two musical strings, differing in. 
their times of vibration, happen to vibrate in 
concert, they do not occasion two distinct sounds 
in the opinion of this gentleman, because the 
strings agitate the air in conjunction; conse. 
quently the pulses, which one of them would 
actually form in an undisturbed atmosphere, must 
unavoidably clash with those which the other 
string would produce in similar circumstances. 
Hence the waves of air belonging to both strings 
are interrupted in their natural progress, and are 
compelled by their mutual interference to coa- 
