of the Position of Sonorous Bodies, 69 
has so considerable a share in the various 
phenomena of sound, as to render geometrical 
speculations on the subject in a great measure 
useless. For instance, if a lofty obstacle inter- 
pose itself between a sounding body placed at 
the foot of it and a person standing at any 
distance on the opposite side, according to geo- 
metry, he will not be able to perceive any sound 
from it, because the pulses which should strike 
his ear with effect, will be intercepted in their 
progress by that obstacle; ‘on the contrary, we 
are convinced by experience, that he will hear 
it, not indeed in the true direction, but as if it 
proceeded from some place elevated above the 
impediment. Sound, therefore, is propagated in 
a manner which neither coincides with the com- 
monly received theory relative to the subject, 
nor with the phenomena of shadows; on the 
other hand, it seems to follow a law, that may 
be said to form a medium between the two. 
The consequences of interrupting the pulses 
of sound, as well as the lateral communication 
of motion being now explained, the topic of the 
essay may be resumed. The annexed diagram 
will be found of use in explaining the phzno- 
mena which arise from the pulses of sound being 
obstructed by the hearers head, as they move in 
the horizontal plane passing through his ears, - 
which case ought to be treated separately from 
