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the particles of air, which are put in motion, and expand 
themselves in an infinite succession of circles; similar in effect 
to that which takes place upon the surface of water, 
previously at rest, when a stone is cast into it: we may 
observe that a number of concentric circles are generated, 
which are constantly enlarging until circumscribed by the 
narrowness of the stream, or some obstacle which prevents 
their perfect formation: for when the undulations meet with 
interruption, the first that recoil resist the progress of those 
which follow in succession. The air is put in motion by 
sound in a similar manner, with this difference, that the 
undulations of the water are made in a plane surface; 
whereas in air they ascend as they extend themselves. 
Thus it is with the voice; when no obstacle interrupts 
the first undulation, the next and those following are perfect; 
and make distinct impressions upon the sense of the spectators 
who are in the upper, as well as those who are in the lower 
seats; without reverberating. 
The architects of former days, therefore, made use of 
a form in the auditory of the theatre, adapted to the 
configurations of air arising from the expansion of sound; 
and by the application of physics to the science of music, 
succeeded in effecting that the sounds which were uttered 
from the stage were conveyed to every part of the theatre, 
clearly defined and better modulated. For as musical 
instruments are formed of thin plates of metal or horn, with a 
view to produce distinctness in the tones of the chords, so the 
principles upon which the theatres of the ancients were 
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