SOUND, 
compressed ; and the like obtains when the 
air less compressed, expanding itself, a new 
compression is generated. Theiefore from 
each agitation of the aii there arises a mo- 
tion in it, analogous to the motion of a 
wave on the surface of the water; which is 
called a wave or undulation of air. In 
each wave, the particles go and return back 
again through very short equal spaces ; the 
motion of each particle being analogous to 
the motion of a vibrating penduhnn while 
it performs two oscillations; and most of 
the laws of the pendulum, with very little 
alteration, being applicable to the former. 
Sounds are as various as are the means 
that concur in producing them. The chief 
varieties result from the figure, constitution, 
quantity, &c. of the sonorous body ; the 
manner of percussion, with the velocity &c. 
of the consequent vibrations; the state and 
constitution of the medium ; the disposi- 
tion, distance, &c. of the organ : the ob- 
stacles between the organ and the sonorous 
object and the adjacent bodies. The most 
notable distinction of sounds, arising from- 
the various degrees and combinations of the 
conditions above mentioned, are into loud 
and 16w (or strong and weak) ; into grave 
and acute (or sharp and flat, or high and 
low); and into long and short. The ma- 
nagement of which is the office of music. 
Euler is of opinion, that no sound making 
ffe'wer vibratioiijs than 30 in a second, or 
more than 7520, is distinguishable by the 
human ear. According to this doctrine, 
the limit of our hearing, as to acute and 
grave, is an interval of eight octaves. The 
velocity of sound is the same with that^of 
the aerial waves, and does not vary much, 
whether it go with the wind or against it. 
By the wind indeed a certain quantity of 
air is carried from one place to another; 
and the sound is accelerated while its waves 
move through that part of the air, if their 
direction be the same as that of the wind. 
But as sound moves vastly swifter than the 
vvind, the acceleration it will hereby re- 
ceive is but inconsiderable ; and the chief 
effect we can perceive from the wind is, 
that it increases and diminishes the space 
of the waves, so that by help of it the 
sound may be heard to a greater distance 
than otherwise it would. 
That the air is the usual medium of 
sound, appears from various experiments in 
rarefied and condensed air. In an unex- 
hausted receiver, a small bell may be heard 
to some distance ; but when exhausted, it 
can scarce be heard at the smallest dis- 
tance, When the air is condensed, the 
sound is louder in proportion to the con- 
densation, or quantity of air crowded in; 
of which there are many instances in Hauks- 
bee’s experiments, in Dr, Priestley’s, and 
others. Besides, sounding bodies commu- 
nicate tremors to distant bodies; for ex- 
ample, the vibrating motion of a musical 
string puts others in motion, whose tension 
and quantity of matter dispose their vibra- 
tions to keep time with the pulses of air, 
propagated from the string that was struck. 
Galileo explains this phenomenon by ob- 
serving, that a heavy pendulum may be put 
in motjon by the least breath of the mouth, 
provided the blasts be often repeated, and 
keep time exactly with the vibrations of the 
pendulum ; and also by the like art in rais- 
ing a large bell. 
It is not air alone that is capable of the 
impressions of sound, but water also ; as is 
manifest by striking a bell under water, the 
sound of which may plainly enough be 
heard, only not soToud, and also a fourth 
deeper, according to good Judges in musical 
notes. And Mersenne says, a sound made 
under water is of the same tone or note, as 
if made in air, and heard under the water. 
The velocity of sound, or the space through 
which it is propagated in a given time, has 
been very differently estimated by authors 
who have written concerning this subject. 
Roberval states it at the rate of 560 feet in 
a second; Gassendus at 14f3; Mersenne 
at 1474; Diihamel, in the History of the 
Academy of Sciences at Paris, at 1338; 
Newton at 968; Derham, in whose mea- 
sure Flamsteed and Halley acquiesce, at 
1142, The reason of this variety is as- 
cribed by Deiham, partly to some of those 
gentlemen using strings and plummets in- 
stead of regular pendulums; and partly to 
the too small distance between the sono- 
rous body and the place of observation ; 
aud partly to no regard being had to the 
winds. But by the account since publish- 
ed by 91. Cassini de Thury, in the Memoirs 
of the Royal Academy of Sciences at 
Paris, 1738, where cannon were tired at 
various as well as great distances, under 
many varieties of weather, wind, arfd other 
circumstances, and where the measures of 
the different places had been settled with 
the utmost exactness, it was found that 
sound was propagated, on a medium, at the 
rate of 1038 French feet in a second of 
time. But the French foot is in proportion 
to the English as 15 to 16: and 'coilse- 
queiitly 1038 French feet are equal to HQ? 
