; MAMMALIA—MAN. AD 
effect always a vibration, or wave-like motion, communicated by other bodies 
to the air, and to our senses, by the air striking on our auditory nerve. 
Every body that strikes against another, produces a sound, which is sir 
ple in such bodies as are not elastic, but which is often repeated in such as 
are. If we strike a bell, for instance, a single blow produces a sound, which 
is repeated by the undulations of the sonorous body, and which is multiplied 
as often as it happens to undulate or vibrate. These undulations succeed 
each other so fast, that the ear supposes them one continued sound; where- 
as, in reality, they form many sounds. Sounding bodies are, therefore, of 
two kinds; those unelastic ones, which being struck return but a single 
sound; and those more elastic, returning a succession of sounds, which 
uniting together form a tone. This tone may be considered as a great 
number of sounds, all produced, one after the other, by the same body, as 
we find in a bell which continues to sound for some time after it is struck. 
A continuing tone may be also produced from a non-elastic body, by repeat- 
ing the blow quick and often, as when we beat a drum, or when we draw a 
bow along the string of a fiddle. 
To know the manner in which musical sounds become pleasing, it must 
be observed, that no one continuing tone, how loud or swelling so ever, 
can give us satisfaction; we must have a succession of them, and those 
in the most pleasing proportion. The nature of this proportion may 
be thus conceived. If we strike a body incapable of vibration with 
a double force, or what amounts to the same thing, with a double mass 
of matter, 1t will produce a sound that will be doubly grave. Music 
has been said, by the ancients, to have been first suggested by the 
blows of different hammers on an anvil. Suppose then we strike an 
anvil with a hammer of one pound weight, and again with a hammer 
of two pounds, the two pound hammer will produce a sound twice as 
grave as the former. But if we strike with a two pound hammer, and 
then with a three pound, the latter will produce a sound one third more 
grave than the former. If we strike the anvil with a three pound hammer, 
and then with a four pound, it will likewise follow, that the latter will be 
a quarter part more grave than the former. Now, in comparing all those 
sounds, it is obvious that the difference between one and two is more easily 
perceived than between two and three, three and four, or any numbers 
succeeding in the same proportion. The succession of sounds will be, 
therefore, pleasing in proportion to the ease with which they may be dis- 
tinguished. That sound which is double the former, or, in other words, the 
octave to the preceding tone, will among all others be the most pleasing 
narmony. The next to that, which is as two fo taree, or, in other words, the 
third, will be most agreeable. And thus, universally, those sounds whose 
differences may be most easily compared are the most agreeable. 
Sound has, in common with light, the property of being extensively 
liffused. Like light, it also admits of reflection. The laws of this reflec- 
