M A N 
MAN 
v/c can neither iudge of the difbance, nor of the 
momentum of the ilroke that gives rife to it; but 
whenever we can afcertain the fpecies of any indi- 
vidual found, we are enabled to guefs both at the 
diflance and the momentum of the ftroke. If, 
for example, v/e hear the report of a cannon, or 
the found of a bell, we compare them with thofe of 
the fame kind we have formerly heard, and thus 
form a pretty exa6l judgment of their diftance and 
force. 
Every body ftriking againft another produces 
a found, which is fimple, and but one in non- 
elaftic bodies, but often repeated in fuch as are 
elaftic. If we ftrike a bell, or a ftretched ftring, 
which are both elaftlc, a fingle blow produces a 
found, which is repeated by the undulations of the 
fonorous body, and is multiplied as often as it 
happens to vibrate: thefe undulations each ftrike 
their ov/n peculiar blow; but they fucceed each 
other fo rapidly, that the ear fuppofes them one 
continued found, whereas they are many. A per- 
fon who flhould, for the firft time, hear the toll of 
a bell, would very probably be able to diftinguifli 
thefe breaks of found; and, in fa6l, even conftant 
experience cannot prevent us from perceiving an 
intenfion and remiffion in the found. After this 
manner, fonorous bodies are of two kinds; thofe 
non-elaftic ones, which being ftruck, return but a 
fingle found ; and thofe more elaftic, returning a 
fucceffion of found; which uniting together, form 
one tone. This tone may be confidered as a great 
number of founds, all produced after each other 
by the fame body; as we find in a bell, or the 
firing of a harpfichord, which continue to found 
fome time after they are ftruck. A continued 
tone may alfo be produced from a non-elaftic 
body, by repeating the blow quick and often ; as 
when we beat a drum, or draw a bow along a 
fiddle-ftring. 
Regarding the fubjed in this light, if we fiiould 
multi)3ly the number of blows, or repeat them at 
quicker i,ntervals on the fonorous body, it is evi- 
dent tha'c this will have no effeft in altering the 
tone; if will only render it more even, or more 
diftin6t But it will be otherwife if we increafe 
the force of the blow : if we ftrike the body v/ith 
double weight, this will produce a tone twice as 
loud as the former. If, for inftance, a table is 
ftruck with a rod, the found will be very different 
from that produced by ftriking it with a cudgel. 
Hence, therefore, we may infer, that all bodies 
•emit a graver and a louder tone, not in proportion 
to the number of times they are ftruck, but to the 
force that ftrikes them: and if this be true, it is 
evident that thofe philofophers v/ho make the tone 
of a fonorous body to depend only on the number 
of it's vibrations, and not on the force, have mif- 
taken what is only an effecft for a caufe. A bell, 
or an elaftic ftring, can only be confidered as a 
drum beaten; and the frequency of the blows can 
make no alteration v^^hatever in the tones. The 
largeft bells, and the longeft and thickeft ftrings, 
have the moft forcible vibrations; and therefore 
their tones are the loudeft and graveft. 
In order to difcover how founds thus produced 
become harmonious, it muft be obferved, that no 
one continuing tone, however loud and fwelling, 
can give us pleafure: we muft have a fucceffion of 
tones, and thofe in the moft pleafing proportion. 
The nature of this proportion may be conceived 
thus: if we ftrike a body incapable of vibration 
with a double force, or, what amounts to the fame 
thing, with a double mafs of matter, it will pro- 
duce a found that will be doubly grave. Mufic 
has been faid, by the ancients, to have been firft 
invented from the blows of different fized hammers 
on an anvil: fuppofe then we ftrike an anvil with 
a hammer of one pound weight, and again with 
one of two pounds, it is plain that the latter will 
produce a Ibund twice as grave as the former; 
But if we ftrike with a two-pound hammer, and 
then with a three, it is evident that the latter will 
produce a found one-third more grave than the 
former ; and if we ftrike an anvil with a three- 
pound hammer, and then with a four, it will like- 
wife follow, that the latter will be a quarter part 
miore grave than the former. Now, on comparing 
thefe founds, it is obvious that the diff'erence be- 
tween one and two is more eafily perceived thari 
between two and three, three and four, or any 
numbers fucceeding in the fame proportion. The 
fucceffion of founds will therefore be pleafing in 
proportion to the eafe with which they may be 
diftinguiflied. That found which is double the 
former, or, in other words, the odave to the pre- 
ceding tone, will of all others be the moft pleaf- 
ing: the next to that, which is at two or three, or, 
in other words, the third, will be moft agreeable i 
and thus univerfally, thofe founds whofe difference 
may be moft eafily compared, are the moft melo- 
dious. The profefi^ors of mufic have therefore 
contented them.felves with feven different propor- 
tions of found, which are called notes, and fuffi- 
ciently anfwer every purpofe of pleafing harmony : 
not but that they might adopt a greater diverfity 
of proportions, and fom.e have aftually done fo; 
but in thefe the differences of the proportions are 
fo imperceptible, that the ear is rather fatigued 
than charmed in making the diftin£tion. How- 
ever, in order to give variety, they have admitted 
half tones: but in all countries where mufic is yet 
imperfeftly underftood, fuch are rejefted, as the 
natives can perceive no mufic but in the obvi6us 
tones. The Chinefe, for inftance, have neither 
flats nor fharps in their mufic ; but the intervals 
between the other notes are in the fame proportion 
with ours. 
Sound, like light, is not only propagated at a 
diftance, but capable of being reflefted; but the 
laws of this refledicn, it muft be confefi^d, arc 
but imperfeftly underftood. Ail we know is, 
that found is principally reflefted when it's aiotion 
is interrupted by hard bodies: a mountain, a 
houfe, or a wall, reflecTcs found, and fometimes in 
fuch perfefcion, that we imagine it proceeds from 
a quite contrary dire£tion to that of it's original 
motion. Smooth concave fubftances, fuch as 
vaults and hollow rocks, increafe the reverbei-a- 
tion, but no art can conftitute an echo; and fome 
who have been at great labour and expence to 
effefl fuch a projeft, have only ereiled fhapeleis 
buildings, whofe filence proved a mortifying lec- 
ture on their prefumptuous folly. 
The internal cavity of the ear, which is fitted 
for the purpofe of echoing found with the greateft 
precifion, is hollowed out of the hard part of the 
temporal bone, like a cavern in a rock. In this 
cavity founds are repeated and articulated: this 
repetition of found excites vibrations in the folid 
parts of the lamina of the cochlea, which are com-' 
municated to the membranous part of the lamina; 
and this membranous part is an expanfion of the 
auditory nerve, which conveys the difierent vibra- 
tions to the niind. As the ofifeous parts are folid 
and 
