MECHANIC S. 
takes for granted, that the clouds move as fad as the 
•wind. It is probable that the velocity is fomething more 
than what is here dated. 
Sound is a fenfation excited by the vibrations of the 
air upon the tympanum or drum of the ear. That the 
air is the instrument by which found is conveyed from 
the fonorous body, is manifeft from hence, that no found 
can be produced if the body be in a vacuum, or if there 
be a vacuum between the body and the ear. 
By percuffion, the parts of a fonorous body, as a bell, 
a mufical itring, &c. are put into a date of vibration ; and, 
as long as the vibrations are continued, correfponding vi¬ 
brations are communicated to the air; and found is heard 
as long as the vibrations are drong enough to produce 
the fenfation. All fonorous bodies are therefore elaftic. 
The manner in which the vibrations are excited in the 
air is fo clearly deferibed by Mr. Cotes, that we cannot 
do better than give the account in his own words. 
“ The parts of the fonorous body, being put into a tre¬ 
mulous and vibrating motion, are by turns moved for¬ 
wards and backwards. Now, as they go forwards, they 
mud of neceffity prefs upon the parts of the air to which 
they are contiguous, and force them alfo to move for¬ 
wards in the fame direction with themfelves; and confo- 
quently thofe contiguous parts will at that time be con- 
denfed ; then, as the parts of the fonorous body return 
back again, the parts of the air which were jud before 
condenfed will be permitted to return with them, and by 
returning they will again expand themfelves. It is mani- 
fed, therefore, that the contiguous parts of the air will 
go forwards and backwards by turns, and be fubjeCt to 
the like vibrating motion with the part of the fonorous 
body. And, as the fonorous body produces a vibrating 
motion in the contiguous parts of the air, fo will thefe 
parts, thus agitated, in like manner produce a vibrating 
motion in the next parts, and thofe in the next, and fo 
on continually. And as the fird parts were condenfed in 
their progrefs, and relaxed in their regrefs, fo will the 
other parts, as often as they go forwards, be condenfed, 
and, as often as they go backwards, be relaxed. And 
therefore they will not all go forwards together, and all 
go backwards together; for then their refpeitive didances 
would always be the fame, and confequently they could 
not be rarefied and condenfed by turns; but, meeting 
each other when they are condenfed, and going froin each 
other when they are rarefied, they mull neceffarily one 
part of them go forwards whilft the other goes backwards, 
by alternate changes from the firft to the laft. Now the 
parts which go forwards, and by going-forwards are con¬ 
denfed, conrtitute thofe pulfes which ltrike upon our or¬ 
gans of hearing, and other objeCts they meet with ; and 
therefore a fucceffion of pulfes will be propagated from 
the fonorous body. And, becaufe the vibrations of the 
fonorous body follow each other at equal intervals of 
time, the pulfes which are excited by thofe feveral vi¬ 
brations, will alfo i'ucceed each other at the fame equal 
intervals.” 
As, when a fluid is put in motion, that motion is com¬ 
municated in all directions, fo found muft be propagated 
in all directions from a fonorous body as a centre, in con¬ 
centric iuperficies, or ihells of air, called aerial pulfes , or 
waves of air, analogous, as fuppofod by fome, to the cir¬ 
cular waves produced on the furface of water when a ftone 
is thrown in. If the found be impeded by a body which 
has a hole, the waves pafs through, and diverge from it 
as a new centre, and the found is heard on all parts on 
the other fide of the body. 
The law by which the force of found decreafes as you 
recede from the fonorous body, is not eafy to be deter¬ 
mined by theory. It has been ufually eftitnated, by di¬ 
viding the furrounding air into (hells of an equal thick- 
nefs, and fuppofing thefe fhells to aCt upon each other, 
as fo many elaftic bodies would ; but it is probable, that 
this is a fuppofiiion very far diftant from the truth. The 
759 
utmoft diftance at which a found has been heard, is about 
200 miles. This was obferved in the war between Eng¬ 
land and Holland, in the year 1672. The unafiiftcd hu¬ 
man voice has been heard from Old to New Gibraltar, 
a diftance .of 10 or 12 miles; the watch-word, All's well, 
given at the latter, in a fiill night, having been heard at 
the former. In both thefe cafes, the found pafied over 
the water; and it is found, that found will always be 
conveyed much further along a fmooth than a rough 
furface. 
The velocity of found, produced by all bodies, is found 
by experiment to be 1342 feet in a fecond, fubjeCt to a 
fmall variation from the courfe of the wind. Dr. Derham 
determined this very accurately, by placing cannon at 
different diflances, and firing them, and obierving the 
interval between the flafh and the report. And thus he 
alfo found that found (or rather the pulfes of air which 
excite it) moves uniformly; it being always found, that 
the interval was in proportion to the diftance. Sir Ifaac 
Newton determined the velocity of found by theory; with 
which if the reader with to be acquainted, he may con- 
fult the Principia, lib. ii. Prop. 47. See alfo our article 
Acoustics, vol. i. p. 85-90. 
Having bad an opportunity, when refident at Ramfgate, 
(fays Mr. Vince,) to fee the flafh of the evening-gun at 
the Downs, and to hear the report, I made a great many 
obfervations on the interval between the flafh and the 
report, in order to difoover how much the velocity of 
found varied in different directions of the wind. When 
there was no fonfible wind, the interval was 32" of time. 
When a very ftrong wind, amounting to a hurricane, 
blew' direttly from the Downs to Ramfgate, the interval 
was 30"; and, when it blew in the oppolite direction, it 
was 34". It appears, therefore, that a very ftrong wind 
will increafe or diminifli the velocity of found by fa part 
of its mean velocity, according as the direction of the 
wind coincides with oroppofes that of the found. When 
the wind blew at right angles to the direction of the 
found, however ftrong the wind might be, it did not 
appear to produce any fonfible efteCt upon the velocity 
of the found. 
Sound is conveyed to the greateft diftance by a trum¬ 
pet, called a fpeaking or itentorophonic trumpet, the 
form of which is like that figure which would be gene¬ 
rated by any part of the logarithmic curve revolving 
about its axis, the mouth being applied the fmalier end. 
The fame found is always excited, when the air is put 
into the fame fate of vibration ; that is, if a bell and 
rnufical Itring make the air vibrate the fame number of 
times in a focond, they excite the fame tone. And, as 
the fame fonorous body performs all its vibrations, whe¬ 
ther greater or lefs, in the fame time, the fame body will 
always give the fame tone, whether the percuffive Itroke 
be greater or lefs. The flower the vibration, the deeper 
or graver is the tone. But we mean not here to enter 
into the inveftigation of the times of vibration of mufical 
ftrings. If the reader wifh for any information upon the 
fubjeCt, we refer him to Mr. Parkinfon’s Hydroftatics, or 
Dr. Smith’s Harmonics. 
The reflection of the vibrations of the air from any- 
fixed objeCl to the ear, will caufe a found diftinCt from 
that which is caufod by the vibrations coming riireCtly to 
the ear; and this is called an echo. If the diitance of the 
objeCt which returns the echo be great, it will return fe¬ 
veral fyllables. A tingle fyllable will not be clearly re¬ 
turned, unlefs the diitance of the objeCt be at iealt 120 
feet; and fo in proportion. Hence, an echo returning 
ten fyllables muft come from an objeCt 1200 feet diftant. 
More fyllables, however, will be returned by night than 
by day, becaufe the air, being then colder, is denfor, in 
which cafe the return of the vibrations become (lower, 
and confequently more fyllables may be heard. 
Vapours are raifod from the furface of the water; the 
principal caufe of which is, probably, the heat of the fun, 
tbs 
