ACOUSTICS. 
89 
found can either increafe or diminish its gravity, in any 
degree whatever. 
The fubjeCt next to be confidered in the order of the 
prefent work, is the propagation of found; but, as the 
doctrine of Sir Ifaac Newton is very generally known and 
acknowledged, we refer to the clear, explanatory, and 
able, defence of his 47th propoiition, by Mr. Young, of 
Trinity College, Dublin. 
Of the Velocity, &c. of Sound. 
Experience has taught us, that found travels at about 
tire rate of 1142 feet in a fecond, or near 13 miles in a mi¬ 
nute ; nor do any obftacles hinder its progrefs, a contrary 
wind only a fmall matter diminifhing its velocity. The 
method of calculating its progrefs is eafily underftood. 
When a gun is difcharged at a difiance, we fee the fire 
long before we hear the found. If then we know the 
diitance of the place, and know the time of the interval 
between our firft feeing the fire and hearing the report, 
it will Ihew us exaCtly the time the found has been in tra¬ 
velling to us. For infiance, if the gun is difcharged a 
mile oft, the moment the flafii is feen, you take a watch 
and count the feconds till you hear the found; the number 
of feconds is the time the found has been travelling a mile. 
Again, by the above axiom, we are enabled to find the 
difiance between objects that would be otherwife immea- 
furable. For example, fuppofe you fee the flafii of a 
gun in the night at fea, and tell fe.ven feconds before you 
hear the report, it follows therefore, that the difiance is 
feven times 1142 feet, that is, 24 yards more than a mile 
and a half. In like manner, if you obferve the number 
of feconds between the lightning and the report of the 
thunder, you know the difiance of the cloud from whence 
it proceeds. 
Derham has proved, by experiment, that all founds 
whatever travel at the fame rate. The found of a gun, 
and the firiking of a hammer, are equally fwift in their 
motions; the fofteft w'hifper flies as fvviftly, as far as it 
goes, as the loudeft thunder. 
To thefe axioms v r e may add the following:—Smooth 
and clear founds proceed from bodies that are homoge¬ 
neous, and of an uniform figure ; and harlh or obtufe 
founds, from fuch as are of a mixed matter and irregular 
figure. 
The velocity of found is to that of a brifk wind as fifty 
to one. 
The firength of founds is greateft in cold and denfe air, 
and leaft in that which is warm and rarefied. 
Every point againft which the pulfes of found ftrike, 
becomes a centre from which a new feries of pulfes are 
propagated in every direction. 
Sound defcribes equal fpaces in equal times. 
Of Reverberated Sounds. 
Sound, like light, after it has been reflected from feve- 
pets is accounted for in the following Manner: When 
a perfon fpeaks through the trumpet, the whole force of 
his voice is fpent upon the air contained in the tube, 
which will be agitated through the whole length of the 
tube; and, by various reflections from the tide of'the 
tube to the axis, the air along the middle part of the tube 
will be greatly condenfed, and its momentum proportiona- 
bly increafed, fo that when it comes to agitate tire air at 
the orifice of the tube, its force will be as much greater 
than what it would have been without the tube, as the 
furface of a fphere, whofe radius is equal to the length 
of the tube, is greater than the furface of the fegment of 
fuch a fphere whofe bafe is the orifice of rhe tube. 
But it is obvious, notwithftanding, that the confine¬ 
ment of the voice can have little effeCt in increafing the 
firength of found, as this firength depends on the veloci¬ 
ty with which the particles move. Were this reafoningcon- 
clufive, the voice fiiould iffue through the fmalleft pofii- 
ble orifice ; cylindrical tubes would be preferable to any 
that increafed in diameter; and the lefs the diameter, the 
greater would be the efleet of the inftrument; becaufe the 
plate or riiafs of air to be moved, would, in that cafe, be 
lefs, and confequently the effeCt of the voice the greater ; 
all which is contradicted by experience.. 
The caufe of the increafe of found in thefe tubes mufi 
therefore be derived from fome other principles : amongft 
thefe we fnall probably find, that what the ingenious Kir- 
cher has fuggefted in his Phonurgia is the molt deferving of 
our attention. Fie tells us, that “ the augmentation of the 
found depends on its reflection from the tremulous tides 
of the tube ; which reflections, eonfpiring in propagating 
the pulfes in the fame direction, mufi increafe its intenfr- 
ty.” Newton alfo feems to have confidered this as the 
principal caufe, in the Scholium of Prop. 50. B. 2. Prin- 
cip. when he fays, “ We hence fee why founds are fo 
much increafed in ftentorophonic tubes, for every reci¬ 
procal motion is, in each return, increafed by the gene¬ 
rating caufe.” 
Farther, when we fpeak. in the open air, the effeCt on 
the tympanum of a diftant auditor is produced merely by a 
fingle pulfe. But when we ufe a tube, all the pulfes pro¬ 
pagated from the mouth, except thofe in the direction of 
the axis, ftrike againft the tides of the tube, and every 
point of impulfe becoming a new centre, fronw hence the 
pulfes are propagated in all directions, a pulfe wall arrive 
at the ear from each of thofe points: thus, by the ufe of a 
tube, a greater number of pulfes are propagated to the 
ear, and confequently the found increafed. The confine¬ 
ment too of the voice may have an effeCt, though not fuch 
as is aferibed to it by fome ; for the condenfed pulfes pro¬ 
duced by the naked voice freely expand every way ; but, 
in tubes, the lateral expanfion being diminifhed, the di- 
reCt expanfion will be increafed, and confequently the ve¬ 
locity of the particles, and the intenfity of the found. 
The fhbftance alfo of the tube has its effect ; for it is found 
