Encyclopaedia Britannica > Volume 15, PLA-RAM

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P N E U M
One pint of water was 8TV ounces.
of air
irf*r veiT
ate
:cn.
64
Kj
imp. weigh 945 grains.
This gives for the fpecific gravity
3 nesince we are thus immerfed in a gravitating fluid, it
on follows, that every body preponderates only with the ex-
■eight cefs of its own weight above that of the air which it
lies difplaces ; for every body lofes by this immerfion the
:rfcd weight of the difplaced air. A cubic foot lofes about
521 grains in frofty weather. We fee balloons even
rife in the air, as a piece of cork rifes in water. A
mafs of water which really contains 850 pounds will
load the fcale of a balance with 849 only, and will be
balanced by about 849t pounds of brafs. This is
evinced by a very pretty experiment, rejprefented iu
fig. 31. A fmall beam is fufpended within a receiver.
To one end of the beam is appended a thin glafs or
copper ball, clofe in every part. This is balanced by
a fmall piece of lead hung on the other arm.. As the
air is pumped out of the receiver, the ball will gradu¬
ally preponderate, and will Regain its equilibrium when
the air is re-admitted.
Some naturalifts have propofed, and actually ufed, a
large globe of light make, fufpended at a beam, for a
barometer. If its capacity is a cubic foot, lTV grains
will indicate the fame change that is indicated by A,
of an inch . of an ordinary barometer.’ But a veffel of
this fize will load a balance too much to leave it fuffr-
ciently fenfible to fmall changes of denlity. Befides,
it is affected by heat and cold, and would require a
very troublefome equation to cor re ft their effects.
It may perhaps be worth while to attend to this in
buying and felling precious commodities ; fuch as pearls,
diamonds, iilk, and fome drugs. As they are generally
fold by brafs or leaden weights, the buyer will have
fome advantage when the air is heavy and the barome¬
ter high. On the other hand, he will have the advan¬
tage in buying gold and mercury when the air is light.
It is needkfs to coniine this obfervation to precious
commodities, for the advantage is the fame in all in
proportion to their levity."
There is a cafe in which this obfervation is of confe-
quence to the philofopher: we mean the meafuring of
time by pendulums. As the accelerating force on a
pendulum is not its whole weight, but the excefs of its
weight over that of the difplaced air, it follows that a
pendulum will vibrate more llowly in the air than in va¬
cuo. A pendulum compofed of lead, iron, and brafs,
may be about 8400 times heavier than the air which
it difplaces when the barometer is at 30 inches and
the thermometer at 32°, and the accelerating force will
be diminilhed about Ty|oo* This will caufe a fecond
pendulum to make about live vibrations lefs in a day
than it would do in vacuo. In order therefore to de¬
duce the accelerative power of gravity from the length
of a pendulum vibrating in the air, we mult, make an
allowance of o ', 17, or AV °f a fecond, per day for
every inch that the barometer Hands lower than 30
inches. But we muff alfo note the temperature of the
air; becaufe when the air is warm it is lefs denfe when
lupporting by its elafticity the fame weight of atmo-
fphere, and we muff know how much its denfity is di-
minifhed by an increafe of temperature. The correftion
is ftill more complicated ; for the change of denfity af-
fe^ts the reflffance of the air, and this affefts the time
ol the vibration,, and this by a law that is not yet well
ift6
A T I C S. in
afeertained. As far as we can determine from any ex- Air rump.
periments that have been made, it appears that the y
change arifing from the altered refiftance takes off about
of the change produced by the altered denfity, and
that a fecond pendulum makes but three vibrations a-
day more in vacuo than in the open air. This is a very
unexpefted refult; but it muff be owned that the expe¬
riments have neither been numerous nor very nicely
made.
The air-pump alfo allows us to Ihow the effefts of the
air’s preffure in a great number of amuflng and inftruc-
tive phenomena. _ 167
When the air is abftrafted from the receiver, it is Experi-
ftrongly preffed to the pump-plate by the incumbent
atmofphere, and it fupports this great preffure in con- efpc(^s 0c
fequence of its circular form. Being equally compreffed the air’s
on all fides, there is no place where it fhould give way preflure,
rather than another p but if it be thin, and not very
round, which is fometimes the cafe, it will be cruftied
to pieces. If we take a fquare thin phial, and apply
an exhaufting fyringe to its mouth, it will not fail be¬
ing cruihed.
As the operation of pumping is fomething like
fucking, many of thefe phenomena are in common dif-
courfe aferibed to fuftion, a word much abufed ; and
this abufe mifleads the mind exceedingly in its contem¬
plation of natural phenomena. Nothing is more ufual
than to fpeak of the fuftion of a fyringe, the fuftion
and draught of a chimney, &c. The following expe¬
riment puts the true caufe of the ftrong adheiion of the
receiver beyond a doubt.
Place a fmall receiver or cupping-glafs on the pump-
plate without covering the central hole, as reprefented piate
in fig. 32. and cover it with a larger receiver. Exhauft CCcCUl
the air from it; then admit it as fuddenly as poffible.
The outer receiver, which after the rarefaftion adhered
ftrongly to the plate, is now loofe, and the cupping-
glafs will be found {ticking faff to it. While the rare¬
faftion was going on^ the air in the fmall receiver alfo
expanded, efcaped from it, and was abffrafted by the
pump. When the external air was fuddenly admitted,
it preffed on the fmali receiver^ and forced it down to
the plate, and thus {hut up all entry. The fmall re¬
ceiver muff now adhere; and there can be no fuftion,
for the pipe of the pump was on the out fide of the
cuppiug-glafs.
This experiment fometimes does not fucceed, becaufe
the air fometimes finds a paffage under the brim of the
cupping-glafs. ■ But if the cupping-glafs be preffed
down by the hand on the greafy leather or plate, every
thing will be made fmooth, and the glafs will be fo little
raifed by the txpanfion of its air during the pumping,
that it will inffantly clap clofe when the air is re-ad- :~
mitted.
In like manner, if a thin fquare phial be fnrniffied 16&
with a valve, opening from within, but {hutting when
preffed from without, and if this phial be put uiider a
receiver, and the air be abftrafted from the receiver, the
air in the phial will expand during the rarefaftion, will
efcape through the valve, and be at laft in a very rare¬
fied Hate within the phial- If the air be now admitted
into the receiver, it will prefs on the flat fides of the
included phial and crufh it to pieces. See fig. 33.
If a piece of wet ox-bladder be laid over the top of
a receiver whofe orifice is about four inches wide, and
5 tire