Encyclopaedia Britannica > Volume 15, PLA-RAM

5)8
ioi
Generally
adopted.
p N E
A^ump. the top of the barrel, and the air below K is equally
^ rare with that in the receiver. Theteiore the preffure
of the external air on the piiton K is nearly equal to
that on the piiton Id. Both, therefore, are aft.ng m
eppofite directions on the wheel which gave them mo¬
tion ; and the force neceflary for railing H is only
the difference between the elafticxty of the air m the
barrel H and that of the air in the barrel K. i his
is very fmall in the beginning of the ftroke, but
gradually increales as the pilton K defeends, and
becomes equal to the whole excels of the air’s preflure
above the elafticity of the remaining air of the receiver
when the air at K of the natural denfity begins to
open the pifton valves. An accurate attention to the
circumftances will (how us that the force requisite for
working the purnp is greateft at firff, and gradually
dirainifties as the rarefaction advances; and when this
is nearly complete, hardly any more force is required
than what is neceffary for overcoming the fridtion of
the piftons, except during the diicharge of the air at
the end of each ffroke. # .
This is therefore the form of the air-pump which is
moft generally ufed all over Europe, borne traces of
national prepoffefiion remain. In Germany, air-pumps
are frequently made after the original model of Gue¬
ricke’s (Wolff Cyclomathefis) ; and the French gene¬
rally ufe the pump made by Fapin, though extremely
aukward. We fhall give a defcription of Boyle’s air-
pump as finally improved by Hawkefbee, which, with
fome fmall accommodations to particular views, Hill
remains the moft approved form.
Here follows the defcription from Defaguliers.
It confifts of two brafs barrels a a (fig. 19.), 12
provements jnc^es an(l 2 wide. The piftons are railed and de-
rrlu u Puffed by turning the winch b l. This is faftened to an
L ~ axis paffing through a ftrong toothed wheel, which lays
hold of the teeth of the racks cccc. Then the one
is railed while the other is depreffed ; by which means
the valves, which are made of limber bladder, fixed in
the upper part of each pifton, as well as in the open¬
ings into the bottom of the barrels, perform their office
of difeharging the air from the barrels, and admitting
into them the air from the receiver to be afterwards dif-
eharged; and when the receiver comes to be pretty
well exhaufted of its air, the preffure of the atmofphere
in the defeending pifton is nearly fo great, that the
power required to raife the other is little more than is
neceffary for overcoming the fndiion of the pifton,
which renders this pump preferable to all others, which
require more force to work them as the rarefaftion of
the air in the receiver advances.
The barrels are fet in a brafs difh about two inches
deep, filled with water or oil to prevent the infinuation
of air. The barrels are ferewed tight down by the nuts
ee>ee, which force the frontifpiece ff down on them,
through which the two pillars ggygg pafs.
Br»fs° Ve Frora between the barrels rifes a fiender brafs pipe h h>
&.c!b 1 ’ communicating with each by a perforation in the tranf-
verfe piece of brafs on which they ftand. The upper end
of this pipe communicates with another perforated piece
of brafs, which ferews on underneath the plate i i i i, of
ten inches diameter, and furrounded with a brafs rim
to prevent the fhedding of water ufed in fome experi¬
ments. This piece of brafs has three branches: ift,
An horizontal one communicating with the conduit-
U M A T I C S.
ICi
Hawkef-
hec’s im-
103
Barrels.
10J
Stopco
pipe h h. 2. An upright one ferewed into' the middle A!r.pw
of the pump-plate, and terminating in a fmall pipe k, —
riling about an inch above it. 3d, Is a perpendicular
one, looking downwards in the continuation of the
pipe k, and having a.hollow ferew in its end receiving
the brafs cap of the gage-pipe ////, which is of glafs,
34 inches long, and immerfed in a glafs cittern m m
filled with mercury. This is covered a-top with a cork
float, carrying the weight of a light wooden fcafe di¬
vided into inches, which are numbered from the furface
of the mercury in the cittern. This fcale will there¬
fore rife and fall with the mercury in the cittern, and
indicate the true elevation of that in the tube.
There is a ftopcock immediately above the infertion
of the gage-pipe, by which its communication may be
cut off. There is another at », by which a communi¬
cation is opened with the external air for allowing its
readmiffion; and there is fometimes another immediately
within the infertion of the conduct-pipe for cutting off
the communication between the receiver and the pump.
This is particularly ufeful when the rarefaction is
to be continued long, as there are by thefe means
fewer chances of the infinuation of air by the many
joints. . 10
The receivers are made tight by limply fetting them Recti'
on the pump-plate with a piece of wet or oiled leather
between ; and the receivers, which are open a-top, have
a brafs cover fet on them in the fame manner. In
thefe covers there are various perforations and contri¬
vances for various purpofes. The one in the figure has
a flip wive paffing through a collar of oiled leather, ha¬
ving a hook or a ferew in its lower end for hanging any
thing on or producing a variety of motions. k
Sometimes the receivers are fet on another plate, which Conti
has a pipe ferewed into its middle, furnifhed with avi"“
ftopcock and a ferew, which fits the middle pipe Lths
When the rarefaClion has been made in it, the cock is
fhut, and then the whole may be unferewed from the
pump, and removed to any convenient place. This is
called a tran [porter plate.
It only remains to explain the gage IIIL In the^
ordinary ttate of the air its elafticity balances the pref- ,(,e{
fure of the incumbent atmofphere. We find this from coni
the force that is neceffary to fqueeze it into lefs bulked,
in oppofition to this elafticity. Therefore the elaftici¬
ty of the air increafes with the vicinity of its particles.
It is therefore reafonable to expedl, that when we al¬
low it to occupy more room, and its particles are far¬
ther afunder, its elafticity will be diminiftied though
not annihilated; that is, it will no longer balance the
whole preffure of the atmofphere, though it may ftill
balance part of it. If therefore an upright pipe have
its lower end immerfed in a veffel of mercury, and com¬
municate by its upper end with a veffel containing rare¬
fied, therefore lefs elaftic, air, we ftiould expedl that the
preffure of the air will prevail, and force the mercury
into the tube, and caufe it to rife to fuch an height that
the weight of the mercury, joined to the elafticity of the
rarefied air adding on its upper furface, lhall be exadtly
equal to the whole preffure of the atmofphere. The
height of the mercury is the exadd meafure of that part
of the whole preffure which is not balanced by the ela¬
fticity of the rarefied air, and its deficiency from the
height of the mercury in the Toricellian tube is the
exadd meafure of this remaining elafticity.