A I R 
performed with it, was publiftied at Amfterdam in the 
year 1787 ; from which experiments it appears that, by a 
coincidence of the feveral gauges, a rarefadlion of 1200 
times was fhewn; but, when the atmofphere was very dry, 
the exhauftion has been fo complete, that tire gauges have 
fhewn the air in the receiver to be rarefied above 2400 
times. 
Fig. 2, in the annexed Plate, is a perfpedtive view of 
Mr. Cuthbertfon’s air-pump, with its twp principal gau¬ 
ges fcrewed into their places. Thefe need not be ufed 
together, except in cafes where the utmoft exaftnefs is 
required. In common experiments one of them is remo¬ 
ved, and allop-fcrew put in its place. Fig. 3, reprefents 
a crofs-bar, for preventing the barrels from being lhaken 
in working the pump, or by any accident. Its proper 
place in the pump is fhewn ' y the dotted lines; where it 
is confined, and kept clofe down on the barrels, by two 
flips of wood A A, which murt be drawn out, as well_as 
the fcrews B B, when the pump is to be taken afunder. 
There are made alfo, by different perfons, portable or 
fmall air-pumps, of various conftructions, tofet upon a 
table, to perform experiments with, as reprefented in the 
Plate,4. In thefe, the gauge is varied according to 
the fancy of the maker, but commonly it confifts of a bent 
glafs tube, like a fyphoii, open only at one end. The 
gauge is placed under a fmall receiver communicating, by 
a pipe, with the principal pipe leading from the general 
receiver to the barrels. The clofe end of the gauge, of 
three or four inches long, before the exhauftion, lias the 
quickfilver forced clofe up to the top by the preffure of 
the air on the open end; but, when the exhauftion is con- 
fiderably advanced, it begins to defcend, and then the 
difference of the heights of the quickfilver in the two 
legs, compared with the height in the barometical tube, 
determines the degree of exhauftion. So if the difference 
between the two be one inch, when the barometer (lands 
at 30°, the 'air is rarefied thirty times; but, if the diffe¬ 
rence be only half an inch, the rarefaction is fixty times, 
and fo on. 
In whatever manner or form this machine be made, the 
ufe and operation of it are always the fame. The handle, 
which works the pifton, is moved up and down in the bar¬ 
rel, by which means a barrel of the con-ained air is drawn 
out at every ftroke of the pifton, in the following manner : 
by pufhing the pifton down to the bottom of the barrel, 
where the air is prevented from efcaping downwards, by 
its elafticity it opens the valve of the pifton, and efcapes 
upwards above it into the open air; then raifing the pif¬ 
ton up, the external atmofphere (huts down its valve, and 
a vacuum would be made below it, but for the air in the 
receiver, pipe, See. which now raifes the valve in the bot¬ 
tom of the barrel, and ruffes in and fills it again, till the 
whole air in the receiver and barrel be of one uniform 
deniity, but lefs than it was before the ftroke, in propor¬ 
tion as the fum of all the capacities of the receiver, pipe, 
and barrel, together, is to the fame fum wanting the bar¬ 
rel. And thus is the air in the receiver diminiftied at each 
ftroke of the pifton, by the quantity of the barrel or cy¬ 
linder full, and therefore always in the fame proportion : 
fo that, by thus repeating the operation again and again, 
the air is rarefied to any prop.ofed degree, or till it has not 
elafticity enough to open the valve of the pifton or of the 
barrel, after which the exhauftion cannot be any farther 
carried on : the gauge, in comparifon witli the barometer, 
(hewing at any time what the degree of exhauftion is, ac¬ 
cording to the particular nature and conftruftion.of it. 
But, fuppofing no vapour from moifture, &c. to rife in 
the receiver, the degree of exhauftion, after any number 
of ftrokes of the pifton, may be determined by knowing 
the refpedtive capacities of the barrel and the receiver, 
including the pipe, &c. For as we have feen above that 
every ftroke diminiflies the denfity in a conftant propor¬ 
tion, namely, as much as the whole content exceeds that 
of the cylinder or barrel; and confequently the fum of as 
many diminutions as there are ftrokes of the pifton, will 
Vol. I. No. 15. 
A I R 225 
fhew tire whole diminution by all the ftrokes. So, if the 
capacity of the barrel be equal to that of the receiver, in 
which the communication-pipe is always to be included; 
then, the barrel being half the fum of the whole contents, 
half the air will be drawn out at one ftroke; and confe¬ 
quently the remaining half, being dilated through the 
whole or firft capacity, will be of only half the denfity of 
the firft : in like manner, after the fecond ftroke, the denfi¬ 
ty of the remaining contents will be only half of that after 
the firft ftroke, that is, only one-fourth of the original 
denfity; continuing this operation, it follows that the den¬ 
fity of the remaining air will be one-eighth after three 
ftrokes of the pifton, one-fixteenth after four ftrokes, ^ 
after five ftrokes, and fo on, according to the powers of 
the ratio ^; that is, fuch power of the ratio as is denoted, 
by the number of the ftrokes. 
Some of the principal effetts and phenomena of the air- 
pump, are the following:—That, in the exhaufted recei¬ 
ver, heavy and light bodies fall equally fwift; fo, a gui¬ 
nea and feather fall from the top of a tall receiver to the 
bottom exaftly together. That moft animals die in a mi¬ 
nute or two : but, however, That vipers and frogs, though 
they fwell much, live an hour or two; and, after being 
feemingly quite dead, come to life again in tire open air. 
That fnails furvive about ten hours; efts, or flow-worms, 
two or three days ; and leeches five or fix. That oyfters 
live for twenty-four hours. That the heart of an eel, ta¬ 
ken out of the body, continues to beat for good part of 
an hour, and that more brifkly than in the air. That 
warm blood, milk, gall, &c. undergo a confiderable intu- 
mefcence and ebullition. That a moufe or other animal 
may be brought, by degrees, to furvive longer in a rarefied 
air than it naturally does. That air may retain its ufual 
preffure, after it is become unfit for refpiration. That 
the eggs of (ilk-worms hatch in vacuo. That vegetation 
(lops. That fire extinguifhes; the flame of acandle ufu- 
ally going out in about one minute; and charcoal in about 
five minutes. That red-hot iron, however, feems not to 
affected.; and yet fulphur or gunpowder are not lighted 
by it, but only fufed. That a match, after-lying feem¬ 
ingly extinct a long time, revives again on re-admitting 
the air. That a flint and fteel ftrike fparks of fire as co- 
pioully, and in all directions, as in air; That magnets, 
and magnetic-needles, a6t the fame as in air. That the 
fmoke of an extinguifhed luminary gradually fettles to the 
bottom in a darkifh body, leaving the upper part of the 
receiver clear and tranfparent; and that on inclining the 
veffel fometimes to one fide, and fometimes to another, the 
fume preferves its furface horizontal, alter the nature of 
other fluids. That heat may be produced by attrition. 
That camphire will not take fire; and that gun-powder, 
though fome of the grains of a heap of it be kindled by 
a burning-glafs, will not give fire to the contiguous grains. 
That glow-w orms lofe their light in proportion as the air 
is exhaufted, and at length become totally obfeure; but, 
on re-admitting the air, they prefently recover it all. 
That a bell, on being (truck, is not heard to ring, or very 
faintly. That,water freezes. But that a fyphon will not 
run. That eleftricity appears like the aurora borealis. 
With multitudes of other curious and important particu¬ 
lars, to be met with in the numerous writings on this ma¬ 
chine, namely, befides the Philofophical Tran factions of 
moft Academies and Societies, in the Writings of Torri¬ 
celli, Pafcal, Merfenne, Guericke, Schottus, Boyle, Hook, 
Duhamel, Mariotte, Hauklbee, Hales, Mufchenbroeck, 
Gravefande, Defaguliers, Franklin, Cotes, Helfham, and-a 
great many other authors. See alio the article Pneu ma- 
tics. :. 
Air-shafts, among miners, denote holes or (hafts let 
down from the open air to meet the adits and furnifh freili 
air. The damps, want and impurity of air, which occur, 
when adits are wrought thirty or forty fathoms long, make 
it neceffary to let down air-fhafts, in order to give the air 
liberty to play through the whole work, and thus difeharge 
bad vapours, and furnifli good air for refpiration: the 
3 M expencc 
