A I R 
the fpr'ing and elafticity of the air; as that on which the 
common or water pump is formed, is the gravity of the 
fame air; the one gradually exhaufting the air from a vef- 
fel by means of a pifton, with a proper valve, working in 
a cylindrical barrel or tube; and the other exhaufting wa¬ 
ter in a (imilar manner. 
The air-pump has proved one of the principal means of 
performing philofophical difcoveries, that has been in¬ 
vented by the moderns. The idea of fuch a machine oc¬ 
curred to feveral perfons nearly about the fame time. 
But the firft it feems was completed by Otto Guericke, the 
celebrated conful of Magdeburg, who exhibited his firft 
public experiments with it, before the emperor and the 
ftates of Germany, at the breaking up of the imperial 
diet at Ratifbon, in the year 1654. But it was not till the 
year 1672, that Guericke publiflied a defcription of the 
inftrument, with an account of his experiments, in his 
Experimcnta Nova Magdcburgica de P'acuo Spacia ; though an 
account of them had been publiflied by Schottus in 1657, 
in his Mcchanica Hydraulico Pneumalica. 
Dr. Hook and M. Duhamel afcribe the invention of the 
air-pump to Mr. Boyle. But that great man frankly con- 
feffes that Guericke was beforehand with him in the exe¬ 
cution. Some attempts, he affures us, he had indeed made 
upon the fame foundation, before he knew any thing of 
what had been done abroad : but the information he after¬ 
wards received from the account given by Schottus, ena¬ 
bled him, with the afliftance of Dr. Hook, after two or 
three unfuccefsful trials, to bring his defign to maturity. 
The product of their labours was a new air-pump, much 
more eafy, convenient, and manageable, than the German 
one. And hence, or rather from'the great variety of ex¬ 
periments to which this illuftrious author applied .the ma¬ 
chine, it was afterwards called Machina Eoyliana, and the 
vacuum produced by it Vacuuvi Boylianum. 
Molt of the air-pumps that were firft made, confided 
of only one barrel, or hollow cylinder, of brafs, with a 
valve at the bottom, opening inwards; and a moveable 
embolus or pifton, having likevvife a valve opening up¬ 
wards, and fo exadtly fitted to the barrel, that when it is 
drawn up from the bottom, by means of an indented iron 
rod or rack, and a handle turning a fmall indented wheel, 
playing in the teeth of that rod, all the air will be drawn 
up from the cavity of the barrel; there is alfo a fmall 
pine opening into the bottom of the barrel, by means of 
which it communicates with any proper vefiel to be ex¬ 
haufted of air, which is called a receiver, from its office 
in receiving the fubjefts upon which experiments are to 
be made in vacuo : the whole being fixed in a convenient 
frame of wood-work, where the end of the pipe turns up 
into a horizontal plate, upon which the receiver is placed, 
juft over that end of the pipe. 
The other parts of this machine, being only accidental 
circumftances, chiefly refpedfing conveniency, have been 
diverfified and improved from time to time, according to 
the addrefs and feveral views of the makers. That of 
Otto Guericke was very rude and inconvenient, requiring 
the labour of two ftrong men, for more than two hours, to 
extraft the air from a giafs, which was alfo placed under 
water; and yet allowed of no change of fubjefts for ex¬ 
periments. 
Mr. Boyle, from time to time, removed feveral of thefe 
inconveniences, and leflened others; but ftill the working 
of his pump, which had but one barrel, was laborious, 
by reafon of the prelfure of the atmofphere, a great part 
of which was to be removed at every lift of the pifton, 
when the exhauftion was nearly completed. Various im¬ 
provements were fucceflively made in the machine by the 
philofophers about that time, and foon after, who culti¬ 
vated this new and important branch of pneumatics; as 
Papin, Merfenne, Mariotte, and others. But ftill they 
laboured under a difficulty of working them, from the 
circumftance of the Angle barrel, till Papin, in his farther 
improvements of the air-pump, removed that inconveni¬ 
ence, by the ufe of a fecond barrel.or pifton, contrived to 
AIR 223 
rife as the other fell, and to fall as it rofe; by which, and 
the great improvements made by Mr. Haiikfbee, the pref- 
fure of the atmofphere on the defcending pifton always 
nearly balanced that of the afcending one; fo that the 
winqh, which worked them up and down, was ealily mo¬ 
ved by a very gentle force with one hand : and, befides 
this, the exhauftion was hereby made in lefs than half the 
time. 
Some of the Germans, and others likewife, made im¬ 
provements in the air-pump, and contrived it to perform 
the counter office of a condenfer, in order to examine the 
properties of the air depending on its condenfatiort. 
Mr. Boyle contrived a mercurial gauge or index to the 
air-pump, which is defcribed in his firft and fecond Phy- 
fico-Mechanical Continuations, for meafuring the degrees 
of the air’s rarefaction in the receiver. This gauge is fi- 
milar to the barometer, being a long giafs tube, having 
its lower end immerfed in an open bafon of quicklilver, 
but its other end, which was open alfo, communicating 
with the receiver; which, being exhaufted, this tube is 
equally exhaufted of air at the fame time, and the external 
air preftes the quicklilver up into the tube, to a height 
proportioned to the degree of exhauftion. 
Mr. Vream, an ingenious pneumatic operator, made an 
improvement in Hauklbee’s air-pump, by reducing the 
alternate up-and-down motion of the hand and which to 
a circular one. In his method, the winch is turned quite 
round, and yet the piftons are alternately raifed and de- 
prelfed; by which the trouble of fhifting the hand back¬ 
wards and forwards, as well as the lofs of time, and the 
fliaking of the pump, are prevented. 
The air-pump, thus improved, is reprefented in the an¬ 
nexed Plate, fig. 1. where A,,is the receiver to be exhauft-' 
ed, ground truly level at the bottom, fet over a hole in 
the top of the pump, from which defcends the bent pipe 
B, to the ciftern C, with which the two barrels D D, 
communicate, in which the piftons are worked by a tooth¬ 
ed wheel, by turning the handle E; by which the racks 
F F, with the piftons, are worked alternately up and down. 
G G, is the gauge-tube, immerfed in a bafon of quicklil¬ 
ver H, at bottom, and communicating with the receiver 
at top; from which however it may be occafionally difen- 
gaged, by turning a cock. And 1 , is another cock, by 
turning.of which, the air is again let into the exhaufted 
receiver; into which it is heard to rufh with a confiderable 
hiding noife. 
Notwithftanding the great excellency of Mr. Haukfbee’s 
air-pump, it was ftill fubjeft to inconveniences, from 
which it was in a great meafure relieved by fome contri- 
vances of Mr. Smeaton, which are defcribed at large in 
the Philof. Tranf. for the year 1752. The principal im¬ 
provements fuggefted by Mr. Smeaton, relate to the guage,, 
the valves of the pifton, and the pifton going clofer down 
to the bottom of the barrel; for his pump has only one. 
By the laft of thefe, the air was extracted more perfectly 
at each ftroke. By the fecond, he remedied an inconve¬ 
nience arifing from the valve-hole of the pifton being too 
wide properly to fupport the bladder-valve which cover¬ 
ed it: inltead of the ufual circular orifice. Mr. Smeaton 
perforated the pifton with feven fmall and equal hexagonal 
holes, one in the centre, and the other fix around, form*, 
ing together the appearance of a tranfverfe fection of a 
honey-comb ; the bars or divilions between which, ferved 
to fupport the preflure of the air on the valve. His gauge 
confifts of a bulb of giafs, of a pear-like fhape',_ and ca 
pable of holding about half a pound of quicklilver : it is^ 
open at the lower end, the other terminating in a tube 
hermetically fealed ; and it has annexed to it a fcale, divi¬ 
ded into parts of about one-tenth of an inch, and auf- 
wering to the 1000th part of the whole capacity. Du¬ 
ring the exhauftion of the receiver, the gauge is fufpend- 
ed in it by a wire; but when the pump has been worked 
as much as necelTary, the gauge is pufhed down, till the 
open end be immerfed in a bafon of quicklilver placed 
underneath. The air is then let into the receiver again, 
3 and. 
