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AIR-PUM P.

operator having fixed the lower shank of the stop-cock into the upper orifice of the cylinder, turns the handle, and thus forces the sucker to the top of it, so that no air may be left in its upper part. Then shutting the valve with the plug, and turning the handle the other way, he draws down the sucker to the bottom of the cylinder, and thus its cavity, into which no air By is admitted, will be in an exhausted state. turning the stop-cock, and opening a passage between the cylinder and the reservoir, the air contained in the one will descend into the other; and this air being prevented from returning, by turning back the key of the stop-cock, will be made to open the valve, and to escape into the external air, by forcing the sucker to the top of the cylinder; thus, alternately moving the sucker upwards and downwards, turning the key, and stopping the valve, as occasion requires, the exhaustion may be carried on to any extent.We now pass to

AIR-PUMP, BOYLE'S, AS IMPROVED BY HAWKESBEE. The description of this machine as given by Desaguliers, in vol. ii. of his Exper. Philos. is as follows: It consists of two brass barrels aa, aa, fig. 22, twelve inches high and two wide. The pistons are raised and depressed by turning the winch bb. This is fastened to an axis passing through a strong-toothed wheel, which lays hold of the teeth of the racks cccc, the one being raised, while the other is depressed; by which means the valves, which are made of limber bladder, fixed in the upper part of each piston, as well as in the openings into the bottom of the barrels, perform their office of discharging the air from the barrels, and admitting into them the air from the receiver to be afterwards discharged; and when the receiver becomes pretty well exhausted of its air, the pressure of the atmosphere in the descending piston is nearly so great, that the power required to raise the other is little more than is necessary for overcoming the friction of the piston; this renders the pump preferable to all others, which require more force to work them as the rarefaction of the air in the receiver advances. The barrels are set in a brass dish about two inches deep, filled with water or oil to prevent the insinuation of air. The barrels are screwed tight down by the nuts e e, e e, and thus force the frontispiece ff down on them, through which the two pillars, gg, gg, pass. From between the barrels rises a slender brass pipe hh, communicating with each by a perforation in the transverse piece of brass on which they stand. The upper end of this pipe communicates with another perforated piece of brass, which screws on underneath the plate iiii, of ten inches diameter, and surrounded with a brass rim to prevent the shedding of water used in some experiments. This piece of brass has three branches: first, an horizontal one communicating with the conduit-pipe hh. Second, an upright one screwed into the middle of the pumpplate, and terminating in a small pipe k, rising about an inch above it. Third, a perpendicular one, pointing downwards in the continuation of the pipe k, and having a hollow screw in its end receiving the brass cap of the gage-pipe 111, which is of glass, thirty-four inches long, and VOL. I.

immersed in a glass cistern, mm, filled with mer-
cury. This is covered on the top with a cork
float, carrying the weight of a light wooden scale
divided into inches, which are numbered from
the surface of the mercury in the cistern; and the
scale will therefore rise and fall with the mercury
in the cistern, and indicate the true elevation of
that in the tube.

There is a stop-cock immediately above the in-
sertion of the gauge-pipe, by which its communi-
cation may be cut off; and another at n, by which
a communication is opened with the external air
for allowing its re-admission; there is sometimes
also another, immediately within the insertion of
the conduit-pipe, for cutting off the communica-
tion between the receiver and the pump. Ths
is particularly useful when the rarefaction is to be
continued long, as there are by these means
The receivers are made tight by
fewer chances of the insinuation of air by the
many joints.
simply setting them on the pump-plate with a
piece of wet or oiled leather between; and the
receivers, which are open at the top, have a brass
cover set on them in the same manner. In these
covers there are perforations and contrivances
for various purposes. The one in the figure has
a slip wire passing through a collar of oiled
leather, having a hook or a screw in its lower
end for hanging any thing on, or for producing a
variety of motions. Sometimes the receivers are
set on another plate, which has a pipe screwed
into its middle, furnished with a stop-cock and a
screw, fitting the middle pipe k. When the
rarefaction has been made in it the cock is shut,
and then the whole may be unscrewed from the
pump, and removed to any convenient place.
This is called a transporter-plate.

It only remains to explain the gage 1111. In the ordinary state of the air its elasticity balances the pressure of the incumbent atmosphere. We find this from the force that is necessary to press it into less bulk in opposition to this elasticity. Therefore the elasticity of the air increases with the vicinity of its particles; consequently it is reasonable to expect, that when we allow it to occupy more room, and its particles are further asunder, its elasticity will be diminished, though not annihilated; that is, it will no longer balance the WHOLE pressure of the atmosphere, though it may still balance part of it. If, therefore, an upright pipe have its lower end immersed in a vessel of mercury, and communicate by its upper end with a vessel containing rarefied, and therefore less elastic, air, we should expect that the pressure of the air will prevail, force the mercury into the tube, and cause it to rise to such a height that the weight of the mercury, added to the elasticity of the rarefied air, acting on its upper surface, shall be exactly equal to the whole pressure of the atmosphere. The height of the mercury is the measure of that part of the whole pressure which is not balanced by the elasticity of the rarefied air, and its deficiency from the height of the mercury in the Torricellian tube, is, in like manner, the exact measure of this remaining elasticity. It is evident, therefore, that the pipe will be a scale of the elasticity of the remaining air, and will indicate the degree of rarefaction; for there must be some analogy be

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tween the density of the air and its elasticity, and we have seen, (except in extreme cases of condensation and rarefaction) that they increase and diminish together and according to the same law. This gage must be considered as one of the most important improvements introduced into this machine by Hawkesbee; at the same time it must be acknowledged to have certain imperfections and inconveniences attending it, which are obviated in other contrivances of a like kind.

It will be immediately perceived, that airpumps merely serve to rarefy the air to a greater or less degree, and that none of them can produce a complete exhaustion. But, independent of this theoretical impossibility, there are mechanical defects which prevent us from carrying on the exhaustion according to the law laid down in theory; few pumps will bring the mercury, in such a gage as that above described, to within one-tenth of an inch; and that of Hawkesbee's, fitted up according to his own instructions, will seldom bring it within one-fifth. Pumps with cocks of the best construction, and under the most favourable circumstances, will bring it within one-fortieth; but this degree of rarefaction has never been attained, when valves fitted up with wet leather were employed: in such cases, one-fifth of an inch is the least that the gage falls short of the Torricellian column. Pumps with stop-cocks, when well made and newly put together, are generally found to rarefy the air to a greater degree than those which are made with valves; but after being used for some time they become less accurate than those with valves. The valves, however, are also imperfect, because the external air, pressing upon that in the piston, prevents its rising when the elastic force of air in the receiver under exhaustion is much diminished; and attempts have therefore been made, particularly by the Abbé Nollet and Gravesande, to perfect the construction of cocks.

In the latter author's double-barrelled-pump, the cocks at the bottom of the pistons are turned by an apparatus that is moved by the handle of the pump. The piston has no valve, and the rod is connected with it by a stirrup, as in a common pump. This rod has a cylindrical part, which passes through the stirrup and moves stiffly in it, through the space of about half an inch, between a shoulder above and a nut below. The stirrup supports a round plate, which has a short square tube that fits tight into the hole of a piece of cork, and which has also a square shank that goes into the square tube. Between the plate and the cock is put a piece of thin leather, soaked in oil, and another is placed between the cork and the plate, which forms the sole of the stirrup. When the winch is turned to raise the piston from the bottom of the barrel, the friction of the piston against it keeps it in its place, and the rod is drawn up through the stirrup. The wheel has thus liberty to turn about an inch, and this is sufficient to turn the cock, so as to cut off the communication with the external air, and to open that with the receiver. When this is done, the continued motion serves to raise up the piston to the top of the barrel. When the

winch is turned in the opposite direction, the piston remains fixed till the cock is turned, so as to shut the communication with the receiver and open that with the external air. The cock has one perforation diametrically through it, and another in a perpendicular direction to this, and after reaching the centre, it passes along the axis of the cock, and communicates with the external air. By means of this communication, when it is opened, the air rushes in and balances the pressure on the upper side of the piston in this barrel, so that the pressure on the other must be counteracted by the person who works the pump. In order to obviate this inconvenience, Gravesande put a valve on the orifice of the cock, by tying over it a slip of wet bladder, or oiled leather; and by means of this the piston is pressed down as long as the air in the barrel is rarer than the outward air, just as if the valve were in the piston itself. This is all that is necessary to be described in Gravesande's pump; its performance is highly extolled by him, as far exceeding his former pumps with valves; and the same preference was given to it by his successor Muschenbroeck. Hawkesbee's pump, however, maintained its pre-eminence in this country, and, indeed, pretty generally on the continent, except in France, till about the year 1750, when Smeaton, so celebrated for his mechanical skill, undertoook to improve and perfect the cock-pumps, which, however, he at length, after various attempts, gave up, finding it absolutely impossible to bring them to that degree of accuracy he had in view; and he then turned his attention to the valve-pumps; and the great advantages, gained by his construction, have been thus explained:

His first object was to diminish the resistance to the entry of the air from the receiver into the barrels, which he rendered almost nothing, by enlarging the surface on which this feebly elastic air was to press. Instead of making these valves to open by the pressure of the air on a circle of one-twentieth of an inch in diameter, he made the valve hole an inch in diameter, thus enlarging the surface 400 times; and to prevent this piece of thin leather from being burst by the great pressure upon it, when the piston in its descent was approaching the bottom of the barrel, he supported it by a delicate but strong grating, dividing the valve-hole like the section of a honey-comb; and that the points of contact between the bladder and grating might be as few as possible, the holes were made hexagonal, and the partitions filled almost to an edge. The breadths of these hexagons are three-tenths of an inch, and, consequently, the surface nine times larger than common; and, as the circumference is three times greater than that of the common valve, and the cohesion to be overcome is, in the first moment of the air exerting its force, proportional to the circumference of the hole, the valve over any of these holes will be raised with three times more ease. Beside, the raising of the valve over the centre hole is aided on all sides by those that are placed round it; and, as they all contribute as much to raise the bladder over the centre hole, as the air acting immediately under it, the valve will be raised with

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