PREUMATICS,

condenser: Fig. 72.

So that the remaining space, being to the total space, as the firft preffure is to the total preffure; three of these terms being known, it will be eafy to find a fourth, by the rule of proportion. For instance, in our gage, let the tube ED, be the total space, into which the air is comprefs'd, by the ufual preffure of the air, which, in England, is equivalent to thirty inches of mercury; the first preffure, therefore, will be thirty inches of mercury. Now, if that preffure be increased, and the air reduced into a lefs fpace, fuppofe into the space NE; to find the quantity of this preffure, I measure the remaining space NE, and conftitute that, fuppofe fix inches, for the first term of the proportion; then, the fecond term, will be the total space DE, fuppofe twelve inches; the third term, the height of thirty inches of the mercury, which was the first preffure; and fo the fourth term, or total preffure, will be found to be fixty inches of mercury: whence I conclude, that the preffure of the air in the receiver can fuftain the mercury to the height of fixty inches; and fo of the reft.

From the fame principle, it will be cafy to find, what ought to be the proportion, between the fize of the tubes AB, and ED. For that depends on the length of the legs, which, the higher they are, fo much the better they reftrain, and keep in the air, but little dilated, in the feal'd part. For instance, let the length AB, be ten inches, which height of the mercury is one third of the accuftom'd preffure, and it is fufficient, that the tube HB, be twice as big, as the tube ED; for, after the mercury hath afcended to the top of the tube AB, the air included in the other leg, expanding itself into the space forfaken by the mercury, will poffefs three times more than its former fpace; and fo one half of the firft preffure, which is ten inches, will be fufficient to curb its fpring. But, if the legs were fhorter, the mercury would be expell'd, by the included air, at least in part. And, therefore, the magnitude of the tube AB, ought to have a greater proportion to the magnitude of the tube ED, that the afcending mercury may afford more space to the air, to be dilated; fo that the fpring of the air being weaken'd, the weight of the mercury cannot be overcome. And, thus it would happen, if the height of the gage were to the height of thirty inches, in the fame proportion with the first fpace of the air, to the total space it would poffefs in vacuo.

The height of the tube, fhould rather be too long, than too fhort; becaufe, if it be too fhort, the mercury will be expell'd in part, and fo, not fhew all the degrees of rarifaction; but, if it be too long, the mercury will, only, not reach to the top, and fo the gage will, nevertheless, fhew all the variations, tho they be lefs fenfible.

But the tube DC, ought to contain a fufficient quantity of mercury, at the leaft, to fill the tube AB, before any paffage be open'd for the air included in the tube ED.

In our engine to compress the air, AA, is a glass-vessel, whofe orifice is exquifitely fitted to the flat plate BB.

BB, is a flat plate of brafs, made to close the veffel AA exactly.

CC, a

CC, a fmall tube of brafs, paffing thro' the middle of the plate, and PREUMATIC faftened thereto.

E, a little valve, opening inwardly, to fhut the fmall tube C.

F, the fpring depreffing the valve E.

GGG, the gnomon faftened to the plate BB, made for restraining the fpring F.

II, a fquare lath, fuftaining the plate BB, and bored thro' in the middle, to tranfmit the little tube C.

LLL, LLL, two iron-wires, which, paffing thro' the holes in the lath II, and compaffing the upper part of the iron-plate KK, hinder the plate from being much moved from the lath.

KK, an iron-plate, with a hole in the middle, formed into a femalefcrew, to receive the male-fcrew MM.

MM, an iron-fcrew, ftraitly to conjoin the receiver AA, with the plate BB; and, left the brafs-veffel fhould be broken, it is proper to put fome wood and leather between the fcrew, and the upper part of the receiver: leather, alfo, is to be put upon the plate BB, both to prevent the breaking of the glass, and the more exactly to fhut the receiver.

NN, a pump faftened to the tube C, below the plate BB.
OO, the fucker of the pump NN.

P, a little hole in the lower part of the pump, by which the air enters into it, when the fucker is brought to the lowest part thereof.

To comprefs the air by means of this engine, we put the bodies, whereon the experiment is to be made, into the receiver AA; and laying it on the plate BB, firmly bind it thereto, by help of the fcrew MM. This done, the fucker or plug OO, is to be drawn, till the external air, by the hole P, can fill all the upper part of the pump; then, if the fucker be drawn upwards, the air finding no other paffage, will open the valve E, and enter into the receiver AA; from whence there is no regrefs, becaufe the valve E, is prefently depreffed by the fpring F, and fhuts the hole C. And fo we may repeat the compreffion of the air into the veffel AA, at pleafure; whilft the quantity thereof is eafily known by the mercurial gages.

But I fo fafhion the pump, that it may be fitted, by a fcrew, to the tube C; for, thus, when one receiver is full, we may take away the pump, and use it to fill others.

Now, because, in thefe engines, mercurial gages ferve to fhew the degrees of compreffion; there is no occafion for the gages before defcribed; for thofe are made with more difficulty, and, befides, afford but a small fpace, wherein to note the degrees of compreffion. It is, therefore, better to bend the glass-tube, feal'd at one end, in feveral places, as in the figure T, that a long tube may be contain'd in a fhort receiver; fo that the Fig. 73. mercury, being put in thro' the open end, as much as will fuffice to fill the length of one inch; all the reft of the space, fill'd with air, will ferve for marking the degrees of compreffion, much more fenfibly than can be done in a fhorter tube.

Here

PNEUMATICS. Here we must note, that when the mercury tends downwards, in fuch an inflected gage, the weight thereof forwards the external preffure; but when it is impell'd upwards, the fame weight refifts it: a difference to be regarded in very accurate experiments.

To mix liquors or In order to make mixtures in compreffed air, let the receiver be AA, in powders in com-which we would mix either liquors, or powders. prefs'd air. Fig. 7a.

To make and remove artificial air from

One receiver into another.

Fig. 74. & 75.

Let QQ, RR, be two tubes, each of them feal'd at one end, and open at the other.

Let RQS, be a veffel of brafs, to be laid upon the orifice of the tubes, as in the figure.

The liquors to be mixed, must be poured into the tubes QQ, RR, each liquor in its own tube; and let the vefiel RQS, being inverted, be laid on the orifices of the tubes; and, in that pofture, let all be cover'd with the receiver AA; let the fcrew be driven, and the air intruded after the manner juft defcribed: and when the gage TT, fhews, that the compreffure is arrived at the degree intended, the engine is to be inverted, and fo the liquors will flow down from the tubes into the veffel RQS, and be mix'd there. If more liquors, or powders, are to be mix'd, the number of the tubes is to be increas'd accordingly.

To tranfmit air out of one receiver into another, we use the following contrivance.

AA, is a flat plate of metal, with a hole in the middle.

BB, is the ftop-cock, faftened to the hole in the middle of the plate AA, one of whofe ends is form'd into a male-screw.

DC, is a copper-funnel, open below, with a broad orifice, (that it might be easily fet upon the pneumatic engine, and there ftand firm;) and the upper part of the orifice D, is fashion'd into a female-fcrew, to receive the male-fcrew of the ftop-cock BB.

EE, is a small tube, open at both ends, which are cut into a femalefcrew, to receive the male-fcrew of the ftop-cock BB.

FF, is the receiver laid on the plate AA, and exquifitely fitted thereto. Now, to make factitious air, we must put the matter which is to produce the air, into the receiver FF; and placing that on the plate AA, by means of the fcrew, we ftrongly faften it thereto, as in our engine for compreffing the air; the ftop-cock BB, we infert into the femalefcrew D: then the orifice C, and with it the receiver, is to be placed upon the pneumatic-engine, and the stop-cock B, being open'd, the air is to be extracted. When the receiver FF, is emptied of air, the ftop-cock B, is to be fhut, that all paffage to external air into the receiver may be denied; and the stop-cock, being taken out from the female-fcrew D, the receiver is prefently to be immers'd in water; fo that, at least the plate AA, with the ftop-cock, may be cover'd therewith: thus no air from without can find entrance; and the air, produced out of the matter in the receiver, will be preferv'd unmix'd; whilft the degrees of its rarifaction, or compreffion, are known, as thofe of common air.

But

But if we would tranfmit that air into another receiver; another re- PNEUMATICS. ceiver FF, with another plate AA, and a ftop-cock BB, is to be procured, Fig. 74. and evacuated: then, by means of the fmall tube EE, we join the ftop-cocks BB of, both receivers, when all fufpected places are to be ftopp'd with cement, that no external air may find entrance. Then, the ftop-cocks being open'd, the air, produced in the former receiver, flows into the latter; and the ftop-cocks being again fhut, and pluck'd out from the tube EE, the receivers may be kept a-part: when if there be any matter included in the latter receiver, we may eafily view what influence the factitious air hath upon it.

But, because the mercurial gages, lately defcrib'd, are fpoil'd, if they be inverted, and the crooked gages prefently expel their mercury, if the air be rarify'd in their receivers; and, fince the operation, here defcrib'd, cannot be perfected, but both receivers must be inverted, and both, likewife, emptied of air; gages of another fort are to be made, after the manner following.

AA, is a glafs vial, fill'd with mercury to the fuperficies DD.
BB, is a glafs tube, very well cemented, in the orifice of the vial.

CC, is another tube, tranfmitted thro' the tube BB, and reaching to the bottom of the glafs. This tube must be feal'd above, and open below; neither muft it fo exactly fill the tube BB, but that paffage may be given to the external air, within the glass AA.

If this inftrument be put into a receiver, from which, the air must be, afterwards, extracted, both tubes will be exhaufted of air; and, when you invert the receiver, to take in new air, as in Fig. 74. the mercury will How down to the orifices of the vial, and be there kept, below the orifice of the tube BB; when the new air entring, will easily fill both tubes, and the vial then, the receiver being erected, the mercury will again reft, in the bottom of the vial, and the orifice of the tube CC, will be plung'd in it. And, if any air be produc'd, out of the bodies included in the fame receiver, the mercury will afcend into the tube CC, and there, reducing the air into a narrower fpace, thew the degrees of compreffion.

Fig. 76.

The inftrument wherewith we filtred air thro' water, was thus con-To filter air trived.

thro' water.

AA, is a glafs receiver, whofe orifice, laid upon the plate BB, agrees Fig. 77. exquifitely therewith.

BB, is a plain plate with a hole in the middle, to tranfmit the tubes CC, DD.

CC, DD, are two tubes cementedto the plate BB; one of which is no higher than the plate, but the other reacheth almost to the top of the recei

ver.

EEEE, is a stop-cock, to whofe holes the extremities of the tubes CC, DD, are faftned.

FF, is the key of the ftop-cock unperforated, wherein is only one chink

GG.

VOL. II.

Cccc

HH

PNEUMATICS.

Fig. 78.

HH, is the receiver, compaffing the end of the ftop-cock, and faftned to it, preventing the entrance of the outward air, and communicating with the pump II.

LL, is a glafs veffel.

M, is a hole in the top of the receiver, whose stopple is fastned with a fcrew.

The next figure exhibits a ftop-cock, cut tranfverfly, that the two tubes CC, DD, may be the better diftinguished, and their insertion into the ftop-cock be perceiv'd.

This inftrument is thus to be ufed: we put the thing about which the experiment is to be made, into the vessel; and the receiver AA, being laid on the plate BB, we pour water into the hole M, till the receiver be about half full, and the veffel LL, with the matter contain'd therein, fwims on the top thereof; then we ftop the hole exactly, and faften it with a fcrew. The key is afterwards to be fet fo, that the chink GG, may communicate with the tube CC; then the plug being brought to the lowest part of the pump, the air of the receiver AA, entring through the upper orifice of the tube CC, will flow down thro' the chink GG, into the receiver HH, and into the pump. Then the key being inverted, fo that the chink GG, may anfwer to the infertion of the tube DD, the plug is to be impelled upward; when the air will be expelled from thence, and, finding no other paffage, be driven through the chink GG, into the tube DD; and from thence it will emerge to the upper part, through the water ftagnant in the receiver. And by repeating this procefs, we ftrain the air thro' the water, as often as we pleafe; and thence know whether it acquires any new qualities, in refpect of the body included with it.

Let the receiver AA be placed upon the plate BB, and fcrewed on

How to condenfe and rarify the Same parcel of to it.

air.

Fig. 79.

▲ wind-gun.

Fig. So.

CC, is the ftop-cock, faftned to the hole in the midst of the plate BB.
DD, is a pump joined to the ftop-cock C, with a screw.

E, is a veffel, fo large, that it may fluctuate in the receiver AA, without danger of being over-turn'd.

Let fome animal be put into the veffel E, and let the receiver AA, be put upon it, and screwed to it, as the figure fhews. Then let the pump be fill'd with water, and, by a fcrew, be fitted to the ftop-cock; the ftopcock, being then open'd, let the plug C, be forced upwards, and the water afcending through the ftop-cock will, in part, fill the receiver AA, and reduce the air, contained therein, into a narrower space, without any addition of new air: if, then, you draw the plug downwards,, the fame numerical air will be again rarified. Thus you may both condense and rarify the fame air as often as you pleafe; and, by this means, you may find, whether the condenfation of the air contributes to prolong the life or health of animals.

In our wind-gun AA, is a hollow copper globe.
BB, a tube, faftned to the globe.

F, a valve opening inwardly, and shutting the tube BB.

G, the