Phyfico-mechanical EXPERIMENTS, To fhew the Spring and Effects of the AIR. T SECT. I.. HE air is fo neceffary to life, that moft creatures which The origin of breathe cannot fubfift for many minutes without it; and moft the air-pump. of the natural bodies we deal with, being, as well as our own, almost perpetually contiguous to it, the alterations thereof have a manifeft fhare in many obvious effects, and particularly in diftempers; wherefore a farther inquiry into the nature of this fluid, will probably fhew, that it concurs to exhibit abundance of phenomena, wherein it has hitherto feem'd little concern'd.. So that a true account of any new experiment, upon a thing whereof we have fuch a.conftant and neceffary ufe, may prove advantageous to human life.. With this view, before ever I was inform'd that Otto Gueric, the ingenious conful of Magdeburg, had practifed a way in Germany of emp tying glafs veffels of the air, I had made experiments on the fame foundation; but as that gentleman firft produced confiderable effects by this means, I acknowledge the affiftance and encouragement which the report of his performances afforded me. But as few inventions happen to be compleat at the firft, fo the engine employ'd by the conful, feem'd very defective in its contrivance; whence but little more could be expected from it, than thofe very few phenomena obferved by the author, and related by Schottus. I therefore put Mr. Hook upon contriving an air-pump, more manageable and convenient, that might not, like the German-engine, require to be kept under water; and after fome unfuccefsful attempts, he fitted me with one, confifting of two principal parts, a glass veffel, and a pump to evacuate the air. The PNEUMATICS Fig. 30. The air-pump defcribed. The first is a glafs A, with a large mouth, a cover thereto, and a stopcock fitted to the neck below. This would contain 30 quarts of water. B C, the mouth of it, is about four inches in diameter, and furrounded with a glass lip, almost an inch high, for the cover to reft on; wherein D E, is a brafs ring, to cover, and be cemented on to the lip B C. To the internal orifice of this ring, a glafs ftopple is fitted, to keep out the external air. In the middle of this cover is a hole H I, half an inch in diameter, incircled with a ring, or focket; to which is adapted a brass stopple K, to be turned round, without admiting the leaft air. In the lower-end of this, is a hole 8, to admit a ftring, 8, 9, 10; which alfo paffes thro' a fmall brass ring L, fixed to the bottom of the ftopple F G, to move what is contained in the exhausted veffel, or receiver. That the flopcock N, in the firft figure, might perfectly exclude the air, we faftened a thin tin-plate, MTV W, to the shank of the cock X, all along the neck of the receiver, with a cement made of pitch, rofin, and wood-afhes, poured hot into the cavity of the plate; and to prevent the cement from running in at the orifice Z, of the shank X, it was ftopt with a cork fixed to a ftring, that it might be drawn out at the upper orifice of the receiver; and then the neck of the glass, being made warm, was preffed into the cement, which thus filled the interstices betwixt the tin-plate and the receiver, and betwixt the receiver and the fhank of the cock. The lower part of our engine confifts of a fucking-pump, fupported by a wooden frame, with three legs 111, fo contrived, that, for the freer motion of the hand, one fide of it may ftand perpendicular; and a-crofs the middle of the frame we nail'd a piece of board 222, to which the principal part of the pump is fixed. The pump confifts of an exact strong concave cylinder of brafs, fourteen inches long, its cavity three inches in diameter; to which a fucker, 4455, is adapted, made up of two parts; one of which 44, is less in diameter than the cavity of the cylinder, with a thick piece of tann'd leather nailed on it, whereby it excludes the air. The other part, a thick iron plate 55, is firmly joined to the middle of the former, and is a little longer than the cylinder; one edge of it being fmooth, and the other indented, to receive the teeth of a small iron-nut a By, fixed by two staples to the underside of the board nailed a-cross 22, on which the cylinder ftands; and is turned by the handle 7. The laft part of the pump is the valve R, a hole at the top of the cylinder, and taper towards the cavity; to this is fitted a brass-plug, to be taken out as occafion requires. The engine being thus contrived, fome oil must be pour'd in at the top of the receiver upon the ftop-cock, to fill up the interftices of its parts, and that the key S, may turn with the greater eafe. A quantity of oil, alfo, must be left in the cylinder, to prevent the air from getting betwixt that and the fucker; for the like reafons, fome muft, likewife, be applied to the valve. And here it is proper to obferve, that when we ufed oil, or water, feparately, for this purpofe, and they have not answered the end, a mixture of the two has afterwards proved effectual. And, that the air may not PNEUMATICS enter betwixt the brass-cover and the ring, 'twill be convenient to lay fome diachylon plaifter on their edges with a hot iron. That no air, alfo, may remain in the upper part of the cylinder, the handle is to be turn'd till the fucker rifes to the top; and then, the valve being fhut, it is to be drawn down to the bottom; by which means, the air being driven out of the cylinder, and a fucceffion from without prevented, the cavity of the cylinder must be empty of air; fo that when the ftop-cock is turn'd to afford a communication betwixt the receiver and the cylinder, part of the air before lodg'd in the receiver, will be drawn down into the cylinder; which by turning back the key, is kept from entering the receiver again, and may, by unftopping the valve, and forcing up the fucker, be driven into the open air; and fo, by repeated exfuctions out of the receiver, and expulfions out of the cylinder, the veffel may be exhausted as the experiment requires*. The air-pump has receiv'd great improvements fince the time of Mr. Boyle, and feems brought to its utmost degree of fimplicity and perfection, by the late, and the prefent Mr. Hauksbee. This inftrument, as 'tis now made, by Mr. Hauksbee, conFig. 31. fifts of two brafs-cylinders, a a aa, twelve inches high, and two their internal diameter. The emboli are raised, and depreffed, by turning the winch bb, backward and forward. This winch is faften'd to a spindle, paffing through a lanthorn, whofe pins ferve for cogs, laying hold of the teeth of the racks cccc; fo that one is depreffed, and the other elevated reciprocally. By this means the valves, made of limber bladder, and fix'd on the upper part of each embolus, and at the bottom of the cylinders, mutually exhauft and difcharge the fame air from the receiver: which becoming nearly empty, the preffure of the external air on the descending embolus is fo great, that the power required to raise the other, need but little furmount the friction of the moving parts; whence this pump becomes preferable to all others. The bottoms of the barrels lie in a brafs difh dd, its fides two inches high, containing water to keep the leather-collars, on which the cylinders ftand, moift; whereby the air is precluded. The cylinders are fcrew'd hereon by the nuts eeee, which force the frontispiece ff, down upon them; thro' which pass the two pillars gggg. Each pillar has an iron VOL. II. 1. Upon belonging to it, paffing from them in the the PNEUMATICS 1. Upon drawing down the fucker of our engine, whilft the valve is Some phenome- fhut, the cylindrical space deferted by it will be left empty of air; and, na of the engine therefore, upon turning the key, the air contain'd in the receiver rushes folved. into the cylinder, till, in both veffels, it be brought to an equal dilatation; fo that, upon fhutting the receiver, turning back the key, opening the valve, and forcing up the fucker again, almoft a whole cylinder of air will be driven out after this firft exfuction; but, after every fucceeding ftroke, lefs air will come out of the receiver into the cylinder: fo that, at length, the fucker will rife almoft to the top of the cylinder, before the valve need be open'd. And if, when it is fo exhausted, the handle of the pump be let go, and the valve be stopp'd, the fucker, by the force of the external air, which is an over-balance to the internal rarify'd air, will be forced to the upper part of the cylinder, and higher, in proportion, as the air is more exhaufted*. We obferv'd, alfo, that, whilft any confiderable quantity of air remains in the receiver, a brifk noife is immediately produced, upon turning the key. The Spring and But to render our experiments the more intelligible, we must premife, preffure of the that the air abounds in elaftic particles, which being preffed together by air explain'd. their own weight, conftantly endeavour to expand and free themfelves from that force; as wool, for example, refifts the hand that fqueezes it, and contracts its dimenfions; but recovers them as the hand opens, and endeavours at it, even whilft that is fhut. It may be alledg'd, that tho' the air confifts of elaftic particles, yet this only accounts for the dilatation of it in pneumatical engines, wherein it hath been compreffed, and its fpring violently bent, by an external force; upon the rem oval whereof, it expands, barely to recover its natural dimenfions; whilft, in our experiments, the air appears not to have been compreffed, before its fpontaneous dilatation. But, we have many experiments to prove, that our atmosphere is a heavy body, and that the upper parts of it prefs upon the lower. And I found a dry lamb's bladder, containing two thirds of a pint, and compress'd by a pack-thread tied about it, to lofe, in a very tender balance, 1 grain of the mercury afcends or defcends in the gage. its former weight, by the recefs of the air, upon pricking it. Suppofing, PNEUMATICS therefore, that the air is not deftitute of weight, 'tis eafy to conceive, that the part of the atmosphere wherein we live, is greatly comprefs'd by thofe directly over it, to the top of the atmosphere. And tho' the height of this atmosphere, according to Kepler, fcarce exceeds eight miles, yet later aftronomers extend it fix or feven miles farther. The learned Ricciolo makes it reach fifty miles high. So that a column of air, feveral miles in height, preffing upon fome elaftic particles of the fame fluid here below, may easily bend their little fprings, and keep them bent; as if fleeces of wool were piled to a vaft height upon one another, the hairs of the loweft locks would, by the weight of all the incumbent parts, be ftrongly comprefs'd. Hence it is, that upon taking off the preffure of the incumbent air, from any parcel of the lower atmosphere, the particles of the latter poffefs more space than before. If it be farther objected against this condenfation of the inferior air, that we find this fluid readily yields to the motion of flies, feathers, &c. we may reply, that as when a man fqueezes wool in his hand, he feels it make a continual refiftance; fo each parcel of the air, about the earth, conftantly endeavours to thrust away fuch contiguous bodies as keep it bent, and hinder the expanfion of its parts: which will fly out towards that part, where they find the leaft refiftance. And, fince the corpufcles whereof the air confifts, tho' of a fpringy nature, are fo very small, as to compose a fluid body, 'tis eafy to conceive, that here, as in other fluids, the component parts are in perpetual motion, whereby they become apt to yield to, or be difplaced by other bodies; and that the fame corpufcles are fo variously mov'd, that, if fome attempt to force a body one way, others, whofe motion had an oppofite determination, as ftrongly prefs it the contrary way; whence it moves, not out of its place; the preffure, on all fides, being equal. For if, by the help of our engine, the air be drawn only from one fide of a body, he, who thinks to move that body, as easily as before, will, upon trial, find himself miftaken. 2. Thus, when our receiver is tolerably exhausted, the brass stopple in the cover is fo difficult to lift, that there feems to be fome great weight faften'd to the bottom of it: for the internal air being, now, very much dilated, its fpring must be greatly weakned; and, confequently, it can but faintly prefs against the lower-end of the ftopple, whilft the fpring of the external air keeps it down, with its full natural force. And, as the air is gradually admitted into the receiver, the weight is manifeftly felt to decrease; till, at length, the receiver being again fill'd with air, the ftopple may be eafily lifted. It may feem furprizing, that we fpeak of the air fhut up in our receiver, as of the preffure of the atmosphere; though the glafs manifeftly keeps the incumbent pillar of air from preffing upon that within the veffel. But, let us confider, that if a fleece of wool, by preffure, be thus directly reduced into a narrow compafs, and convey'd into a close box, tho' the former force ceases to bend its numerous fpringy parts, yet they continue Ggg 2 as |