ed to the top of an high mountain, where the incumbent cylinder of the PNEUMATIC atmosphere is fhorter, and its weight the lefs; and will, for the contrary reafon, grow more flaccid, the nearer it approaches to the foot of the moun

tain.

Mr. Hobbs's

Mr. Hobbs attempts to explain the phenomena of our engine, by fuppofing, that many earthy particles are interfpers'd with the air, which explanation ef have a fimple, circular motion, congenite to their nature; and that there the phenomena of the air-pump, is a greater quantity of thefe particles in the air, near the earth, than examin remote from it." But this affumption, to me, feems very precarious; for, I know no unquestionable example, or experiment, whereby it can be made out, that any fmall parcel of matter, has fuch a fimple, circular motion, as he afcribes to each of thefe innumerable earthy, and, as he adds, aqueous particles. The only argument he here brings to prove, that each atom would have this motion, if all the reft of the earth were annihilated, does not feem clear to me. For, it is not always true, that each minute part of a homogeneous body, has, in every refpect, the fame qualities with the whole as the roundnefs which a fmall drop of water, or quick-filver, is obferv'd to have upon a dry plain, is not to be met with in a large portion of either of thefe fluids, tho' plac'd upon the fame plain. And Mr. Hobbs, as well as we, makes the terreftrial atoms in the air to have gravity; a quality that does not properly belong to the whole globe of the earth: nor is it manifeft why, becaufe the terrestrial globe moves in a vaft circle about the fun, each particular atom of it muft defcribe a small circle in the air, about I know not what center. And, tho' he afferts, that the air, near the earth, abounds with fuch terreftrial corpufcles, 'tis not likely they should have fuch a regular motion, as he attributes to them; but, striking against one another, they muft, in probability, be put into, almost, as various, and confused a motion, as Des Cartes afcribes to his tereftrial particles, fwimming in the atmosphere.

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Mr. Hobbs farther endeavours to prove, that, by the exhauftion of our cylinder, no vacuum is produc'd; and gives a very different account of the experiment itfelf: he fays, that" while the fucker is drawn back, the more fpace is left within, the lefs is left to the external air; which being "thruft backwards, by the motion of the fucker, towards the outer"moft parts, moves, in like manner, the contiguous air; and that, the next; and fo forwards: fo that, of neceffity, at laft, the air must be compell'd into the space deferted by the fucker, and enter between the convex furface of the fucker, and the concave of the cylinder. For, the parts of the air being infinitely fubtile, muft infinuate themselves that way by which the fucker is drawn down; fince the contact of "thofe furfaces cannot be perfect in all points, because the furfaces them"felves cannot be made infinitely fmooth: and then, that force, which is applied to draw back the fucker, in fome meafure diftends the cavity of the cylinder; and if, betwixt the two furfaces, one fingle hard atom fhould enter, pure air will enter by the fame way, tho' with a finall force. And thus air, for the fame reafon, infinuates itself through

Sfff2

the

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PNEUMATICS." the valve of the cylinder; and, therefore, the retraction of the fuder "will not prove a vacuum. It follows, alfo, that the air, which is driven up into the space deferted by the fucker, because it is forcibly impell'd, has a very fwift, and circular motion, betwixt the top and the bottom of the cylinder; becaufe there is nothing there to weaken its "motion; and nothing can give motion to itself, or diminish it." But, many exceptions may be made to this reafoning. And, firft, I know not why Mr. Hobbs fhould, here, confine his difcourfe to the pump, without taking notice of the glafs it is defign'd to evacuate. We will, therefore, confider how he can account for the exhauftion of the receiver, as well as of the cylinder, fince we ufually employ them both together.. And he being obliged to explain the exhauftion of the one, as well as the other, it will be convenient to take into confideration the receiver, becaufe that being of glafs, and tranfparent, we can better fee what happens in it, than in the opake cylinder. This premis'd, I do not clearly perceive, by this explanation, how he avoids a vacuum; for, according to his first words, the external air is difplac'd by the motion of the fucker outward, and this difplac'd air muft move that which is next to it; and that the next, and fo onward, till, at length, the air muft be compell'd into the space deferted by the fucker: fo that till this returning air get in betwixt the fucker and the cylinder, how appears it, from this difcourfe, that the deferted fpace was not empty for fome little time? Certainly all these motions of the air, forward and backward, could not be perform'd in an inftant; as may appear by the motion of founds and echoes, whofe velocity is reducible to meafure. Secondly, tho' he take his adverfaries to be vacuifts, yet he here fuppofes the plenitude of the world. I wish, thirdly, that Mr. Hobbs had declar'd, from whence the return of the air's impulfe fhould begin; for that may well be requir'd from one, who, making the world full, and, for ought appears, fluid, allows us to believe it infinite, if the magiftrate fhall enjoin us that belief. Fourthly, I demand, what neceffity there is for fo forcible a return of the impulfe, as is requifite to thruft in the air at fo narrow a paffage as that between the fucker and cylinder? For, why may not that impulfe, when diffused in the vast ambient medium, be fo communicated, and blended among the different motions of the other parts of it, as not to return again from whence it begun? As a voice, tho' ftrong, will not move the air, beyond a certain diftance, fmartly enough to be reflected in an echo, to the fpeaker ; and, as a stone caft into a lake, will have the waves, it makes, diverted from returning to the place they began at. Fifthly, I do not, likewife, fee, that 'tis probable, what Mr. Hobbs affirms of fo thick a cylinder as ours, that it should be diftended by depreffing the fucker. But this I infift not on; the principal thing, peculiar in Mr. Hobbs's explanation, is, that as much air as is driven away by the fucker, presently gets in again, betwixt that and the cylinder. But, by the air thus fuppos'd to get in, he either means in the ufual fenfe, and in ours, the common air, fuch as we live and breathe in ; er,, he does not.

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If he speaks of fuch air, I can plainly prove, by feveral experiments, PNEUMATICS that our engine is, in great part, deftitute of it. For, firft, if there be a contrivance made, whereby the whole pump may be cover'd with water, we may, as we have try'd, plainly fee the air that is drawn out of the receiver, at each reciprocation of the fucker, pafs, in great bubbles, out of the valve thro' the water..

Next, it appears, by the Magdeburgic experiment, that, by reafon of the recefs of the air, the globe of glafs, whence it went out, was diminish'd in weight, above an ounce. Thirdly, the fame truth may be prov'd by the experiments formerly mention'd, of the fwelling of a bladder, and the breaking of an hermetically feal'd glass, upon the recefs of the ambient air; thefe experiments having been already vindicated from Mr. Hobbs's very improbable folutions. Fourthly, the fame may be prov'd, by the breaking of weak, or ill-figur'd receivers, inwards; of which, on our hypothefis, the reafon is clear; but not on his. And, fifthly, what I contend for, may be fufficiently prov'd from this one phenomenon; that tho', if the receiver being full of common air, the key be turn'd under water, the water will not at all afcend at the open orifice; yet the like being done, after the exhauftion of the receiver, we have had feveral gallons of water violently impell'd into the cavity of the glafs: which could not happen, if it were full of air, both in regard there can be no probable caufe affign'd why the water fhould be thus fpurted up; and because the re ceiver being already full of air, either two bodies must be contain'd in one place, and fo we must allow penetration of dimensions; or else common air, to which glass is impervious, muft pafs thro' the water; which, we conclude, it does not, because no fuch bubbles are made in the external water, as would appear, if common air pass'd thro' it. Nay, fo little of this common air was, fometimes, left in the globe ufed at Magdeburg, that when the water was fuffer'd to rush in, it reduced the air into lefs than the thoufandth part of the capacity of the globe; and even if our receiver be unftopp'd, not under water, but in the open air, the ambient air will, violently, prefs in, with a great noife, durable enough to argue, that the glafs was far from being full of fuch air before.

And thus we may argue against Mr. Hobbs, if he would have the engine, when we call it exhaufted, fill'd with common air; as his words feem to intimate. But becaufe, by fome other paffages of this dialogue, he may be favourably thought to mean, that the pure air is that which gets in by the fides of the fucker, into the pump, and fo into the receiver, let us confider his explanation in this fenfe alfo. I defire it may be obferv'd, that if Mr. Hobbs takes the air in this fecond fenfe, he does not oppofe what I have deliver'd; the air, I pretend to be pump'd out of the receiver, being the common air, which confifts, in great part, of groffer corpufcles, than the ethereal fubftance. Yet, even this explanation will be liable to the two firft inconveniencies, lately objected against the other, in favour of the vacuifts; and to feveral objections befides. I obferve, again, that tho' the pump be kept all the while under waters.

yet

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PNEUMATICA yet the exhauftion of the cylinder, and receiver, will proceed as well as in the open air. I demand, then, how the pure air gets in by the fides of the fucker, immers'd in water? I prefume, for want of a more plaufible anfwer, Mr. Hobbs will here fay, that the air paffes thro' the body of the water, to fill up the deferted fpace, that muft, otherwife, be void. But then I appeal to any rational man, whether I am obliged to believe fo unlikely a thing, upon a bare affirmation; for he does not fo much as pretend, by any phenomenon, to countenance this affertion: and there are phenomena that make against it. Many experiments fhew us, that when air pafles thro' water, it makes bubbles there, which, in our cafe, do not appear. Befides, why fhould not the outward air, rather impel the water, as we fee it frequently does, than be fuppos'd to dive fo itrangely and imperceptibly thro' it? When, alfo, the throughly exhausted receiver is unftopp'd, under water, he, who obferves how the water rushes in with a ftream, as big as the paffage admits, will hardly imagine, that at the fame time, as much air as water can pass thro' the fame orifice unperceiv'd. But, it may be faid, in Mr. Hobbs's behalf that either his explanation, or a vacuum, must be admitted. To which I reply, firft, that he has not evinc'd there can be no vacuum. Next, that we have made it probable, that, by his explanation, he does not avoid the neceffity of a vacuum. And, thirdly, that a plenift, having recourfe to Mr. Hobbs's precarious diving of the air, may, more probably, decline the neceffity of yielding a vacuum, by faying, that the æther is, by the impulfe of the deprefs'd fucker, and the refiftance of the ambient bodies, fqueez'd thro' the pores of the glafs, or cylinder, into the cavity of the veffel, as faft as room is there made for it. And, I confefs, I wonder that Mr. Hobbs fhould be fo averfe to this way of folving the objection, fince he fupposes the parts of the air to be infinitely fubtile; which, if they are, no pores can be too narrow to admit them. But, to prefs this no farther, I muft here take notice, that whether the cavity of the receiver, be refolv'd to be empty, or full of Mr. Hobbs's æthereal body, or the Cartefian celestial matter; the violent rufhing in of the water, when the veffel is unftopp'd under that liquor; with feveral other phenomena, which cannot be afcrib'd to the fubtile matter within; fufficiently argue, that there is, in the external air, a far greater power of preffing inwards, than there is within of refifting; and, confequently, fuch a weight, or fpring in that air, as we plead for.

Mr. Hobbs, too, will have the air, impell'd by the fucker, to move very fwiftly betwixt the top and bottom of it; as alfo, when it gets into the cavity of the receiver; yet, when a light bladder is fufpended in the cavity of the receiver, it betrays no fuch motion: nay, the flame of a taper was not blown out, nor ftirr'd by this fuppofed wind; and fmoke, produced in the exhaufted receiver, was not, by this vehement motion of the air, blown about the receiver. But, if the common external air be admitted at the ftop-cock, that, indeed, will rush in with noife and violence, and whirl about the bladder, which hung quietly before.

In explaining the Torricellian experiment, he fpends many words to PNEUMATICS. prove, that the place deferted by the fufpended mercury, is full of air. But this expofition fuppofes a plenum: and, if he takes the air in the common fenfe of the word, 'tis manifeftly repugnant, to feveral phenomena; as that, if the experiment be carefully made, we may, by inclining the tube, impel the mercury from its wonted ftation to the top; which will not happen, in cafe the air were, before inclination, let. into the deferted fpace; that if, when the mercury is fettled at its ufual itation, the tube be lifted up out of the ftagnant quick-filver, the outward air will drive up the heavy mercurial cylinder, oftentimes, with force enough to beat out the feal'd end; and, laftly, the quick-filver refting at its ftandard height, if you carefully ftop the lower orifice, under the furface of the ftagnant quick-filver, and then lifting up the tube into the air, keep it well ftopp'd, and firft deprefs one end, and then the other; the quick-filver will fall against the deprefs'd end of the tube, with a furprizing force and fwiftnefs: whereas, if unftopping the tube, whilst the fame quantity of mercury remains in it, you let the outward air into the cavity, unpoffefs'd by the meraury; and. then, again, ftop the orifice with your finger, and proceed as before, you fhall perceive the motion of the included fluid, to be much flower, and lefs violent than formerly, by reafon of the refiftance of the admitted air; which, alfo, manifeftly difclofes itfelf, by the conflict, and bubbles produced betwixt the air and quick-filver, in haftily paffing by one another, to the oppofite ends of the tube. But, Mr. Hobbs, not pretending that any attraction intervenes in the cafe; I fee not how he can poffibly make out, to omit other phenomena, the gradual defcent of the mercury, in the tube, beneath its wonted ftation, upon the exhauftion of the receiver; and the re-afcent of the fame, in the fame tube, as we let in more or lefs of the outward air, without admitting as much of fpring or preffure in the air, as I contend for. The weight of the terreftrial particles, by which he endeavours to account. for the quick-filver's falling lower at the top, than at the bottom of a hill, will by no means ferve his turn; it being utterly improbable that the air, contain'd in fo little a veffel as one of our receivers, can, by its weight, counter-balance fo ponderous a cylinder of quick-filver : whence we may be allow'd to argue, that the air fuftains it by fuch a preffure, or fpring, as we plead for, whether that proceed from the texture of the aerial particles, from their motion, or from both.

The laft of Mr. Hobbs's principal explanations, is of the experiment wherein above 100 pound weight, being hung at the deprefs'd fucker, the fucker was, notwithstanding, impell'd up again, by the air, to the top of the cylinder. This phenomenon Mr. Hobbs accounts for thus. The air being beaten back by the retraction of the fucker, and finding no "void place, wherein to difpofe of itself, befides that which it may make,. by driving out other bodies, is, by perpetual trufion, at length, forc'd into the cylinder, with fo great fwiftnefs, between the concave furface "of the cylinder, and the. convex furface of the fucker, as may anfwer

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