while he walks in the midft of it,he is preferv'd from the fatal Effects thereof; the rather, if he does at all reflect upon the following wonderful manner of fuch Prefervation. As first, that a very small Quantity of the Air, and which is hardly worth naming, fhould be capable of making a Refiftance, and of balancing an unspeakable greater Quantity thereof, and hinder it from crufhing most of the things that are under it. Secondly, that befides fuch a Refiftance, the aforefaid fmall Quantity of the Air does equally operate and gravitate with all the reft of the Air extended even to the Clouds and higher. Now as the first hinders every thing from being deftroy'd, the fecond is no lefs ufeful to Men, tho they are capable of, ufing but a very little thereof.

SECT. XVI. A little Air refifts a greater quantity. tance

ONE may fee an Inftance of the first in Tab. XIII. Fig. 3.where a Glafs A B, impervious to the Air, is placed upon a little Veffel A B CD; which ftanding upon the Brafs Plate and its moiften'd Leather HI, is thereby closed at Bottom, as it may be after another manner, if People will, fo that the little Air at K, remaining inclofed therein, makes fo equal and fo compleat a Refiftance against the Air EA BF (which otherwife, as we have fhewn above, breaks the Glafs, and being extended from the Top of the Clouds down to the Earth, does a thoufand times furpafs the Air at K, both in Quantity and Gravity) that the Glafs A B, tho' never fo thin and brittle, is not in the leaft hurt thereby.

SECT.

SECT. XVII. A little Air gravitates as ftrongly as a great deal.

THE Second, by which we fee that a fmall quantity of Air (befides the Resistance abovemention'd) does likewife gravitate and prefs equally with the whole external Air, may be firft proved by Tab. XIII. Fig. 2. where the Quickfilver in the Barometer A I, with its little Glafs B X, standing in the open Air, is thereby raised and fufpended to the Heighth F 1. Now if you cover the whole with the Glafs Receiver HG L, fo that no Air befides that which is in the Receiver can act upon the Quickfilver at BD; yet you will fee that that which is in the Tube will preferve the faid Height of FI. So that it is here proved unanswerably, that the Air in the Receiver, how little foever it be, gravitates as ftrongly, yea even more upon the Quickfilver B D, than the whole external Air had done before.

But in Tab. XIII. Fig. 5. you may have an occular Demonftration of it, if you place a long Tube FO (like that of a Barometer, but open at both ends) in a little Glafs Veffel GKPQ, thrufting it thro' the Covering of the faid Glafs Veffel GK at I, and clofing it round about; into this Veffel you must pour thro' the little Hole at N, which was ftopp'd before with a Screw) fome Quickfilver, till it rife up to B D, a good deal higher than the End of the Tube O, whilft the rest of the Veffel BDGK, has nothing but Air in it. Then stopping again the little Hole at N with the Screw, fet the whole Apparatus under the Receiver HSL, and exhaufting the AirV V, you will fee that the little included Air at G BDK, will lofe its Refiftance, and preffing upon BD, by its rarifying and expanfive Faculty, will force the QuickC 2 Tilver

filver in the Tube to afcend to the Heighth of F; which was about the fame with that at which the Quickfilver remained standing in a Barometer, when fufpended by the Preffure of the whole Air.

SECT. XVIII. The Difference between the Gravity and Elafticity of the Air.

Now the first (that is to fay, the Resistance which a small Quantity of the Air makes against a greater) is common to all other Liquids, according to the wonderful Laws of Hydroftaticks, to which the weight of all Fluid Matters fubmits itfelf in its Operations. Accordingly, we fee that all Liquors prefs'd upon, do either prefs reciprocally, if they be elaftical, or otherwife refift like folid Bodies; as may be experienced in a closed Syringe or Air-Pump, in which there is either Water or Air; this laft Effect however, ought to be rather ascribed, as we think, to the Air's Elaftick Faculty, than the Weight thereof, which appears from hence, that the weight of the included Air GBDK, does hardly bear any Proportion to that of the Quickfilver in the Tube FI; and again, because if we should fill the space G BDK, where the Air is, with a heavier Matter, or with Quickfilver itself, the Quickfilver in the Tube (tho the Air were exhaufted out of the Bell) would not rise higher than I.

SECT. XIX. How the Elaftick Power of the Air works by the Gravity thereof.

Now in order to understand in fome manner, how the Weight of the Air and the Spring thereof, do produce thefe their Operations with one another, we must represent to ourselves, that in Tab. XIII. Fig. 6. there is a Column of Air, A,H,

confifting

confifting from Top to Bottom of a great number of Air Particles, fuch as A, B, C, D, E, F, G, P, &c. each of which have a certain Weight, whereby they gravitate upon thofe that are under them. We muft likewife fuppofe, that in each of them (of what Figure foever they be) there is an inherent Elaftical Power, by which, like the Steel Springs of Watches, &c. being bent together, they endeavour to expand themfelves again with the fame force wherewith they were bent.

From hence it follows, that the lowermoft Parts of the Air, G and P, &c. bearing the Weight of all those that are above 'em, must be more bent than those that are higher and bear a leffer Burden, as A BC; for which reafon the undermoft, P G, endeavouring_more forcibly to reftore themselves, will prefs the Body IK, that fupports them, with more violence, as those that stand above the Body NO, do the same.

And fo far the Point H bears no more than the Weight of all the Air-Particles A,B,C,D,E,F,G,P, c. which ftand upon one another, without any remarkable Alteration of the Elaftick Power.

But if we proceed further, and place another folid Body between thefe Air Particles, thereby cutting off thofe that are at P and G from the aforefaid Column, and likewife encompass the place LIK M by folid Bodies, in fuch manner, that the Air Particles, P and G, are entirely feparated from the others. If now (as in Water which has little or no Elafticity) the Parts P and G did prefs by their Weight only upon the Body I K at H, the faid Body IK, would be fo much lefs preffed than before, that the Body L M was placed above G; forafmuch as I K does now only bear the Weight of P and G; whereas it had born before, the weight of all the Parts of the Air of which the whole Column AP confifted.

But

But fuppofing on the contrary, that the Parts A, B, C, D, E, F, G, P, had all, like the Air, an Elaftick Faculty, and fhould again endeavour to expand themselves in Proportion to the Preffure of thofe above them, the Body IK; will then be preffed as much by these two Parts P and G, as it was before by the whole Column of Air from A to P; for fince the Parts P and G, that were cut off, are continued in the fame Inflection, by the Refiftance of the folid Body LM, which they had acquired by the weight of the incumbent Parts A, B, C, D, E, F; their Expanfive Faculty, and confequently the Gravitation or Preffure which they make upon the Body IK at H, will remain equally great.

And thus we fee, that the weight of the Air Particles, bearing upon one another from A to P, do prefs the lowermoft P G; and bending the fame, do encrease their Elaftick Force; fo that how little foever they might have been, whilft by the Refiftance of a folid Body ILMK, they were hinder'd from expanding themselves farther, thefe few Parts P G, that are cut off and excluded from the reft, do prefs the Body I K, upon which they act, as much as if the whole Column of Air A P remained over them.

Now, that this laft obtains in the separated Parts of the Air, has been lately fhewn in §. xvii. from the Effects of the included Air in the Place G B D K.

SECT. XX. The Air that bears moft Weight is moft comprefed.

WHAT We have juft now faid, namely, that the undermoft Parts of the Air, P and G, being preffed by a greater weight of those that are above them, will be more compreffed than those of D and E, which have the horter Column of