exactly to the Depth of the Water a L, which was the Column of Water incumbing upon PQ and L M.

For pouring gently fome Water into the Vessel, till it afcended on the outfide of the Tube to A B, we likewise observed the Water within the Tube, to rife from de to RS.

But on the contrary, when by fucking or letting out fome of the Water out of the Veffel, it was reduced to the Depth of a b or lower, the internal Water did also fubfide to de or yet lower, but fo as always to continue of the fame Height with the External.

VI. Now it has been fhewn before, and it alfo follows from Sect. XXXIV. that if the Curve Tube Y X Q were extended from PQ to NO, or higher, and then fill'd up to the Height of g h or a b of the Veffel, the Preffure of this Column of Water PQbg would be great enough to fuftain the Water in the other Shank of the Tube to de.

VII. So that we may conclude from hence, that the whole Mass of Water in the great Veffel a L M b, gravitates as much, but neither more nor lefs, upon P Q, than this fame Column of Water P Q bg.

VIII. Now fince this Column PQbg, is equal to a Column whofe Bafis was the Part PQ of the Horizontal Plane L M, and whose Height is the perpendicular Height Pg or Qh (or otherwise La or Mb) or the Water incumbing from ab upon the Horizontal Plane LM: A famous Propofition in Hydroftaticks is deducible from hence, namely, That if we fuppose a Horizontal Plane paffing thro' a ftagnating or quiefcent Fluid, the Force whereby a Part thereof

thereof, as PQ, is gravitated upon, or prefs'd down, is equal to the Weight of the aforefaid Column PQgh, whofe Bafis is the Area of P Q Part of the Horizontal Plane LM, and the Height of which is a L, or Mb, or the whole Height of the Fluid imcumbent upon the faid Horizontal Plane, measuring the fame directly upwards.

IX. This Column (because it extends itself from the fuppos'd Horizontal Plane to the uppermoft Superficies of this fingle Fluid, and, if there be more Fluids imcumbent on each other, to the uppermoft Superficies of that Fluid which is higheft, and contains all the perpendicular Heights of all the Fluid Matters impreffing or incumbing on each other) we fhall hereafter, for Brevity fake, call the Column of Altitude.

X. Now to fhew that there happens not only to this one, but to all equal Parts, as PQ, of the fame Horizontal Plane L M, one and the fame Preffure, and each equal to the Weight of this Column, we remov'd the little Piece of Wood ET, with the Curve Tube Y X Q that was tied to it, from one Part of the Veffel to the other, fo that the Orifice P Q fill'd at every Turn a new Place of the faid Horizontal Plane, but we always found the Water stopping at de, or at the fame Height; and confequently that every Part equal to the Area P Q of a Horizontal Plane L M, is always prefs'd down with an equal Force, which is alfo equal to that of the Column of Height.

XI. And to fhew farther, that the different Figures of Veffels did not alter the Cafe, or that it is not neceffary that this gravitating Column PQbg, fhould be always directly perpendicular to the Part P Q that it preffes, we thruft a

Piece

Piece of Wood IK, GH, with a flat Bottom GH, or a Beer-Glafs, or a Phial with the Bottom downwards, to a certain Depth as GH, under the Superficies of the Water ab, and held it there immoveable; after which we turn'd the Tube YXQ quite about, bringing the Orifice PQ to pq directly under the aforefaid Bottom, and we obferv'd, that notwithstanding the gravitating perpendicular Column over pq, could not extend itfelf higher than to G H, yet the Water remain'd in the Tube at d'e, and confequently at the fame Height, as if the whole Column of Altitude P Qbg, were fupported or refted on PQ

XII. So that it appear'd from thence, that each Part PQ, q, &c. of the Horizontal Plane L M, was not always juft prefs'd by the Column of Altitude itself, but by a Weight equal to that of the faid Column; and confequently that this Eaw obtains in Veffels of all Figures, of which, tho there be here but one fingle Inftance given, and numberless Veffels might be propos'd for Tryal, fufficiently confirms our Pofition with the Concurrence of all that are vers'd in Hydroftaticks, and abundance of Experiments in all kinds. of Veffels.

XIII. I must however endeavour to remove one Difficulty, which it may be renders what we have juft now faid obfcure to fome People, and then pafs on to fomewhat else.

It is this, If a Drinking-Glafs or Cup kl 7, 8, be filled with Water, and then inverted fuddenly, fo that the Mouth 7, 8, defcends below the Superficies ab; and if one continue the Cup or Glafs in the faid Pofture, it will be found:

Firft, That the Water will defcend either to k L or cf, according as there was more or lefs of it in the Glass, but by no Means so low as 9 or 10, or as the external Water a b

Secondly, That in cafe the Curve Tube YXQ, in which the Water is at the Height of de, be turn'd about in its String and fhoy'd forwards, if neceffary, with the Piece of Wood E F, fo that the faid Tube YXQ be brought to 1 46 and its Orifice P Q to 5 6 directly under the Glafs k178 (continuing ftill in the Horizontal Plane LM) we fhall find that the Water will remain in the faid Tube immoveably at 23, the fame Height as de, and as the external Water a b.

Now fince each part P Q and 5:6 of the Horizontal Plane L M is prefs'd by the Column, the Height of which is equal to the Height of the Water, and forafmuch as there is no more Weight upon PQ than the Column PQ gb, and fince there feems to be incumbing on 56 a Gravitating Column 6, fc, of a greater Depth, and confequently of a greater Weight than that of PQ bg; it seems that it ought to follow likewife, that the Preffure upon 5 6 fhould be much greater than that upon PQ; and therefore that the Water in the Tube at 1 4 6 fhould afcend much higher than 23 or die, but on the contrary, the Water at 2 3 or de continues at an equal Height with the external ab.

This Experiment would be a notable Objection against what we have advanced, were it not that all who are any ways vers'd in Hydroftatics, know, that what is faid before, is only meant when there is no other gravitating Fluid upon the Water a b; and that the Preffure of the Air, which always gravitates upon the Water ab, is only the Cause here that the Water continues fu-spended in the Glafs or Cup at cf. That in

cafe

cafe no Air prefs'd upon the Water ab, the Water in the faid Glafs k 1 8 7 would not continue higher than the external Water ab or 9 10, tho' the Glass be inverted; as is well known to those that use Air-Pumps.

So that this Objection is properly of no weight against what we have afferted, fince we only treat of Cafes in which the Preffure of the Air produces no remarkable Alterations, or at leaft, in which we may suppose them.

SECT. XXV. Experiments proving that Fluids prefs upwards.

XIV. To proceed now to the Preffure of Fluids upwards:

That in Water and other Fluids a Preffure upwards has likewife Place, may be inferr'd from many Water-Works and Fountains that throw up Water.

This will alfo appear by the ftrait Tube Zrt: For unless the Water at the Part rt of the Horizontal Plane L M were prefs'd upwards, it would not be poffible that the Column rtnm, which lies upon the Superficies of the External Water ab in the Veffel, could keep its Station at nm, fince it is continually prefs'd downwards by its own Weight.

To give then an Inftance thereof: Stop the empty Tube Zrt with your Finger at Z, and thruft or put it down into the Water as far as rt, you will thereupon find that the faid Tube will remain empty from Z tort or thereabouts; excepting perhaps, that by the Preffure upwards of rt, the Water may rife a little, or fo much higher in the Tube than rt (if let down into a great Depth where the Preffure upwards is ftronger) as

the