Edges, as to prevent the Water from oozing out at the Joints.

This Veffel being fill'd with Water up to B D, we knew that there was the Weight of a Foot of Water preffing laterally upon the Piece of Wood HDRS, because the Height of the Water was equal to DR, or to the Height of the uppermoft Part of the faid Piece of Wood.

Having then examin'd into the Force of this lateral Preffure of the Water by Means of a Ballance A E F, the Arms of which A E and EF were Rectangular at E, to which the Weight Y was fufpended, we found that this Wood being prefs'd inwards at E A by Y, could refift a Weight of between 31 and 32 Pounds; but upon adding more it immediately burft inwards.

So that the Rule mention'd in Sect. LII. was fufficiently verified hereby, fince a Rynland Foot of that Water was about twice as heavy.

It is to be observ'd, that the preffing Part A of the Ballance AE F, must be placed exactly upon the Point of the Wood A, where the Centre is, or where the lateral Preffure is strongest.

In order to do fo, Care was taken that by the Means of the cross Piece of Wood V W, the Axis E of the Ballance might be thruft up or down; and we found that when the Ballance prefs'd higher or lower than A, the flat Piece of Wood (the Water being at higheft) yielded to a much smaller Weight or Preffure; which fhew'd that the true Point was at A.

XLVII. We then fill'd the Veffel quite full of Water up to GK, so that the upper Part DR of the wooden Plane DR SH, which bears the leteral Preffure, was not equal to the upper Superficies of the Water, but was below it the Length of K D, or of of a Foot, that is, 8 Inches. Where

Wherefore if what is faid before be true, and that HK were 1 of a Foot, or 20 Inches, we fhould have seen the lateral Preffure on HDRS refift a greater Weight.

Having therefore rais'd the Axis a little higher, fo that A, the End of the Ballance, being hereby apply'd to the Center of the Preffure, was alfo higher, we found that by putting on the Weight Y of 77 Pounds, the wooden Plane immediately gave way, but that the lateral Preffure held good against 73 or 74 Pounds.

This agreed pretty near with the foregoing Rule, as we found by computing after the fol lowing Manner :

25

Let Tab. XXVII. Fig. 2. be transferr❜d to Fig.3. and that the Water is from HS to K Z. Then HK equal to 1, or of a Foot is equal to P H (if we fuppofe this fame H K to be lying horizontally) and HS is a Foot: So that the whole Body of Water of this Breadth, Length and Depth of a Cubical Foot; and the half of it, which preffes laterally upon K HZS will, be of a Foot. If now we fubftra&t from thence the Water's lateral Preffure upon KDRZ, to wit, of a Cubical Foot, it being the Half of a Foot of Water (which laft expreffes the Magnitude of a Body, the Length whereof R D is 1 Foot, the Breadth m D, and the Height KD likewise of a Foot) there will remain the lateral Preffure of or of a Foot, or otherwife 1 of a Cubical Foot of Water, that is (fuppofing fuch a Foot of Water to be 63 Pounds, or fomewhat more) of 73 Pounds against HDRS; which fufficiently agrees with the Rule mention'd Sect.

LII.

SECT.

SECT. XLIV. The Lateral Preffure adapts itself to the Height, and not to the Breadth of Water.

XLVIII. WE likewife obferv'd, upon placing a flat Board or Partition (Tab. XXVII. Fig. 2.) Tab, after fuch a manner, that the Water, which before prefs'd upon DHRS, became divided, or had not above the half a K of its former Breadth KG, that the lateral Preffure was not leffen'd, whilft the Water continued at its first Height K G.

So that from hence alfo it appear'd, that the Powers of the lateral Preffure remain the fame, whether the Breadth of the Water be increas'd or diminish'd; but that upon augmenting or leffening the Height of Water, thofe Powers are accordingly augmented or leffen'd.

SECT. XLV. The Lateral Pressure of Water, with Air preffing upon the fame.

XLIX. But forafmuch as in thefe Experiments of Lateral Preffures we have fuppofed Water to be the uppermoft Fluid, fo that, for inftance, in the Veffel A B CD (Tab. XXVII. Fig. 4.) we are to fuppofe there is no other Fluid Matter above the Superficies of the Water A C, the following Difficulty feem'd to arife, namely, that the lateral Preffure of the faid Water A C upon A B would be much greater than it is found to be by thefe Experiments; because the Air between AC and and ooo, &c. actually preffing upon A B with the Weight of 30 Foot of Water (according to the Barometer) the lateral Preffure against A B would appear to be confiderably encreas'd.

But

But upon the whole Matter, we fhall fhow by the following Calculation, that the lateral Preffure of Water in the Veffel A BDC upon A B, is not fo much increas'd by the Weight of the Air above A C, as that the Force which A B withstands, or which fhould prefs inwardly against C D (like the Weight in the foregoing Ballance, Fig. II.) could be fenfibly augmented thereby.

Let the Veffel ABDC (Tab. XXVII. Fig. 4.) be fill'd with Water up to A C; above which let us fuppofe a Column of incumbing Air as high as oooRooo; we know that the faid Air being alfo on the other Side of AB, at A, B, i, n, will prefs likewise against A B laterally from the Side of ni.

Now to find what Preffure AB undergoes by the Water on the Side D C, and by the Air on the Side n i, and how much the first Pressure exceeds the laft:

Let us call the Weight of the Air gravitating upon each Point in the Planen N, to wit, n O, mO, EO, &c. by the Name of a.

Let the Gravitation of the Water K F on the Point F be b, then L G is 2 b, and M H 3 b, &c.

Let the Air-Weight of kf on the Point ƒ be c; then is Lg, 2 c; m b, 3c, &c.

Whereupon (according to the Calculation in Sect. LI. if we do here alfo fuppofe five Points of Preffure) the Force of the fuperior Air, and of the Water in A B C D, which prefs A B laterally towards ni, will be 5 a with Iob. And on the contrary, the Force of the upper Air, and of that Air which is at ni BA, which prefs'd A B laterally back towards D C, will be 5 a with 10 c. So that these two Powers preffing laterally against each other, being drawn by each other, the Force wherewith A B is prefs'd laterally towards n i, will be rob lefs 10 c.

Now

Now without the fuperior Air, the lateral Preffure of the Water (according to Sect. LI and LII.) would be equal to 10 b; and c is equal to about ... part of b, if we fuppofe Water to be 1000 times heavier than Air. So that the furrounding Air does only leffen the lateral Preffure of the Water part; which in the foregoing Experiment can make no fenfible Alteration, and confequently the abovesaid Difficulty is obviated.

SECT. XLVI. The Augmentation and Diminution of the refifting Force of Fluids produces Motion, and the Force thereof.

L. HITHERTO we have confidered the up'ward and downward Preffures of Fluids which are quiefcent or ftagnant, either by reafon of Powers really acting upon and against each other, or elfe by the Refiftance and Obstructions of Motion; we should now naturally proceed to inquire into the Powers of thofe Fluids that are in actual Motion; but forafmuch as this would require a whole Book to account for it, and fince what we propos'd to fhew here concerning the Law of Altitude or Depth, may likewife be conceiv'd by the Preffure of stagnating Fluids; we will not inlarge this Digreffion, which to those who understand Hydroftatics, may feem already much too long.

We fhall only observe further concerning the Powers preffing upon each other, that if in Tab. XXVII. Fig. 5. the Tube b kg is equally fill'd with Water up to a and f, fince d is prefs'd upwards by a b, and downwards by fd, the Part d will quiefce or ftand ftill, if the Powers ab and df are equal; but if one of 'em be leffen'd to e d or the other increas'd to hb, d will be removed towards that Side where the Force is fmalleft, or in this cafe tog; and even with as much Force, as

the