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Water as a Fluid that has no other upon or over it. But forafmuch as there is another fluid Matter which is ufually incumbent on it, namely the Air, and which in fome Cafes does often and notably exert its Power, as we have shown above, Sect. XIII; it will not perhaps be difpleafing to fuch as have not been much converfant in Hydroftatics, if we fhould here confider fome of the Properties of different Fluids lying upon each

other.

The Experiment therefore which we made fome Years ago for this Purpose, may be perform'd with all unmixable Liquors; as thefe two are in which we fee Brandy that has ftood a while upon Pot-Afhcs, to which we may joyn Oil of Turpentine for a third unmixable Fluid. We only

try'd the fame with two, to wit, with Pickle or Water, in which there was as much common Salt diffolv'd as could be, and with Oil of Turpen

tine.

h

We therefore took two little Sticks cd, and ef, and bound them to a Piece of Wood & (Tab. XXV. Fig. 3.) which, as in the foregoing Experiment, we placed upon the Horizontal Rim of a pretty large and deep Glafs-Veffel, fo that the faid Sticks made right Angles with the Piece of Wood, and defcended Perpendicularly into the Veffel; to one of 'em we ty'd the Curve Tube CGB, to the other the Streight one DE F, in fuch manner, that the upper Orifice of each, Cand D, was a good way below the Rim of the GlassVeffel, and the lower Orifices A B, and EF (which as the Tubes themfelves were of about the fame Size) were placed as near as poffible in the fame Horizontal Plane HE.

XX. Now to give a rough Notion of the Freffure of the Air; we pour'd fome Pickle into

the

the Veffel up to HI, which running into the Tube BGC, at the Orifice A B, ascended in the oppofite Leg up to the Height k 1, being Parallel to the external Liquor H I, and there it ftaid.

When we remov'd the Tube, as in the former Cafe, from one part of the Superficies of the Pickle HI, we always found that the Liquor within the Glafs remain'd unchangeably at kl.

From whence it appear'd, in cafe the Air gravitates, which we fuppofe here, that it preffes upon equal Parts, fuch as A B, a b, &c. of the Surface of the Pickle with equal Force: Since otherwife, if the faid Preffure were greater on fome Parts, and lefs on others, the Fluid kl in the Tube, would have rifen or fallen; if otherwife (which follows from what has been fhown already) the Air in the Tube C G incumbent upon kl, and higher, does continually prefs upon the faid kl with equal Force. But this by the way.

XXI. HAVING pour'd in more Pickle, up as high as LM, we found the Liquor in the Tubes to rife to x y and z 4; or as in the firft Experiment, to the fame Height as the external Liquor.

But when we pour'd upon the faid Pickle L M, Oil of Turpentine to the Height NO, the Liquor in the Tubes rofe from xy to no, and from z 4 top q; but remain'd below ON, or the Superficies of the Oil.

From whence we may infer,that a lighter Fluid, like the faid Oil NOLM, being incumbent upon a heavier, as the Pickle, produces both an upward and downward Preffure: Since A B must be prest more downwards to make the Pickle rife from xy to no; and E F with a greater Force upwards, to raise the Pickle from z 4 to p q, and keep it there fufpended. XXII. AFTER

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XXII. AFTER this we thruft down a Craner or Syphon with a long narrow Tube thro' the Oil NOLM, till it came below L M, or till it reach'd the Pickle; of which as we drew fome part out, we obferv'd, that the Depth of the fame Fluid in both the Tubes decreas'd proportionably, falling below n o and p q, but rofe again as more Pickle was put in.

The fame Thing happen'd upon taking out, and letting in again any part of the Oil N O.

And fo did the laft, with lifting upwards the Orifices of the Tubes A B, and E F.

From whence it appears, that in feveral Fluids incumbent upon each other, as well as in one alone, the Preffures which raise and fall the Parts of any Horizontal Plane in each of the Fluids, are likewife adapted to the Depths of the incumbent Fluids.

But they are no ways affected by the Breadth of Fluids, fince it is fufficiently known to all that understand Hydroftatics, that in cafe the Height of the Fluids over A B and E F continues the fame, the Effect would be the fame; and the Pickle at no, and pq, would continue at the fame Height in Veffels of all kinds of breadths.

XXIII. WE likewife found,that if we carry'd the Orifices of these Tubes A Band E F laterally along the Parts of the Horizontal Plane of the Pickle HI (the fame may be faid of the Oil too) the Liquor in the Tubes preferv'd the fame Depth or Height in every new Place upon the fame Horizontal Plane, as was obferv'd in the first.

From whence it may be concluded, as before, that all equal Parts of a Horizontal Plane, whether the faid Plane runs thro' the uppermoft Fluid, or thro' any other which has more Fluids in

cumbent

cumbent on it, are preft upwards and downwards with equal Force.

SECT. XXIX. Oil gravitates or preffes on Pickle, in the fame manner as Air does on Water.

XXIV. To reprefent now in a fhort Sketch, how Air operates upon a Fluid below it; we need only pour in Oil till it rifes up to PQ, or above the upper Orifices of the Tubes Cand D; which by that means running into the faid Tubes, it will caufe the Pickle within them to fubfide from no and p q, to xy and z 4, or to an equal Depth with the external Pickle at LM, just as if the Air prèst upon it.

SECT. XXX. The Greatness of the Preffure Upwards and Downwards in feveral Fluids incumbent upon one another.

XXV. Laftly, LET us again fuppofe NO to be the upper Superficies of the Oyl. Now to fhow with how much Force every equal Part of a Horizontal Plane HI, fuch as AB, EF, &c. are preft upwards and downwards, when there are feveral Fluids incumbent on each other; we pour'd Quickfilver into the Curve Tube A BG C, till it rofe to an equal Height in both Legs, namely, to A B, and kl. Then we pour'd into the great Veffel, Pickle as high as LM, and Oyl of Turpentine as high as NO, and fo thruft down the Tube B G C thro' the Oyl into the Pickle, as far as HI; and when all was quiet and fettled, we found that the Quickfilver had fubfided in one Shank from A B to TU, and rifen in the other from kl to rs.

This remaining fo, we filled another Curve Tube of equal Legs (Tab. XXV. Fig. 4) 7 8 9,

first,

first below to a certain Height (as here alfo A B and k) with Quickfilver, and bringing forwards the Curve Tube C G B, Fig. 3. close to the fide of the great Veffel; we measured with Compaffes as nicely as we could, the Height of the Pickle U W above the Quickfilver T U.

We also pour'd Pickle into the other equal Leg'd Tube 789, Fig. 4. till it rofe to the fame Height as WU above the Quickfilver that was in it, equal to the Pickle in the Veffel. After which, measuring in the like manner the Oyl in the Veffel at MO; we alfo pour'd Oyl into the Equicrural Tube to the fame Height WX; fo that the Pickle and the Oyl, as well in the faid Tube, as in the Veffel, were both fufpended at an equal Height above the Superficies of the Quickfilver TU.

I fhould add here, that in order to bring Pickle and Oyl into the Equicrural Tube Fig. 4, with thofe in the Veffel, you must take care you do not pour them in at firft to the Height required; because the Tube not being very large, when that which sticks to the Sides at pouring in comes to fubfide, it will caufe it to rife higher than in the Veffel. This may be obviated perhaps by first filling the Tube Fig. 4, and then by pouring Pickle and Oyl into the Veffel. They that have a mind may make the Tryal.

Afterwards having meafured with Compaffes the Height r u (which the Superficies of the Quickfilvers in one Leg, had above the Superficies TU in the other) in both Tubes in and out of the Veffel, we found the fame Height ru to be equal in both.

XXVI. FROM whence we plainly perceiv'd, that all the Fluids incumbent on each other in the Veffel above T U, and confequently alfo

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