the Difference of both Powers is, that act on each Side upon d. This may be eafily try'd, and wants no further Proof.

SECT. XLVII. Fluids are moved by, or rather after taking away á Refiftance, and the Force thereof.

LI. MOTION may likewife be produced, by removing a Refistance which obftructed Motion. For Inftance, blow into a Tube at g, which is fill'd in both its Legs with Water up to a and f, till the Water fubfides from f to e, and rifes from a to h on the other Side; then presently

ftop the Orifice g with your Finger, whereupon all will ftand ftill, and your Finger will become the Impediment or Refiftance of Motion. This appears by removing your Finger, upon which the Motion will enfue.

Now by what has been faid it is obvious, that whilft the Finger continues upon g, and the Fluid is stagnant, the Part d is prefs'd upwards by hb, and downwards by dg; and that the refifting Power or Finger is prefs'd upwards by the difference of the Powers hb and dg. So that this Refiftance or Finger being taken away, a Motion will be made towards the Side of the Refiftance; and even with that Force with which the Refiftance was prefs'd when the Fluids were quiefcent, unless upon removing the Refiftance, no other actual Force begins to operate.

SECT. XLVIII. Tranfition to Hydraulics, or fome few Hydroftatical Examples.

Now that Fluids do fo exactly obferve these Laws; that First, every thing which is deduced from thence by good and juft Confequences (all Circumstances being rightly obferv'd) is likewise

Expe

experimentally verify'd in the fame; and Secondly, that thefe Fluids, before they recede from this Law of Altitude or Depth, do moreover produce Effects which, to fuch as are unexperienced in Hydrostatics, appear to be so many Wonders, and of whofe manner of Operating, even the greatest Mathematicians acknowledge themselves ignorant, or at least uncertain, we will prove by fome few Inftances.

SECT. XLIX. Calculation of the Force of a
Syphon.

LII. THE First Inftance, that we may begin with one that is fimple, fhall be the Operation of a Syphon.

ABCD is a Veffel fill'd to the Brim with Water (Tab. XXVIII. Fig. 1.) in it there is placed a Curve Tube or Syphon E G H K, likewise full of Water, the Orifice whereof 1 K is for that purpose stopt with the Finger or otherwife.

If now you remove your Finger from I K, every Body knows experimentally, that the Water will run out from I K to Z, rifing up in the mean while in that Part of the Syphon E G which is shortest, and coming down in the longest HK, as long as the Water in the Veffel continues higher than the Mouth of the shortest Leg E F.

Now to know from the foregoing Principles, the Force and Manner whereby, and whereon this Operation of the Syphon is brought about:

Stop the Syphon again with your Finger at IK, by which means the Water in that, and in the Veffel will stagnate.

Suppofe then W X to be the upper Place of the Air which preffes here upon the Water, and pro

duce

duce the Horizontal Plane of the Water A D, thro' P Q to R S, whereof L M, NO, PQ and R S are equal Parts; thereupon according to the preceding Rules, the Part L M will be prefs'd with the Weight of the Column of Air incumbing on it.

Let us for brevity fake call the Preffure of Weight thereof upon L M, a, or if you please 100 Pounds more or lefs; especially if People be not used to this way of Letters.

Accordingly we will exprefs the Weight of the Water-Column PQIK by b or 10 Pounds, and that of the Air RST V, being of the fame Height, by cor i Pound.

Now fince L M, N O, PQ, which are all equal Parts of the fame Horizontal Plane A Q and all Water, and to all which we may suppose, that a Line or Thread may be drawn, without paffing thro' a folid Body, or any other Fluid befides Water.

And fince by the Action of the Syphon the Plane L M moves, or is press'd downwards, that of N O upwards, and that of P Q again downwards, if every Thing be reduced to Reft by ftopping the Orifice I K, the Powers whereby the faid Planes were prefs'd up and downwards will be equal, according the Sect. XLI, &c. and LM being prefs'd downwards by the Weight of the Air-Column L W M, that is, by a, or by 100 Pounds, NO will be prefs'd upwards, and P Q downwards by the fame.

If now we joyn to the Weight of this Air-Column of a or 100 Pounds which preffes P Q downwards, the Water-Column PQIK of bor 10 Pounds, by which I K is likewife prefs'd downwards; the Force or Weight that preffes IK will confift of a joyn'd to b, or of 100 and to Pounds, to wit, of the Air and Water-CoVOL. III.

Nnn

lumns

lumns together. And fo it is with this Force, that the Water gravitates downwards to Z.

If now the Horizontal Plane paffing thro' I K be extended to V, and TV fuppos'd equal to IK, then will T V be prefs'd downwards by the whole Air-Column TV X, that is by RSX of a or 100 Pounds (the fame being equal to LMW) and by R S T V of c or 1 Pound, that is, of a and cor 100 and 1 Pounds together.

Now with just so much Force (according to Sect. XLIII.) is the Part I K, or rather the Air preffing against IK, or the Finger (if we do not confider the thickness thereof) prefs'd upwards.

So that we see here two Powers preffing against each other on I K, or the Separation of the Water and Air, operating and acting against one another.

Of which, that that preffes I K downwards, has been found already to confist of a added to b,or 100 and 10 Pounds. And that which preffes 1K upwards, to be no more than of a and c, or 100 and 1 Pounds; fso that this last, a and c, or 101 Pounds, (i. e. the smallest Sum) being Subtracted from a and b, or 110 Pounds, the Remainder is b lefs c, or 10 lefs 1, that is 9 Pounds.

And this fhows the Force wherewith IK is prefs'd downwards more than upwards, and it is equal to the Weight by which the Water-Column RQKI, b or 10, exceeds the Air-Column PST V, c or 1.

So that if you remove your Finger from IK, and fuffer these two reciprocally graviating Powers to act against each other, it is plain, that the Water at the Orifice of the Tube IK, is by the lately mention'd difference of Weight between the two Columns PQK I and R S T V, that is b lefs e, or 9 pound Weight (fuppofing all the Num

bers

bers to be as above) prefs'd or protruded downwards to Z.

Thus we see the Force wherewith a Syphon flows, deduced from the aforefaid Principles, and the Fact is obvious to every one.

But we must beg our Readers to take notice here, as well as to remember hereafter, that we do by no means pretend that thefe Numbers of 100, 1, 10, &c. are the juft Proportions of the Weight of Water and Air; but that we only mean thereby, to fhow that a Column of Water is much heavier than a like Column of Air, and to confult the ease of those who are not accuftom'd to Letters in Calculations.

SECT. LI. Of a Fountain that Spouts or Springs higher than the Water that fupplies it.

LIII. Let us now propose an Inftance that is a little more compounded than the former.

How to make a Fountain whofe Stream rifes much higher than the Water above in the Cistern, which caufes it to Spring out without the Application of any Force Pumps, Bellows, or other Inftruments, and without any other Means whatever, faving the Gravity or Weight of the Water itself.

This may be done after the following manner. ABCD (Tab. XXVIII. Fig. 2.) is an open Ciftern, from which an open Tube N R is carried downwards thro' the Covering EH of another Cistern E F G H, fhut fo clofe that no Air can get in, paffes down to R, almoft to the Bottom of the Cistern F G.

From the upper Part of this lowest Cistern E H, there rifes a fecond Tube ST, paffing on almost as high as TD, or the Lid of a fecond Ciftern DCKI, which is likewife closed; and from thence there is again derived a third Tube to Nnn 2

L.MQ,