they alledge That the intendment thereof at first was no more but this That the tendency of a heavy Body being to the Earths Center when there it is its Heaviness if not otherwise pressed will not endeavour any further motion for to move further were to move from the Center And accordingly if the tendency of any other Body be to a certain place as its term when there it is that Principle will not endeavour a motion from thence for so to do were to move contrary to its own nature And if it be carried further it must be from some other cause as when a Pendulum swings beyond the perpendicular it is not from weight simply considered which would there have stayed but from an Impetus imnpressed by a precedent motion And thus far that Principle is just and good But the Objection perverts it to a sense never intended by the first Introducers Next I would ask What is meant by the Waters own Place And particularly Whether water in a Pond artificially contrived on the top of a Tower be in its own Place If so then though a hole were in the bottom it ought not to run out If not in its own place then the Reason fails for even there a Diver shall no more feel the weight of the water than if in the Thames So that it is not its being in its own Place but somewhat else that makes the weight not to be felt To avoid this therefore and the like Instances they now explain their meaning to be That it doth not Gravitate on any thing which is not specifically lighter than it self And to this Explication it is that we are to apply our Answer But neither will this hold For it is manifest to use an ordinary Instance that a Vessel pierced near the bottom in Fig. 12 13. will run with a fuller and stronger stream than if at the middle or near the top and more when it is full than when half out or almost empty Which argues a Pressure of the upper parts upon those near the vent And to say they press not on the intermediate parts but onely on the Air without is a meer evasion For the remoter parts of the water cannot press that Air but by pressing that which is between like as in a crowd he that is at a distance cannot thrust him that is at the door but by thrusting those that are between And with a Pole we cannot thrust that at the end of it but by thrusting the Pole Nor with a Rope draw that which is fastened to it but by drawing the Rope Where yet there is a signal difference between Trusion and Traction In Trusion it sufficeth that the thing be contiguous though there be no Connexion but in Traction there must be a Connexion and that strong enough else the string will break and the weight not follow And though a Heap of Sand will suffice to press down the Scale yet a Rope of Sand will not serve to draw it up And therefore Mr. Line 's Funiculus in his Explication of the Torricellian Experiment must have somewhat of Texture as well as Contiguity to give it strength without which it will not be able to sustain the weight of the suspended Quick-silver But certainly if the parts of a Fluid be able to Draw one another much more will they be able to Thrust one another that is the one to Gravitate upon the other It is therefore much more conceivable in the inverted Syphon how the water at A Fig. 9. should thrust up that at B than how the Air at E ascending should draw up the water at B and thereby draw down that at A. For in the first case there needs only a Contiguity in the latter there must be a Connexion of all the Parts And therefore if we should allow that Mr. Line 's Funiculus or Rope of Sands if granted would equally solve the Phaenomenon by way of Traction yet since the Hypothesis of Trusion as is acknowledged will do it also it is much rather to be chosen than that of Traction by a Rope of Sands shall I call it or a Rope of Nothings But further it is confessed by a very learned Author the Author of two Treatises the one intituled An Essay touching the Gravitation or Non-Gravitation of Fluid Bodies the other Observations touching the Torricellian Experiment who is pleased to conceal his Name that defends the Funiculus and denies our Hypothesis that not onely Water but even Oyl in the Pipe A Fig. 5. will force up the water at B And if the Pipe being empty Oyl were poured on B it would force up water into the Pipe A not to a Level but to an Equipois that is as his own words are to such a proportion of height in the Tube as will countervail the weight of a like Cylinder of oyl and gives the same reason for it that we do The disparity of pressure causing Motion or Elevation of the water in that part nor equally pressed So that here a lighter Body doth Gravitate on a Heavier Oyl upon Water And that not onely ad pondus but ad motum as Himself admits that is in our Language it doth not onely Gravitate but Pregravitate not onely Weigh but Out-weigh So that here the notion of a Fluid not Gravitating on a Heavier than it self or one as Heavy is quite destroyed And it is manifest also that not the Level but the Equipois is that which is here attended For the surface of the Oyl without the Pipe because specifically Lighter will be somewhat Higher than that of the Water within it and just so much as to make up the Equipois And contrary-wise if that in the Pipe were Oyl and that without it were Water that within the Pipe would be higher and in such proportion higher The same would be if that at B were stagnant Quick-silver and that in the Pipe A were Oyl or Water or some Lighter Fluid A pound of Water poured into the Pipe would it self stand higher because it would take up more room but would raise the stagnant Quick-silver just as high as if a Pound of Quick-silver had been poured on without any respect had to the specifick Gravity or Levity And a Ship laden Fig. 8. will draw just as much water if laden with so many hundred weight of Timber as with so many hundred weight of Lead though that be Lighter and this Heavier than a like quantity of Water And a piece of Wood Fig. 7. though Lighter than Water yet doth not float on the very top but sinks so far into the Water till it possess the place of so much Water as is of equal Weight with it self that is till the Horizontal Plain passing by the bottom of the Wood be in all places Equally pressed partly with Wood partly with Water Which being known Experiments and confessed on all hands do quite destroy the notion of Non-Gravitation of Fluids on what is not specifically Lighter than
Bodies it is that I intend principally to speak Of which the first and great Phoenomenon is this That they will if undisturbed reduce themselves by their own weight to a Level that is to an Horizontal Plain or what as to sense is such and will so continue if either not pressed at all or equally pressed on all parts As if the surface by any means be Undulous as ABAB in Fig. 2. the Prominences at A will sink to fill up the Cavities at B till all come to the Level of LE. And this they will do partly by spreading abroad and flowing into those Cavities as lower places And partly the whole being fluid by pressing down what is under A and pressing up what is under B in Fig. 3. For though onely the former of these would happen in case all under LE in Fig. 4. were a firm Solid surface like as when water overflows the dry ground and fills up all the furrows and onely the latter in case such Prominences whether one or more were contained within Solid Pipes in Fig. 5. so as that they could not flow laterally into the adjacent Cavities Yet in the present case where both occasions happen both Causes will operate For Nature doth not work by Election but ad ultimum virium and all the ways it can where one doth not oppose the other And like as if a Vessel have two holes the one at the side the other at the bottom the water will run out at both So the Prominences at A being not hindred of either will partly by lateral Fluxion partly by direct Depression fill up the Cavities of B in Fig. 5. It 's true That a Solid Body having opportunity of both because by reason of the coherence of parts it can move but one way will move that way only which is most Declive But a fluid Body being partible in every Point divides it self every way as there is opportunity Now such fluid Body being thus reduced to a Level if undisturbed it will so remain in Fig. 6. For there be now no Prominences as at A in Fig. 2. to sink or flow down nor Cavities as at B to receive them Nor is any part of it more pressed than other whereby that should sink or this rise But if at some part as at D in Fig. 7. by weight or other force it be pressed but not in others or more at D than at others it will at D subside or be depressed and rise elsewhere in Fig. 8. And what is thus shewed of the Level LE holds equally of any other Level as F G. within the Fluid at what depth soever in Fig. 8. If all parts of it be equally pressed it keeps its level but if some parts of it be more than others those will subside and these rise because the weaker force must give way to the stronger The like happens in a Syphon inverted in Fig. 9. where if the water be higher in the one leg at A than in the other B that will sink and this rise till they come to a Level at LE And when so it will there rest if there be no other force to put it in motion So in an Ewer Fig. 10. or other Vessel with a nose the water in the Vessel if higher will sink it self till that in the nose be raised to the same height if that in the nose be higher this will sink and that rise till they come to a Level at LE. The Reason of it if we do not study to perplex the Phaenomenon is very evident Because while the Fluid supposing it uniform stands at the Level LE no part of the same Horizontal Plain at what depth soever is more pressed than other whereby it should be inabled to thrust any other out of place Upon the same account that of two Scales equally charged neither can descend or force up the other but do mutually sustain each other in Equipois and are at rest For though both do Ponderate yet neither doth Preponderate And no Power is able to over-bear another Power unless stronger than it But in case the Fluid be higher at A than at B the parts under A are more pressed than those under B and therefore those thrust these away On the same account that if the Scale A be heavier charged than B though both press downwards yet the heavier prevails and forceth up the lighter For of contrary Powers the greater always over-powers the lesser It will yet be not amiss that I may not in the sequel be mistaken to give notice by the way That what I have said of this Level in Heavy Fluides is not so to be understood as if this Level were in all cases Mathematically exact For though it ought so to be if nothing else did intervene than what we have hitherto taken into consideration yet many times some little accidents do disturb it As when a Drop of water on a dry board keeps a convex Figure either because of some little Viscosity therein or as shunning the contact of that dry surface and Quick-silver in a Glass-Pipe or like Vessel will have a visibly convex surface as shunning the contact of the Glass and the like would happen in water if the glass were greasie And contrary-wise the surface of water in such a clean Vessel would be rather concave and so I suppose would be the surface of Quick-silver if the Glass were guilded within because of its easie application of it self to Gold It is observable also that water in very slender Pipes will rise visibly higher than the surface of that in the broad Vessel because the Air can more conveniently apply its pressure on that broader Vessel than in the slender Pipe And Fluides will many times upon motion retain an Undulation or dancing up and down sometimes above sometimes below the true Level for a considerable time before they rest Upon a like reason that a Pendulum will swing back and forth beyond the Perpendicular on either side not by its weight simply considered which would rest precisely at the Perpendicular without rising on the other side but by reason of its contracted Impetus But these and other little inequalities which are to be accounted for from divers accidents we here neglect and consider onely what would be the result of the Gravity and Fluidity freed from such other Accidents too copious here to be insisted on Our meaning therefore is that setting aside other Accidents a Fluid Body will by its Gravity reduce it self to such a Level and being so reduced will so by counterpoise preserve it self if not disturbed by other Force But it is here objected That water upon water doth not Gravitate and the like of other not-springy Fluides because an Element say they doth not gravitate in its own place And for instance they tell us That a man under water feels not the weight of the water over him in Fig. 11. Before I directly answer this Objection I have this to say to the Principle
as Water And from him the Torricellian Experiment takes its name The Experiment is thus administred in Fig. 36. A Glass-Pipe closed at the bottom being filled with Quick-silver and then inverted the orifice being stopped with the finger or otherwise till it be immersed in a Vessel of stagnant Quick-silver and then opened If the height of the Pipe above the stagnant Quick-silver be not more than 29 inches or thereabouts it will remain full The cause hereof say they is Ne detur Vacuum For if the Quick-silver should sink there being no way for the Air to enter there would ensue a Vacuum which nature abhors The cause say we is Because the weight of the incumbent Air on A which we have already proved to be Heavy is equivalent to the weight of 29 inches of Quick-silver Which therefore being defended by the closed Glass which we suppose otherwise to be held firm from any other Pressure than its own Weight is by that Counter-pressure sustained But further If the height of the Pipe above the stagnant Quick-silver be more than about 29 inches that in the Pipe will sink to that height as at E leaving space above it in the Glass void of Quick-silver But whether filled by any other imperceptible Fluid we dispute not The Reason why it so sinks our Ancestors have not assigned Because they were not at all aware of this Phaenomenon but thought that ne detur vacuum it would remain full whatever the height of the Tube were Some Moderns with Des Cartes that they might avoid a Vacuum do imagine that a Materia subtilis of which no Sense can make any dicovery piercing the Pores of the Glass supplies that place But if it will so supply the place above E and give the Quick-silver leave to sink so low why it might not as well come-in to relieve the rest and so give it leave to sink to A I do not find Others with Linus imagine that the weight of 29 inches of Quick-silver doth stretch some part of its upper surface into a subtile matter very thin yet so as to fill that seemingly void space but because a less weight will not serve so to stretch it it falls no lower which stretched matter like a Funiculus holds up the rest ne detur vacuum But why this Weight should stretch some very small part of it so prodigiously thin and not stretch the rest at all rather than give some moderate Tensure to the whole they do not that I remember assign any Reason Others suppose this Funiculus to be made not by stretching the upper part of the Quick-silver but by squeezing out the more subtil parts from the whole body of it which like a vapour fills that seemingly void space but that less than such a weight would not so squeeze it and therefore it falls no lower But why it should so fall out that all Liquors whatever of never so different Texture should by the same weight be thus dissolvable and not rather some require a greater some a lesser weight thus to resolve or squeeze them they assign no Reason Yet we find so it is since that the lighter the Liquor is the greater height must be allowed and in such proportion greater to make up an equivalent weight But the cause is say we and it seems the most simple and unforced account because the Counter-pressure of the Air being equivalent to that of about 29 inches so much it is able to sustain but no more and just so much weight it will sustain whatever the Liquor be whether specifically Lighter or Heavier and whether of a more Firm or a Looser texture and therefore to such a height it sinks but no lower And had the Ancients been aware of what we find That the Air hath a positive Gravity and consequently though it be but small in proportion to that of other bodies yet a great height of Air may countervail a lesser height of a Heavier Liquor like as we see that a greater height of Water will countervail a lesser height of Quick-silver They would not I presume have troubled themselves with a Fuga vacui but said roundly That the weight of the Air at its full height is equivalent to that of Water at the height of about 34 foot and of Quick-silver at about 29 inches and proportionably of other Fluids And consequently when in the Pump or Syringe D by the Embolus or Sucker is defended from the Airs Pressure but A exposed to it in Fig. 34. this Pressure on A will raise over B so much weight of Water Quick-silver or other Fluid as is equivalent to that Pressure In the same manner as if A and E were equally exposed to the Airs Pressure a quantity of Oyl poured on A would have raised a weight of Water or Quick-silver equivalent thereunto The like account we give of the Syphon The Pressure on A in Fig. 35. will raise the Fluid to the height of B if not greater than what is before described and from thence to O it falls by its own Weight Yet so that if O were higher than A the Airs Pressure at O would thrust up O to B supposing the Pipe not so big as that the Air could conveniently pass by the Liquor into the Pipe and it would fall down to A by its own Weight For now BO would less gravitate than BA while yet the Airs Pressure would be much the same on both There is yet a considerable Objection to be removed viz. That Air in a closed Vessel though of no great height pressing on A the surface of the stagnant Quick-silver Fig. 37. will sustain as high a Pillar thereof in a closed Tube suppose AE as if A were exposed to the open Air Whereas yet the Weight of AD within the Vessel defended by the Vessel from the Pressure of the incumbent Air cannot be of equal Weight as if it had the whole height of the Atmosphere But the Reason of this is from the Airs Spring which is always equivalent to the Pressure lying upon it And consequently the Spring of the Air in its ordinary constitution with us must be equivalent to the Weight of the incumbent Air. For if it were less the Air incumbent would yet press it closer if it were more the Spring would relax it self by thrusting away what presseth it Which being so the Air included with such a Spring must therefore press with as great a strength as is equivalent to such a Weight Like as in other Springs if ACB in Fig. 38. be pressed by the Weight D to such a Tensure as to bear it and then this Spring so remaining the Weight were taken away and our hand put in the place of it it would press as hard against the hand as before it did to sustain the Weight that is with a Force equal to that of the Weight it sustained And if thus bowed it were put in a Vessel in Fig. 39. it would with just the same Force press against the sides of it And just so it is in the present Case where the air so included doth press by its Spring just with the same Force as was that of the incumbent Air which gave it this Tensure It is yet the more evident because if by the Air-Pump part of this Air be pumped out and thereby the rest less compressed the Quick-silver in the Tube in Fig. 37. will sink from E to a lower Station as to F or G and so lower and lower as more and more Air is pumped out and the Spring thereby relaxed That is as the Spring grows weaker so it is less able to support the Weight And this quite destroys the Evasions but now mentioned That the seeming void space is filled by a thin Substance which can by the Weight of 29 inches of Quick-silver or 34 foot of Water but not by less be stretched to that fineness and that therefore it will sink to that height but not lower For by this last Experiment when the Air is included with its ordinary Tensure it sustains the Quick-silver at the height of 29 inches as if less than that Weight were too little to stretch the Quick-silver into that supposed fine substance But when that Air by pumping is weakned it will sink to 20 10 5 yea less than 1 inch of height as if now less than the Weight of 1 inch were enough so to stretch it as less than 29 inches would not do before Yet is no alteration all this while made in the Texture of the Quick-silver but in the Tensure of the Air onely 'T is therefore from this different Tensure or Spring of the Air not from any difference in the Quick-silver that it stands sometime at a higher sometime at a lower station And what hath been thus said of this Torricellian Experiment is easily applicable to others of like nature And it is confessed that as the notion of Fuga vacui or that of the Airs pressure doth stand or fall as to this Experiment so must it do as to the others also I content my self therefore to have shewed it in this without expatiating to other particulars FINIS