Subtraction of Air and many the like Experiments made by Mr. Boyle an Honourable Member of this Society in that Pneumatick Engine of his Invention But while I name these I do anticipate what I am next to handle which is the Compression of Springy Bodies WE have been hitherto discoursing of such Fluids principally as Water is supposed to be that is Fluids uncapable of Compression because not Elastical or Springy But Springy Fluids such as we suppose the Air to be may by an incumbent Weight not onely suffer a Trusion as Water may into another Place as from A to B and E in an open Pipe in Fig. 17 but a Compression into a less place As for Instance if the Pipe be close stopped at C or Hermetically sealed so as AB be Water and BC Air or other Springy Fluid a superfusion or addition of the Weight AD whether Fluid or Solid will raise B to E and contract the Air BC into the space EC that is so much as till the Spring in CB which was a Strength equivalent to the Pressure of AB becomes by this Contraction equivalent to the Pressure of DB. And if more yet be superfused on D CE will be yet more contracted and so onwards the strength of the Spring being still made equivalent to the pressure of the Weight For while the Spring CB is too weak the Weight being a greater strength will thrust it closer And if CE be too strong it will as a greater strength thrust away that Pressure And can never rest but when the strength of the Spring is just equivalent to the Pressure So in Solids If for Instance a Room or Vessel be filled with Wool as high as BB Fig. 25. and more Wool or other Weight whether Heavier or Lighter than Wool be laid on as to AA the Wool shall be depressed to LE and more yet if more Weight be laid on And in like manner if BCB be Air and this pressed either by the incumbent Air AB supposing Air to be heavy or by a Solid Weight or Force so close on all sides as that the Air cannot pass by or through it And this being granted the Torricellian Experiment with others of the same nature is confessedly solved by the Pressure of the Air which was anciently thought to be by a Fuga vacui For if the Air be heavy it must Gravitate that is endeavour a Descent as other Heavy Bodies do and actually Effect it if not opposed by at least as great a strength And the Spring of the Air allowing it to have a Spring must always be of such a Texture as is equivalent to the Weight or Force which it bears Now as to the Weight of Air or its Positive Gravity the Peripatetick Philosophy doth not acknowledge it but takes it to be Positively Light and consequently to endeavour an Ascent And some others say the same not onely as to Air but as to all Heavy Bodies And whereas we suppose in them a Positive Gravity and that what we call Levity is but comparatively so being onely Gravity in a less degree they take Levity to be Positive and Gravity to be but a less Degree of Levity and consequently those Heavy Bodies not to Affect a Descent but to be Thrust down by Bodies more Light which more strongly affect a Higher Place But against these the one and the other I apprehend as to Philosophy these Inconveniences which to me seem cogent Arguments If this Motion up-ward be natural it must be either an Aversness from the Center as the Terminus à quo or a Propension to some other Place as the Terminus ad quem If they say the Former it is true that then B ought to move from C in Perpendicular Lines as CBA Fig. 26 and the Phaenomenon doth not contradict it But if the first intendmnent of nature be not to be here without any Positive tendency where to be it seems much more intelligible that somewhat should Thrust it thence by somewhat more forcibly pressing between than that it should Fly thence without Affectation of any other Place But if they say as seems more rational if Levity be the Positive Principle that it is an Affectation of some Higher Place suppose A While B is just between C and A the motion 't is true would be in the Perpendicular CBA as the streightest way thither But if it were any where else as at D then its motion to A would not be in DCE the Perpendicular but in DA an oblique line Which is contrary to all Experience For the same Light body where-ever it be moves upward in a Perpendicular as well as a Heavy body in a Perpendicular downward And if to avoid this they would say That it moves not to a certain place as A or E determinately but to that place whatever it be that is just over it I say this is not properly the moving To a place if it be indifferent whether to A or to E but rather a moving From a place that is to be as far from C as it can Which is the former branch of the Supposition and against which we did before urge the former inconvenience Which makes it not likely that there is any such thing as Positive Lightness at all since it will be hard to assign what shall be the Terminus ad quem which such a Mover aims at But waving this Argument from Philosophy at large I shall argue from Experiment as to the Air thus Suppose we Air in the Bladder AA Fig. 27. of the same Tensure with the External Air and therefore such as will not as they speak Gravitate or as I would rather say Praegravitate thereon nor yet Praelevitate being of the same specifical Gravity or Levity with it If this be compressed into a less room as BB it will then retain the same Quantity of Gravity or Levity as before since all that Air is still here with all its positive Quality But because now within less Dimensions it will be Gradually or as now the Language is Specifically more than before Heavier or Lighter according as that Positive Quality was Gravity or Levity For as the same quantity of Heat in lesser space makes the subject Intensively Hotter so the same quantity of Heaviness in a less Room makes it Intensively Heavier and of Lightness Lighter But experience testifies as is confessed that compressed Air is Intensively Heavier or as we now speak Specifically Heavier and on the Ballance is found so to be not Lighter than before Therefore its Positive Quality was Heaviness not Lightness The Positive Gravity of the Air being thus evinced and consequently that the Air Ascends onely because it is Thrust up by Bodies more Heavy like as Water riseth upon the casting in of Earth or other Heavier Bodies the Torricellian Experiment with other the like Phaenomena are easily solved from Statical Principles without having recourse to a Fuga vacui For admitting as before that in a Vessel with a Nose
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
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
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
themselves And himself grants Essay p. 14. that Air in a Bladder doth Gravitate on Water To avoid the Pressure of these Evidences it is now alledged That the Oyl or Water in the Pipe A in Fig. 5. though not intrinsecally Heavier yet it s Higher Position gives it an Accidental weight more than that in the Vessel and hence it comes to pass that That doth depress This. But he doth not consider that this doth destroy the whole design of his second Chapter which is to prove That C doth not Gravitate on D nor D on E in Fig. 14 that is that the Upper parts of the Water do not Gravitate on the Neather Whereas if meerly a Higher Position will make it Gravitate and that not onely ad pondus but ad motum also then must the Water be in perpetual Motion the Upper parts still pressing away the Neather like as on another account it happens in Boiling Water I mean when the Fire is under it for if it be heated by Fire above it the case is much alter'd which perpetual Motion the said Author there urgeth as a great Absurdity Yet I am not ignorant that Mr. Boyle is indeed of opinion That in all Fluids the minute parts are in continual motion making this the specifick nature of Fluidity as contradistinct of Fixedness but that is on another account and concerns not this Point at all It is not therefore Safe for our Antagonist to ascribe it onely to the Accidental Weight of an Higher Position Nor is it Sound so to do 'T is true that a different Position may give to the same Weight a different Ponderation As for Instance a Weight at G in Fig. 15. will Ponderate more than at H not because Higher but because at G it is to descend directly but at H on an Oblique Plain which abates its Force and doth more abate it as it is more Oblique And a Weight at rest in F or E is of less Force to move the Balance than when from A it falls to E and less there than when it is fallen to F and even this less than if it had been violently thrown down Because in the latter cse there is a greater contracted Impetus Again at E or F it will Ponderate more than at I or K because those suspended at A are at a greater distance from the Center C than those suspended at D The different Position in all these and many other the like cases giving to the same Weight an Accidental additional Force But a Higher Position meerly because Higher gives no such advantage at all The Weight at E being but just of the same force as at F and at I as at K. For the length or shortness of the String on which it hangs doth not at all alter the Weight As is agreed by all and Experience testifies The Reason therefore of this Phaenomenon is not because that at A in a higher Position is of a greater Weight than a like quantity at B But because the parts at C in Fig. 5. are more pressed than those at B as bearing the weight of CA which B bears not whereby C is pressed down and B thereby pressed up BUt against this Explication he brings an Experiment on w ch he lays great weight A Porringer filled with Lead c. which in the Air as at A weighed 78 ounces weighed in the water about 68½ ounces and the same Weight it held with some inconsiderable difference which he excuseth whether at C the depth of 40 or 25 inches or at D the depth of but 12 or scarce 1 inch Fig. 16. Where he attempts the account of two Phaenomena First Why it weighs less in Water than in Air And Secondly Why it weighs alike at several depths in Water Why it should weigh less in Water than in Air he ascribes to the Resistance and Crassitude of the Water And he tells us elsewhere that if we strike with our hand the surface of Water we shall find its Resistance not much less than if we struck a Board By which if he mean the Viscosity or Resistance to Separation he speaks not to the present purpose For as to that it is to be so far considered as a Firm Body not a Fluid which is that we are now speaking of But if he mean a Resistance to be displaced and thrust upward to make way for the Porringers descent he says just the same thing with us For such Resistance is properly Gravitation and doth countergravitate to that of the Porringer and take off so much of its Praegravitation Just as when the Scale B Fig. 16. by its Gravitation resists the descent of A because A cannot Descend without the Ascent of B to which by reason of its Gravitation it is averse And because the Porringer cannot descend but by thrusting up so much Water the Water must needs give so much resistance to this Descent as it gives to its own Ascent that is so much as the weight of the Water that must Ascend and hath just the same effect as if so much Water were put into the Scale B. And just so much the Porringer weighs less in the Water than in the Air. And as to what he says of the great resistance which the hand finds when we strike hard on the Water we are to consider not onely the Weight of the Water but the Swiftness requisite to make way for the Hand moving so Fast Like as if a Weight of 10 pounds hang in the Air by a Thread the least touch of the finger will move it slowly But to move it 10 times so Fast will require a force 10 times as strong And if you strike it hard with a swift stroke of the hand that which made very little resistance to a gentle touch will considerably withstand the stroke of a swift hand Not because the Weight is 10 times heavier than before or doth 10 times as much resist Motion but because it doth 10 times as much resist a Motion 10 times as Swift Now so much Strength as is requisite to move so much Water with so much Swiftness as is necessary to make way for so swift a Motion of the hand so much resistance must the Water give to such a stroke from its own Gravity without the assistance of the supposed Crassitude or Viscosity But when in the present case we consider how much the Porringer weighs in Water we consider onely whether it Remove so much Weight though never so slowly not with what swiftness it will remove it and as to that a very little weight more than what it moves will suffice But his main Objection lyes in the other Point That the Porringer weighs as heavy at D the depth of 12 or but of 1 inch as at C the depth of 25 or 40 inches Fig. 16. And just so say I it ought to be For every thing weighs in Water just so much as its Weight is heavier than so much Water As for Instance
if the Plain δδ Fig. 16. be in all parts equally pressed it is confessedly the same as if not pressed at all for so long there is no reason why one part should rise rather than another And so it would be if D were just as heavy as so much Water But if D be heavier then is that part of it over-charged just so much as D is heavier than so much Waters as would fill the place if this were absent And therefore if not relieved by so much Weight in the Scale B it will sink And just so much will serve at C that is it must weigh equally whether at the depth of C or D or any other depth But saith he if the incumbent Water do Gravitate on D it will more Gravitate on C because at a greater depth True it doth so But as the pressure at C is greater than at D so is the Counter-pressure at χ more than at δ and just so much more So that whatever was the Pregravitation at D must be the Pregravitation at C also And it is the Pregravitation onely that is Weighed Just as when the Scale A outweighs B by 5 ounces and into each Scale you put 10 Pounds it will yet outweigh but just 5 ounces as it did before So that his Argument from this Experiment will not hold against us And the Solution he gives will hold as little It is saith he because the Porringer drives up no more Water out of its Place at the one Station than at the other But this is a mistake For while the Pillar α C Fig. 16. to make room for the Porringer drives away the Water from C to χ that at χ thrusts up all above it as high as α to make room for it self as α D doth all that over δ So that the Water displaced is not the same in both And therefore the Porrigner if not assisted by the incumbent Water would not equally weigh in different depths contrary to his own Experiment Which therefore makes against himself But the great plausible Objection is that a Man under Water feels not the weight of it And why saith he but because Mans body being heavier than so much water the water doth not Gravitate on it But this Reason is as the Schools speak non causa pro causa If the Question were Why the water doth not Raise the Body as it would do so much Wood the Reason had been good Because so much water doth not press downward more than the Body doth and therefore is not able to press it away But when the Question is Why a man doth not Feel it that is Why he is not Hurt by it or put to Pain the Answer Because specifically Lighter will not serve For 1. A Man by this Reason should not feel the weight of Wood because proportionably lighter than himself Yet we find a Man will as much sink under a Load of Wood as a Load of Lead if of equal Weight And if it be said This is because though the Man be not yet the Air about him is Lighter than that Wood I say it is so but this should therefore cause onely a Lateral Pressure on that Air not a Direct Pressure on the Man And though a Man stood up to the neck in Water he should yet find the burden of the Wood laid on his shoulder notwithstanding that both the Man and all about him be Proportionably heavier than Wood. And he shall equally feel it as if it were an equal Weight of Lead if both be above the Water So that the circumjacent Air is not that which makes the Wood weigh upon the Man 2. Though the whole Man be Heavier than so much Water yet many Parts of him are Lighter and would of themselves swim in Water though by their Connexion with some Heavier they be made to sink like Wood tyed to a piece of Lead Now all these Parts at least ought to feel Pain if the specifick Gravity were the onely cause of Indolency But do not 3. A Man immersed in Quick-silver which is a Heavier Fluid though he would thereby be boyed up yet would he no more feel the incumbent Weight than a like weight of Water And though the Experiment cannot so conveniently be made in Quick-silver as in Water yet as to part it may be made by thrusting the Hand into Quick-silver which shall no more be pressed by it than if thrust into an Equivalent depth of Water that is about 14 times as deep And Flyes or other small Animals immersed in Quick-silver are not thereby pressed to death but do safely emerge to the Top. So that it is but a Fansie to think that onely the Proportional or Specifick Lightness of the Water is the cause of that Indolence since Liquids Proportionably heavier if not Positively heavier will be felt as little 4. Let us suppose an inverted Syphon Fig. 17. filled from A to B with Quick-silver from thence to C with Water so high as to ballance the Quick-silver at A. If now Oyl which is Lighter than either be poured on A I ask Whether the Quick-silver at A will not be thereby depressed and that at B and C raised Certainly it will But why The Oyl cannot by their Principles Gravitate on AB because this is Quick-silver Nor yet as they speak mediately upon BC for even this is Water and therefore heavier than Oyl No nor on the Air above C for the Oyl at DA is already lower than it and therefore cannot affect to possess its place It should therefore by their Principles not gravitate at all since there is nothing below it lighter than it self on which it should gravitate Yet gravitate we see it will and thrust out of place that whole Body ABC notwithstanding if that be considerable the higher Position of C and its greater Specifick Heaviness And all this while the Animal in BC shall remain unhurt notwithstanding there be not onely Gravitatio ad Pondus but Gravitatio ad Motum too So that the notion of Non-Gravitation on a Fluid not specifically Lighter than it self is quite out of doors And the truth is supposing ABC to be in Equipoise the superfusion of AD will equally depress A whatever the Liquor be if the Weight be equal And Ounce weight will still be an Ounce weight and an Ounce weight will just so much depress the Quick-silver whether it be an Ounce of Wine Water Oyl or Quick-silver that is just so much as to thrust half that Weight out of the Leg AF Fig. 17. into the Leg FC without any regard had to the specifick Gravity or Levity of the Liquor AD which as to this Point is of no consideration at all And if the higher Position of D above A be thought of moment the higher Position of C above both must be so too And there will be nothing steady to fix upon but that the Positive Weight of DF being at least in Proportion to the bigness of the Pipe
abated But here he proceeds upon a mistake also For though it be very true that Water will flow upon a Declivity yet not as Fluid but as Heavy For we see a Bowl runs down a Hill though not a Fluid but a Solid Body And a broad Solid lying on a narrow Pillar in Fig. 22. hath in every part a Lateral Pressure as well as Water and if it be cut in the middest will fall off on either side as Water would do And when it doth not the reason is not a want of Propension but because this Lateral Propension is checked or impeded by a Greater strength of Cohesion like as its Perpendicular Propension is checked by that greater strength of the Pillar And like as the Pillar if too weak will break under the Perpendicular Weight so if the strength of Cohesion be less than its Lateral Propension the Solid will divide as a Fluid would do As when a Solid breaks by its own Weight in Fig. 23. On the contrary Water in a Pail or other Vessel though a Fluid hath its Lateral Propension Restrained by the sides of the Vessel as by a greater strength but doth not Lose it and if the sides chance not to be strong enough will break through doth at least endeavor it though they be strong enough So that both in Solids and Fluids each Particle hath its Lateral Propension as well as Perpendicular though it be sometimes restrained or over-powered there by the Cohesion of Parts here by the Strength of the Sides but in both Cases if those Strengths be too weak that Propension prevails Now as this Lateral Propension of Fluids is kept in by the Sides of the Vessel as to the utmost parts of it so as to the inner parts of it they keep in each other The Lateral Pressure of A Fig. 24. is sustained by that of B and this by that not as by Greater but as by Equal Strengths For A cannot thrust away B without thrusting up a Body as Heavy as it self nor B thrust away A. So that the Lateral Pressure of the Parts being mutually sustained each by other and the Perpendicular Pressure by the Parts under it hence it comes to pass that those under-parts bear onely the Pressure of a Column and no more which is the Expedient that I intimated but now And therefore in the Heap of Wheat but now mentioned though upon an aperture in the Bottom more fall down than such a Pillar because when that is gone the Lateral Pressure of the rest doth operate yet while that Pillar was there that part of the Bottom did bear no more but it But if these Expedients of his do not serve What is the Reason you will ask that the Man under Water feels not the Weight of it I would Answer First That it is not agreed that at a great depth a Man shall feel no Pain at all And I hear that Mr. Gratrix having contrived a way of taking breath at a great depth under Water through long Pipes reaching to the top of it yet found his breast there so compressed by the water that he could not draw breath But in small depths of water I do not deny but that a Man may remain for some time without any considerable Pain The Reason I judge is this Because the man incompassed by a Fluid whether specifically Heavier or Lighter than himself it makes no matter is equally pressed on all sides and thereby suffers no luxation of Parts and consequently no sense of Pain But upon the luxation or laceration of any part especially a Nervous part Pain ariseth Hence it is that our Flesh feels not the hardness of our Bones because so fitted thereunto as to suffer no luxation or laceration by it But if the Bone be broken or dislocated we shall then find it to hurt us and feel it hard and sharp And though the Body by such compression may be contracted into a less room by reason of the Air Blood and other springy Liquids yet these being all uniformly pressed without any tearing of the Nervous parts he suffers nothing of Pain from it And hence it is that the Egg-shell but now mentioned though pressed by a Body specifically Heavier than it self by which therefore according to their Principles it ought to be crushed receives no prejudice because equally pressed on all sides Which it doth the more easily sustain by reason of its round form in the nature of a continued Arch. And we find in experience that a Round Glass though but of equal thickness will bear a much greater Pressure from without than from within and more than if it were flat-sided and more if the Pressure be of all sides than if but in some onely All which concur in the Egg-shell so situated But if pressed onely upon one side a less Pressure would break it I add also That though in perfect Fluids there be no such Arching yet in a Heap of Solids as that of Wheat something there is of that nature and the more as those Grains be bigger and conveniently shaped and may therefore help to bear the burden Like as 4 or 5 Legs in the Table we mentioned if strong enough will supply the defect of one weak one which therefore is not broken though not strong enough of it self to bear its part But the more any such Heap approacheth to the nature of a Fluid the less is there room for such Arching and in perfect Fluids none at all Hence it is also that a Spunge though Lighter than Water and Flaccid also will not yet though fastened to the bottom of a Vessel be crushed together by the Weight of the incumbent Water Because the Water within its Pores doth bear out the sides with as great a strength as that without would press them in And the like we see when the Lungs taken out of Animals are immersed in Water And the same account serves for the pressure of Air on Animals The Air within pressing as strongly outward by its Spring as that without presseth inward there is no hurt to the Animal at all And contrary-wise the pressure of the Air into the Mouth and Throat doth not break open his Brest or Belly because ballanced with as great a Pressure without But if a Hand or Arm be put into the Air-Pump and the Air about it pumped out that there be a failure of the outward Compression to ballance that within the Arm the Spring of that within it will put the Arm to a great torture as divers of this Society have found by experience And many Animals by that means have been killed within the same Pneumatick Engine in a much shorter time than would have been for want of Respiration onely The like is seen in the breaking of Glass Bubbles hermetically sealed and of Lambs Bladders in the same Pneumatick Engine upon the Subtraction of the Ambient Air as also the boiling of warm Water and the strange expansion of Blood into Bubbles upon such
or a Syphon inverted Fig. 28 29. the Fluid at A by sinking it self will raise that at B to the Level LE then in case the Nose at B be not so high the Liquor if not otherwise stopped must needs run over And if any should say the Reason hereof is Because the Air at B flies away by its Levity and the Water follows to avoid a Vacuity he would hardly be assented to by those who see a visible Weight or Force at A to over-press it and thrust it out And for the same Reason if the Nose or Pipe before it comes to the height of E be recurvate Fig. 30 31. and turned down to O that which would have run over at B will now run out at O being thrust up to B by the Weight of A and falling down from thence by its own Weight But in case A be lower than B Fig. 32. and the Fluid uniformly Heavy A will not be able to drive it up to B much less make it there run over or turn about to O But contrary-wise if it were full to B this would praeponderate and raise that at A. Yet if AC were a heavier Fluid suppose Quick-silver CB a lighter as Wine or Water the Effect would follow as before till the greater Height of CB do countervail the greater Heaviness of AC And contrary-wise if AC be specifically lighter than CB Fig. 33. suppose that Water and this Quick-silver then must that be in such proportion Higher than this or else it will not rise to B nor run out at O. But if AC be higher than in such proportion the Effect will follow from the Praegravitation of A without having recourse to a Fuga vacui And thus far the Ancients would agree with us For they never flye to a Fuga vacui so long as there is visible Weight or Force to Thrust up the Fluid BUt that which gave occasion to introduce this Notion of Fuga vacui were but these Two Experiments and such as are reducible thereunto wherein for want of a Force to raise Liquids by way of Trusion they had recourse to this of Traction ne detur vacuum The first that of Suction in Pumps Syringes and other the like occasions The other is that of a Syphon whereby Liquors are carried over considerable heights above their Level For if the nose of a Syringe be immersed in Water as at B Fig. 34. and the handle or Embolus be drawn back the Water or other Fluid will follow it from B into D Which being contrary to the nature of a Heavy Body and no other Force appearing to thrust it up it was imagined that Nature abhorred a Vacuum and this made the Liquor rise contrary to its particular Propension To which Fuga vacui as it was wont to be called Linus of late and some others after him have given the name Funiculus And the like is to be said of all sorts of Pumps and other the like Engines which draw Water by way of Suction And as to the Syphon If the End C be immersed in Water or other Liquor Fig. 35. though B the top of the Syphon be much higher than A the surface of the Liquor yet if O be lower than A though it will not of it self begin to run yet if by Suction or otherwise it be set a running this Current will continue till either A be sunk so low as to let in Air at C or be lower than the outward Orifice O. The reason whereof say they since there appears not any Force to thrust it up must needs be this BO flowing out by its own Weight if CB did not follow it contrary to the Propension of its own Gravity a Vacuum must needs ensue which therefore they suppose Nature doth abhor For Answer I say First There being no other Foundation in Nature to prove this Abhorrence but onely these Experiments and this not otherwise known but being onely invented as an Expedient to serve a turn If we can otherwise solve the Phaenomenon and shew a Force which they did not think of there will be no need of this Expedient at all And this Abhorrence must be either gratis dictum without any cogent proof or some other evidence must be shewed for it than those who did introduce it were aware of For all the subsidiary proofs of late invented were not the grounds of introducing the opinion And therefore without disputing whether Nature can or cannot admit a Vacuum I shall onely shew That there is no need of that Notion as to this business Next That this Fuga vacui is not the cause of Water thus rising in a Pump or Syphon I thus argue For if so it ought to hold to any height whatever A Pump for Instance must draw Water an hundred foot high and a Syphon convey Water over the highest Hills or Towers For the Argument equally holds whether the height of B be two Foot or two hundred Foot if BO flow out and CB not follow a Vacuum must insue equally in either case And the Consequence of this Argument is so clear that in confidence thereof the Ancients did not doubt but that it would be so None that we know of till Galilaeo's time having ever questioned it or assigned any determinate height beyond which a Pump would not draw water or over which a Syphon would not convey it And it was a surprising discovery and wholly unexpected when about the end of the last Century it was first found out by experience that Water could not thus be drawn higher than about 34 Foot I say about 34 Foot not just so much because that alters with the temperature of the Air. When the Air is very light it will not much exceed 32 Foot when very heavy it may reach 35 Foot Which Experiment alone did evidently evince that the supposed Fuga vacui was not of an Infinite but of a Determinate strength Which put Galilaeo upon the inquiry Whether it were not from some other cause than Fuga vacui that it would be drawn so high but not higher And he happily lighted on this Hypothesis of the Counter-gravitation of the incumbent Air. The same hath been since improved by Torricellio and others after him who rationally argued that if such Counter-gravitation of the Air would countervail the Weight of 34 Foot of Water it ought in lighter Liquors countervail a greater height and a less height in Heavier And found upon Experiment that so it was If some little difference chance to be sometime discovered it is to be accounted for from some different constitution of the Air about us or other little accidents too many to be here recounted And particularly that as Water would be so raised about 34 Foot so Quicksilver to the height of about 29 inches and no more I mean 28 29 or 30 as the Airs temperature doth vary Which agrees with the proportion of the specifick weight of those Liquids Quick-silver being near upon 14 times as heavy