A discourse of gravity and gravitation, grounded on experimental observations, presented to the Royal Society, November 12. 1674 by John Wallis ... Wallis, John, 1616-1703. 1675 Approx. 74 KB of XML-encoded text transcribed from 21 1-bit group-IV TIFF page images. Text Creation Partnership, Ann Arbor, MI ; Oxford (UK) : 2004-11 (EEBO-TCP Phase 1). A67384 Wing W574 ESTC R18644 12213272 ocm 12213272 56349 This keyboarded and encoded edition of the work described above is co-owned by the institutions providing financial support to the Early English Books Online Text Creation Partnership. This Phase I text is available for reuse, according to the terms of Creative Commons 0 1.0 Universal . The text can be copied, modified, distributed and performed, even for commercial purposes, all without asking permission. Early English books online. (EEBO-TCP ; phase 1, no. A67384) Transcribed from: (Early English Books Online ; image set 56349) Images scanned from microfilm: (Early English books, 1641-1700 ; 904:15) A discourse of gravity and gravitation, grounded on experimental observations, presented to the Royal Society, November 12. 1674 by John Wallis ... Wallis, John, 1616-1703. Royal Society (Great Britain) [2], 36 p. Printed for John Martyn ..., London : 1675. Reproduction of original in Huntington Library. Created by converting TCP files to TEI P5 using tcp2tei.xsl, TEI @ Oxford. Re-processed by University of Nebraska-Lincoln and Northwestern, with changes to facilitate morpho-syntactic tagging. Gap elements of known extent have been transformed into placeholder characters or elements to simplify the filling in of gaps by user contributors. 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ORDERED , THat a Discourse , made before the Royal Society , the 12 th of November 1674. by Dr. John Wallis , concerning Gravity and Gravitation , grounded on Experimental Observations , be Printed by the Printer of the Royal Society . BROUNCKER , Pres. R. S. A DISCOURSE OF Gravity and Gravitation , GROUNDED ON EXPERIMENTAL OBSERVATIONS : Presented to the Royal Society , NOVEMBER 12. 1674. BY JOHN WALLIS , D. D. A Member of that SOCIETY . LONDON , Printed for John Martyn , Printer to the Royal Society , at the Bell in St. Pauls Church-yard , 1675. A DISCOURSE OF Gravity and Gravitation , GROUNDED ON EXPERIMENTAL OBSERVATIONS , Presented to the ROYAL SOCIETY , The 12 th of November , 1674. IN Compliance with the Commands of this Royal Society , in order to the promoting of Experimental Philosophy , to present you a Philosophical Discourse , either grounded on , or leading to , Experiments ; the Subject I have chosen for this Discourse , is that of Gravity : the Being and Effects of which , is no otherwise known to us than by Experience , or Discourse grounded thereupon . The Subject is copious , and therefore I must single out some few Experiments out of a great many : And they shall be the most simple and unperplexed in their kind , that the Inferences may be the more clear and perspicuous ; and such Experiments onely , as are either commonly known , or have already been made before You , or may easily be , whensoever you please . I will not dispute the Nature of Gravity or Gravitation , what or whence it is ; whether from a Quality within , or a Pressure from above , or a Magnetick Traction from below : But shall take for granted ( what every days Experience testifies ) that there is , ( at least in this our Sublunary World ) such a thing as Gravity and Gravitation ; whereby those we call Heavy Bodies , have a natural Propension to move downwards ( towards the Earth , or its Center ) if not hindered by some more Potent , or at least an Equivalent , Strength . This Motion downward , we call Descent ; the Endeavour so to move , we call Gravitation ; and the Principle from whence this Endeavor proceeds , we call Gravity . And things are said to be more or less Heavy , as they have more or less of Gravity : Which may be understood , either Extensively , according to the Quantity of it ; as when we say a Pound is heavier than an Ounce , though that be Feathers , and this be Lead : Or Intensively , according to the Degree ; as when we say , that Lead is heavier than Cork , or Quick-silver than Water ; that is , gradually heavier , proportionably heavier ( bulk for bulk ) or ( as it is now wont to be called ) Specifically heavier . I say , the Endeavour thus to move , I call Gravitation ; though by reason of some Impediment , there be not any actual Descent . ( And it is allowed me , by those from whom in some other things I differ , that not onely Motus , but Conatus ad motum , is properly Gravitation . ) Which I thought necessary thus to define , that I be not misunderstood in the sequel . But I add further , this Endeavour of Descent , implies an Aversness to Ascent , with equal force ; and that the one and the other are equally the Effects of Gravitation . This Gravitation , or endeavour to descend , is one kind of Strength ; and may be opposed not onely by a contrary Gravitation , but by any opposite strength whatsoever ; whether by way of Impediment onely , or of contrary Force . For , though there be divers kinds of Strength ; yet they are all thus far Homogeneous , as to be compared each with other , as equal or unequal , greater or less , and that in any Proportion . Thus the Gravitation of the Scale A ( in Fig. 1. ) may be opposed , or hindered of its Effect , by a contrary Gravitation at B , ( supposing all the tackle strong enough ; ) or by a Force under it , which Thrusts it up ; or by a Force above it , which Pulls it up , ( or doth at least endeavour so to do ; ) as , for Instance , that of a mans Hand . Every of which , being contrary Forces , if equal to that of Gravitation at A , will stop its Descent ; if less , they will Retard it ; if greater , they will force it upwards : Not by making it cease to Gravitate ; but by defeating the Effect of that Gravitation . But it is opposed also by the Strength and Stiffness of the Beam ; ( for if that either break or bend , A descends : ) And by the Strength , though not the Stiffness , of the Strings that hold it , ( for if they either break or stretch , the weight will descend , at least in part ; ) or by the Hardness , Strength , and Solidity of the Floor or Table on which it rests ; which , if strong enough , will support it ; or , if the Medium be Viscous , this viscosity ( which is a degree of solidity ) will at least retard its Descent . All which do oppose it , not as contrary Forces , but onely as bare Impediments : Which , if strong enough , do hinder the Descent ; but , though more than so , do not Thrust it up . But if the Medium be supposed perfectly Fluid , in every Point , without any aversness to separation ; it may Hinder , or Retard the Descent of A , by a contrary Force , or contrary Gravitation , ( it self also endeavouring a Descent by its own Gravity , or at least to preserve its Station , against an Ascent ; ) but not as a bare Impediment from its Solidity , Firmness , or aversness from Separation ; which is supposed to be none . It hath a resistance to Motion , not , to Separation . And of such Heavy Fluides , I intend principally to discourse , and of Solid consistent Bodies , onely with reference unto such . If it be objected , That there be no such perfectly fluid Bodies ; but that those which we call Fluides , are either made up of very fine , but disunited , Atoms , ( each having its own shape and figure , though very small ) as the Atomists suppose ; or at least in some degree Unctuous and Viscous : I will not dispute that point , as not now necessary ; but only express what it is I mean by Fluid or Liquid Bodies ; and the nearer any thing comes to such a condition , the nearer it is to perfect Fluidity . And where such Viscosity is very little and undiscernable , we consider it as none at all . And even that which is , if it be not stronger than the incumbent weight can break , it doth not wholly hinder the Descent , but onely retard it : The weight sinks , though not so fast . Now , there is in all Heavy Bodies ( whether Firm or Fluid ) and in every part of them a Prospensity , not onely to a Direct Descent , but ( if that cannot be obtained ) to an Oblique Descent , according to any Declivity . ( For a River will run down-hill , and so will a Bowl also ; and a sloping Pole , if not supported , will fall obliquely . ) Which I the rather note , because I find some to put a great stress on the Lateral Gravitation of Fluides , as peculiar to them ; without taking notice , that the same is common to Solides also : The difference being but this ; in Fluides the parts be separable , but not in Solides : But the Tendencies are in both the same . Now , of Fluid 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 ) more than that of FC ; that will thrust this away , till they come to an Equipoise . It 's true , that , if the specifick Gravity of the Liquor AD , were greater than that of the Quick-silver in AB ; there would , upon another account , have been some difference : Because then , the Heavier Liquor being upmost , it would not onely press upon , but press into , the Body of the Lighter ; and they would by little and little shift places ; ( as when Water is poured upon Wine , that will by little and little sink to the bottom , and this rise : ) Because , by such Descent , each Particle thrusts up a Lighter Body than it self . But , if the Upper be Lighter ; though it press on the Hevier , it cannot press into the Heavier , without thrusting up a Heavier Body than it self . And this , I suppose , if they will consider their own Notion , is that they mean , when they say , A Lighter Body doth not gravitate on a Heavier . And if so much Oyl were poured on A , as to thrust the Quick-silver beyond F ; some of that Oyl would pass by it , into the other Leg , as high as C. And , in such Cases as these , the specifick Gravity or Levity is considerable : But not as to the Case in hand ; where an Ounce of Oyl poured on A , shall depress it just as much as an Ounce of Quick-silver would do ; and thrust up C just as High. Beside this , ( of Non-Gravitation on a Heavier Body ; ) the same Learned Author hath two Expedients for salving the Indolence of a Man under Water , or his not feeling Pain by the Weight of it . The First is this : Supposing a Brick-work , as in Fig. 18. but without Mortar ; if some few Bricks were taken out of the bottom , there would not hereupon sink a Pillar of that Base , but onely a Kind of Pyramid ; the rest being , in manner of an Arch , mutually supported . And thence he supposeth , that those middle Bricks did not bear the Weight of a Column , but onely of a Pyramid . Which Pyramid if taken away , the rest would not gravitate upon that Cavity . And in like manner he supposeth it must be , if , for Bricks , were Grains of Wheat ; yea , of Sand ; and , consequently , of lesser Particles ; and , even those of Water ; which he supposeth would thus support each other , without gravitating on those under them . But he proceeds upon several mistakes . First , he supposeth , That , because those middle Bricks being taken away the rest do not fall ; therefore , when they were there , they bore nothing of that Weight . Which is just as if he should argue , Because , when a Beam ( in Fig. 19. ) is supported by three Posts , if the middle Post be removed , it will not fall ; therefore , while it was there , it did bear nothing of the Beams Weight : Or , Because a Table , ( in Fig. 20. ) supported by five or six Legs , will stand , though any one of them be taken away ; therefore that Leg did bear nothing : And consequently , ( because that Leg is any Leg ) therefore none of them did bear any Weight : Whereas , while all were there , each did bear its part , and thereby ease the rest ; which , in the absence of one , must now bear the more . And if the whole space under it were filled up with such Supports , ( or , which would be Equivalent , an intire Body of that breadth , ) each would bear ( without any considerable difference ) just so much as what is just over it . And such is the case of Fluids . Onely this I add , That if one of the Legs should be too weak to bear its Proportional part , yet if the rest be as much more than able to bear their ; that weak one will not break , being relieved by the rest . Secondly , Admitting that in Brik-work it would so be ; yet it is onely upon this account , Because those parts of the Bricks which hang over , are coherent with the parts supported , and cannot fall without breaking the Brick : But , if they were as easily separable part from part , as Brick from Brick , ( which is the case of Fluids ; ) those parts would fall , as well as the middle Bricks : And consequently , not a Pyramid , but a Column , or rather more . Again , Thirdly , Whereas he argues , from Bricks to Grains of Wheat , and from thence , to Sands , The Consequence will not hold . For the shorter his Bricks are , the less will hang over in each Layer . As if now , for Instance , each Brick lye two Inches over ; if the Bricks were but half so long , each would lye over but one Inch ; and consequently ( supposing their thickness the same , ) the Pyramid on that Base would be twice as great ( because twice as tall ; ) and still , as the over-hangings decrease , that Pyramid increaseth ; till at length , when those over-hangings come to nothing ( which is the case of Fluids , ) the Pyramid becomes a Column , or even more than so . And if , in a Heap of Wheat , Fig. 21. ( as here in a Pile of Bricks , ) he remove so much of the bottom ; he will find , that instead of a Pyramid on that Base , there will fall down more than a Column ( part of an inverted Pyramid : ) And the more such Heaps approach to the nature of Fluids ; the more will it be so . So that , by this Argument ( if there were not another Expedient , of which I shall speak by and by , ) the lower parts must bear , not less , but more , than the Column incumbent on them . And , if he found it otherwise in a Tube filled with moist Calice-Sand ; this was not , because that above did not gravitate ; but because it was so wedged in , that it could not fall . Which , in perfect Fluids , we are not to suppose . Lastly , He doth , by this Explication , destroy his own Hypothesis . For he grants , in a Pail of Water ( for Instance , ) that all the Parts , as well upper as lower , do gravitate on the Bottom , though not each on other : Whereas , if those upper Parts be so supported ( as in his Brick-Arch ) as not to Gravitate on the Cavity ; much less will they gravitate on the Bottom , under that Cavity . And if , as he supposeth , a great Heap of Wheat would not break an empty Egg-shell ; it is not , because the Wheat wants Weight , or Gravitation ; but because the Grains are so intangled as not to fall right down , ( like as in a heap of Bushes , one would bear up another , though all do gravitate : ) But in Liquids it is otherwise ; which we suppose partible in every Point . But however , this of the Egg-shell happen to prove ; it serves not his Hypothesis at all . For , the Air in the Egg-shell , being Lighter than the Wheat that lyes on it ; this ought to gravitate ( by his own Principles , ) and to break the Egg-shell . If , by Arch-work , the Egg-shell be defended ; this is not for want of Gravitation ; but because that Gravitation is surmounted by a greater Strength . Like as when a great Weight hangs on a strong Tack ; or a heavy Scale , supported by as great a Weight in the other , or by a support underneath ; and a thousand other the like Accidents . His other Expedient is , from the Lateral Pressure which he supposeth all Fluids to have ; whereby he supposeth the Perpendicular Pressure to be 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 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 ; 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 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 .