Body or of some intervening Body which by its cohesion with it ought in our present case to be consider'd as part of it And thus Attraction seems to be but a Species of Pulsion and usually belongs to that kind of it which for distinctions sake is called Trusion by which we understand that kind of Pulsion wherein the Movent goes along with the moved Body without quitting it whilst the progress lasts as it happens when a Gardiner drives his Wheel-barrow before him without letting go his hold of it But I must not here dissemble a difficulty that I foresee may be speciously urged against this account of Attraction For it may be said that there are Attractions where it cannot be pretended that any part of the Attrahent comes behind the Attracted Body as in Magnetical and Electrical Attractions and in that which is made of Water when 't is drawn up into Springs and Pumps I need not tell you that you know so well as that partly the Cartesians and partly other Modern Philosophers have recourse on this occasion either to screwed Particles and other Magnetical Emissions to explicate Phaenomena of this kind And according to such Hypotheses one may say that many of these Magnetical and Electrical Effluvia come behind some parts of the attracted Bodies or at least of the little solid Particles that are as it were the Walls of their Pores or procure some discussion of the Air that may make it thrust the Moveable towards the Loadstone or Amber c. But if there were none of these nor any other subtil Agents that cause this Motion by a real though unperceived Pulsion I should make a distinction betwixt other Attractions and these which I should then stile Attraction by Invisibles But whether there be really any such in Nature and why I scruple to admit things so hard to be conceived may be elsewhere consider'd And you will I presume the freelier allow me this liberty if since in this place 't is proper to do it I shew you that in the last of the instances I formerly objected that of the drawing up of Water into the Barrel of a Syringe there is no true Attraction of the Liquor made by the external Air. I say then that by the ascending Rammer as a part of which I here consider the obtuse end Plug or Sucker there is no Attraction made of the contiguous and subjacent Water but only there is room made for it to rise into without being expos'd to the pressure of the supeiour Air. For if we suppose the whole Rammer to be by Divine Omnipotence annihilated and consequently uncapable of exercising any Attraction yet provided the superiour Air were kept off from the Water by any other way as well as 't was by the Rammer the Liquor would as well ascend into the Cavity of the Barrel since as I have elsewhere abundantly proved the surface of the Terraqueous Globe being continually press'd on by the incumbent Air or Atmosphere the Water must be by that pressure impell'd into any cavity here below where there is no Air to resist it as by our Supposition there is not in the Barrel of our Syringe when the Rammer or whatever else was in it had been annihilated Which Reasoning may be sufficiently confirm'd by an Experiment whereby I have more than once shewn some curious persons that if the external Air and consequently its pressure be withdrawn from about the Syringe one may pull up the Sucker as much as he pleases without drawing up after it the subjacent Water In short let us suppose that a Man standing in an inner room does by his utmost resistance keep shut a Door that is neither lock'd nor latch'd against another who with equal force endeavours to thrust it open In this case as if one should forcibly pull away the first Man it could not be said that this Man by his recess from the Door he endeavoured to press outwards did truely and properly draw in his Antagonist though upon that recess the coming in of his Antagonist would presently ensue so it cannot properly be said that by the ascent of the Rammer which displaces the superiour Air either the Rammer it self or the expelled Air does properly attract the subjacent Water though the ingress of that Liquor into the Barrel does thereupon necessarily ensue And that as the Comparison supposes there is a pressure of the superiour Air against the upper part of the Sucker you may easily perceive if having well stopt the lower orifice of the Syringe with your finger you forcibly draw up the Sucker to the top of the Barrel For if then you let go the Rammer you will find it impell'd downwards by the incumbent Air with a notable force CHAP. II. HAving thus premis'd something in general about the Nature of Attraction as far as 't is necessary for my present design it will be now seasonable to proceed to the consideration of that kind of Attraction that is employed to raise Liquors and is by a distinct Name called Suction About the Cause of this there is great contention between the New Philosophers as they are stiled and the Peripateticks For the Followers of Aristotle and many Learned Men that in other things dissent from him ascribe the ascension of Liquors upon Suction to Natures abhorrence of a Vacuum For say they when a Man dips one end of a Straw or Reed into stagnant Water and sucks at the other end the Air contain'd in the cavity of the Reed passes into that of his Lungs and consequently the Reed would be left empty if no other Body succeeded in the place it deserts but there are only that they take notice of two Bodies that can succeed the Air and the grosser Liquor the Water and the Air cannot do it because of the interposition of the Water that denies it access to the immers'd orifice of the Reed and therefore it must be the Water it self which accordingly does ascend to prevent a Vacuum detested by Nature But many of the Modern Philosophers and generally all the Corpuscularians look upon this Fuga Vacui as but an imaginary Cause of Suction though they do it upon very differing grounds For the Atomists that willingly admit of Vacuities properly so called both within and without our World cannot think that Nature hates or fears a Vacuum and declines her usual course to prevent it And the Cartesians though they do as well as the Peripateticks deny that there is a Vacuum yet since they affirm not only that there is none in rerum Natura but that there can be none because what Others call an empty Space having three Dimensions hath all that they think belonging to the Essence of a Body they will not grant Nature to be so indiscreet as to strain her self to prevent the making of a thing that is impossible to be made The Peripatetic Opinion about the Cause of Suction though commonly defended by the Schools as well Modern as Ancient
the Air which by its Spring would hinder the ascension of the Mercury as is easie to be tryed The Instrument having been thus fitted I caus'd one of the by-standers to suck at the shorter legg whereupon as I expected there presently ensued an Ascension of four or five Inches of Mercury in that legg and a proportionable Subsidence of the Mercury in the longer and yet in this case the raising of the Mercury cannot be pretended to proceed from the pressure of the Air. For the weight of the Atmosphere is fenc'd off by that which closes the upper end of the longer Tube and the Spring of the Air has here nothing to do since as we have lately shewn the space deserted by the Mercury is not possest by the included Air and the pulsion or condensation of the Air suppos'd by divers modern Philosophers to be made by the dilatation of the Suckers Chest and to press upon the surface of the Liquors that are to be suck'd up this I say cannot here be pretended in regard the surface of the Liquor in the longer legg is every way fenc'd from the pressure of the ambient Air. So that it remains that the Cause which rais'd the Quicksilver in the shorter legg upon the newly recited Suction was the weight of the collaterally superiour Quicksilver in the longer legg which being at the beginning of the Suction equivalent to the weight of the Atmosphere there is a plain reason why the stagnant Mercury in the shorter legg should be rais'd some Inches by Suction as Mercury stagnant in an open Vessel will be rais'd by the weight of the Atmosphere when the Suction is made in the open Air. For in both cases there is a Pipe that reaches to the stagnant Mercury and a competent weight to impel it into that Pipe when the Air in the cavity of the Pipe has its Spring weaken'd by the dilatation that accompanied Suction The Second point formerly propos'd which is That the weight of the Air is sufficient to raise Liquors in Suction may not be ill prov'd by Arguments legitimately drawn from the Torricellian Experiment it self and much more clearly by the first and fifteenth of our Continued Physico-Mechanical Experiments And therefore I shall only here take notice of a Phaenomenon that may be exhibited by the Travelling Baroscope which though it be much inferiour to the Experiments newly referr'd to may be of some use on the present occasion Having then provided an Instrument like the Travelling Baroscope mention'd under the former Head but whose leggs were not so unequally long and having in it made the Torricellian Experiment after the manner lately describ'd we order'd the matter so that there remain'd in the shorter legg the length of divers Inches unfill'd with stagnant Mercury Then I caus'd one vers'd in what he was to do so to raise the Quicksilver by Suction to the open orifice of the shorter legg that the orifice being seasonably and dexterously closed the Mercury continued to fill that legg as long as we thought fit and then having put a mark to the surface of the Mercury in the longer legg we unstopp'd the orifice of the shorter whereupon the Mercury that before fill'd it was depress'd 'till the same Liquor in the longer legg was rais'd five Inches or more above the mark and continu'd at that height I said that the Mercury that had been raised by Suction was depress'd rather than that it subsided because its own weight could not here make it fall since a Mercurial Cylinder of five Inches was far from being able to raise so tall a Cylinder of Mercury as made a counterpoise in the longer legg and therefore the depression we speak of is to be referr'd to the gravitation of the Atmospherical Air upon the surface of the Mercury in the shorter legg And I see no cause to doubt but that if we could have procured an Instrument into whose shorter legg a Mercurial Cylinder of many Inches higher could have been suck'd up it would by this contrivance have appear'd that the pressure of the Atmosphere would easily impel up a far taller Cylinder of Mercury than it did in our recited Experiment That this is no groundless conjecture may appear probable by the Experiment you will presently meet with For if the gravity of an incumbent Pillar of the Atmosphere be able to compress a parcel of included Air as much as a Mercurial Cylinder equivalent in weight to between thirty and five and thirty foot of water is able to condense it it cannot well be denied that the same Atmospherical Cylinder may be able by its weight to raise and counterballance eight or nine and twenty Inches of Quicksilver or an equivalent pillar of water in Tubes where the resistance of these two Liquors to be rais'd and sustain'd by the Air depends only upon their own unassisted gravity To confirm our Doctrine of the Gravitation of the Atmosphere upon the surface of the Liquors expos'd to it I will subjoin an Experiment that I devis'd to shew that the incumbent Air in its natural or usual state would compress other Air not rarified but in the like natural state as much as a Cylinder of eight or nine and twenty Inches of Mercury would condense or compress it In order to the making of this I must put you in mind of what I have shewn elsewhere at large and shall further confirm by one of the Experiments that follows the next namely that about twenty nine or thirty Inches of Quicksilver will compress Air that being in its natural or usual state as to rarity and density has been shut up in the shorter legg of our Travelling or Syphon-like Baroscope into half the room that included Air possess'd before This premis'd I pass on to my Experiment which was this We provided a Travelling Baroscope wherein the Mercury in the longer legg was kept suspended by the counterpoise of the Air that gravitated on the surface of the Mercury in the shorter legg which we had so order'd that it reached not by about two Inches to the top of the shorter legg Then making a mark at the place where the stagnant Mercury rested 't was manifest according to our Hypothesis that the Air in the upper part of the shorter legg was in its natural state or of the same degree of density with the outward Air with which it freely communicated at the open orifice of the shorter legg so that this stagnant Air was equally prest upon by the weight of the collaterally superiour Cylinder of Mercury in the longer legg and the equivalent weight of a directly incumbent pillar of the Atmosphere Things being in this posture the upper part of the shorter legg which had been before purposely drawn out to an almost capillary smallness was Hermetically seal'd which though the Instrument was kept erected was so nimbly done by reason of the slenderness of the Pipe that the included Air did not appear to be sensibly heated though for greater caution
supposes in Nature such an abhorrence of a Vacuum as neither has been well proved nor does well agree with the lately discover'd Phaenomenon of Suction For according to their Hypothesis Water and other Liquors should ascend upon Suction to any hight to prevent a Vacuum which yet is not agreeable to experience For I have carefully tryed that by pumping with a Pump far more stanch than those that are usually made and indeed as well clos'd as we could possibly bring it to be we could not by all our endeavours raise Water by Suction to above 36½ foot The Torricellian Exp t shews that the weight of the Air is able to sustain and some of our Experim ts shew 't is able to raise a Mercurial Cylinder equal in weight to as high a Cylinder of Water as we were able to raise by pumping For Mercury being near 14 times as heavy as Water of the same bulk if the weight of the Air be equivalent to that of a Mercurial Cylinder of 29 or 30 Inches it must be able to counterpoise a Cylinder of Water near fourteen times as long that is from thirty four to near thirty six foot And very disagreeable to the common Hypothesis but consonant to ours is the Experiment that I have more than once tryed and I think elsewhere deliver'd namely That if you take a Glass Pipe of about three foot long and dipping one end of it in Water suck at the other the Water will be suddenly made to flow briskly into your mouth But if instead of Water you dip the lower end into Quicksilver though you suck as strongly as ever you can provided that in this case as in the former you hold the Pipe upright you will never be able to suck up the Quicksilver near so high as your mouth so that if the Water ascended upon Suction to the top of the same Pipe because else there would have been a Vacuum left in the cavity of it why should not we conclude that when we have suckt up the Quicksilver as strongly as we can as much of the upper part of the Tube as is deserted by the Air and yet not fill'd by the Mercury admits in part at least a Vacuum as to Air of which consequently Nature cannot reasonably be suppos'd to have so great and unlimited an abhorrency as the Peripateticks and their Adherents presume Yet I will not determine whether there be any more than many little Vacuities or Spaces devoid of Air in the Cavity so called of the Pipe unfill'd by the Mercury so that the whole Cavity is not one entire empty Space it being sufficient for my purpose that my Experiment affords a good Argument ad hominem against the Peripateticks and warrants us to seek for some other Cause than the fuga Vacui why a much stronger Suction than that which made Water ascend with ease into the Suckers mouth will not also raise Quicksilver to the same height or near it Those Modern Philosophers that admit not the fuga Vocui to be the Cause of the raising of Liquors in Suction do generally enough agree in referring it to the action of the Suckers thorax For when a Man endeavours to suck up a Liquor he does by means of the Muscles enlarge the cavity of his Chest which he cannot do but at the same time he must thrust away those parts of the ambient Air that were contiguous to his Chest and the displac'd Air does according to some Learned Men therein if I mistake not Followers of Gassendus compress the contiguous Air and that the next to it and so outwards 'till the pressure successively passing from one part of the Air to the other arrive at the surface of the Liquor and all other places being as to sense full the impell'd Air cannot find place but by thrusting the Water into the room made for it in the Pipe by the recess of the Air that pass'd into the Suckers lungs And they differ'd not much from this Explication that without taking in the compression of the ambient Air made by the thorax refer the Phaenomenon to the propagated motion or impulse that is imprest on the Air displac'd by the thorax in its dilatation and yet unable to move in a World perfectly fill'd as they suppose ours to be unless the Liquor be impell'd into as much of the cavity of the Pipe as fast as 't is deserted by the Air that is said to be suck'd up But though I readily confess this Explication to be ingenious and such as I wonder not they should acquiess in who are acquainted but with the long known and obvious Phaenomena of Suction and though I am not sure but that in the most familiar cases the Causes assign'd by them may contribute to the Effect yet preserving for Cartesius and Gassendus the respect I willingly pay such great Philosophers I must take the liberty to tell you that I cannot acquiess in their Theory For I think that the Cause of Suction they assign is in many cases not necessary in others not sufficient And first as to the Condensation of the Air by the dilatation of the Suckers Chest when I consider the extent of the ambient Air and how small a compression no greater an expansion than that of the Thorax is like to make I can scarce think so slight a condensation of the free Air can have so considerable an operation on the surface of the Liquor to be rais'd as the Hypothesis I examin requires And that this impulse of the Air by a Suckers dilated Thorax though it be wont to accompany the ascension of the water procured by Suction yet is not of absolute necessity to it will I presume be easily granted if it can be made out that even a propagated Pulsion abstracted from any Condensation of Air is not so necessarily the Cause of it but that the Effect may be produc'd without it For suppose that by Divine Omnipotence so much Air as is displac'd by the Thorax were annihilated yet I see not why the Ascension of the Liquor should not ensue For when a Man begins to suck there is an Aequilibrium or rather Aequipollency between the pressure which the Air contained in the Pipe which is shut up with the pressure of the Atmosphere upon it has by virtue of its Spring upon that part of the surface of the water that is environ'd by the sides of the Pipe and the pressure which the Atmospherical Air has by virtue of its weight upon all the rest of the surface of the stagnant water so that when by the dilatation of the Suckers Thorax the Air within the cavity of the Pipe comes to be rarified and consequently loose of its Spring the weight of the external Air continuing in the mean time the same it must necessarily happen that the Spring of the internal Air will be too weak to compress any longer the gravitation of the external and consequently that part of the surface of the