A Discourse concerning the Air's Gravity, Obserudin the Baroscope, Occasioned by That of Dr. Garden; Presented to the Phil. Soc. of Oxford, by the Reverend Dr. Wallis, President of That Society. April, 14, 1685

A Discourse concerning the Air's Gravity, observed in the Baroscope, occasioned by that of Dr. Garden; presented to the Phil. Soc. of Oxford, by the reverend Dr. Wallis, President of that Society. April, 14, 1685. The Discourse of Dr Garden (read at our last meeting,) concerning the different state of the Air, in respect of its different gravity, hath in it a great deal of very ingenious speculation. And what I then said to it, upon the first reading of it, and what I am now laying again to the same purpose, is not to contradict it, or detract from it, but to add to it; as a notion which I have long since considered, and judge it capable of further improvement. The notion of the Air's weight and spring, hath been so well settled, by innumerable Experiments of this present Age; that hardly any considering Person doth now doubt of it. And it hath chased away from before it, the notion of Fuga vacui, formerly received; by shewing us an efficient cause, of those effects, for which before we could only pretend to a Final cause. The first occasion (that we know of,) of introducing it, was, from Galileo's discovery, that water, by Pumping, was not to be raised higher than about three or four and thirty foot (or not much more,) of our English measure. Which was a certain argument, that the cause of those effects (commonly ascribed to Fuga Vacui,) was but of a finite strength: whereas, if natures shunning a Vacuity had been the true cause, it was to have operated without stint. Whereupon this Lyncean Philosopher (as he was called,) did, out of his great sagacity, guess happily at the counter- ter-ballance of the Air's weight, as the true cause. And that therefore Air, which was before thought to be a light body, was but comparatively so, and had indeed a positive gravity, though less than that of other bodies which we are conversant with. This notion, was happily pursued by (a Successor of Galileo,) Torricellio; who did rationally argue; that, if the Air's counterpois was sufficient to raise and sustain water at that height, and but to that height; then must it be a just counterpois to a lighter Liquor at a greater height, but, to a heavier Liquor, at a lesser height. And making an essay thereof in Quick-Silver, found it to succeed accordingly: and, in a just proportion to the respective gravities of those fluids. And he hath, by this means, made the experiment (commonly called the Torricellian experiment,) much more manageable with Quick-Silver (in Vessels of about 29 Inches, English measure,) than before it was, with water, in much taller Vessels. In pursuance of this notion, we find, by several sorts of Baroscopes (or weight-wipers,) not only that the Air hath gravity, but that it hath a different gravity, at different times and places; according as its counterpois is able to sustain Quick-Silver at different heights: sometimes a little lower than 28 Inches, sometimes a little higher than 30 Inches, and at other times at some middle height between these, here with us. Which different weight of the Air, or Atmosphere, may reasonably be supposed, partly to proceed from, and partly (as Dr. Garden well intimates,) to give occasion of difference in the weather and winds. The particulars of his discourse (having not the Paper with me, and having but heard it once read,) I may possibly mis-remember. But, as to the main drift of it, (being a notion not wholly new to me,) I think I do not mistake. That there is, in our Air, a body more subtle than are the Fumes and Vapours mingled with it, in our lower region, and which with it do make up that heterogeneous mixture which we commonly call Air; I think to be very certain. But whether that subtile body, be (as Dr. Garden seems to suppose,) much heavier, then our common Air; I much doubt. And do rather think it is not; not having hitherto observed any cogent experiment, either to prove it heavy, or elastic. But it may, for ought I know, be void as well of weight as spring: and, what we find of either, in our common Air, may be attributed to the other mixtures with it. I know that this notion (of the greater weight of that subtler matter,) hath, by some, been made use of, to solve that (surprising,) experiment of Quick-Silver (well cleansed of Air,) being sustained in a Tube inverted (after the manner of the Torricellian experiment,) at a much greater height than the usual standard (of 28, 29, or 30, Inches, even to the height of 40, 50, 60, or even 70, Inches; being steadily managed, and with great care. But I confess I am not satisfied with that reason: and think that it may better be accounted for otherwise, or else we are yet to seek for the true cause of it. But we have no need of this particular notion, in the present discourse, and therefore need not be here solicitous about it: and shall therefore proceed without it. The Air being, as hath been laid, of a different gravity, in different times and places, (arising I suppose, from the different kinds and quantities of the Fumes and Vapours and other particles which are ingredients in it, and the different force of the Suns heat acting thereupon, increasing or allaying the spring thereof, and otherwise,) we are therefore to consider of the Air as a fluid who's parts are in some places heavier, and others lighter. And therefore much of a like nature as if they were different fluids, of different specific gravities (as the word is now a-days,) or (as it was wont to be called, and, I think better,) Intensive gravity, one from the other: (as, when we lay, an Ounce of Lead, is intensively, though not Extensively, heavier than a Pound of Cork.) Now when several fluids, or several parts of a fluid, are thus of different weights; they will (from the general nature of heavy fluids,) when undisturbed, change places with one another, till the heavier becomes lowest and the lighter highest. And this, not only as to the minuter parts; as is observed in Chymical precipitations, or the sinking of Sand in water, or its smaller earthy particles which subside in a muddy sediment, and the like of other Liquors when at rest, and the atoms (as they were wont to be called,) flying in the Air when disturbed, but subsiding in the form of dust when at rest. All which according as they be smaller do (Ceteris Paribus,) subside more slowly (as dust slower than a clod of Earth, and Filings of Gold slower than a lump of it, and thin Plates of Lead laid horizontally, then either a round Bullet of the same weight, or those Plates in a contrary position set edgewise,) and ought by statick principles (as is easily demonstrable,) so to do. But, much more, as to larger parcels. As when Oyl, Wine, Water, Beer, or other the like Liquors, are put together in the same Vessel; as will be observable to the Ey, especially when their colours are considerably different. Thus, if a Bucket of Water, be hastily taken out of a Pond, or a large Vessel full of it, the vacant place will be presently supplyed with Air (as the nimbler fluid,) & then with Water flowing (by a declivity,) from the higher parts, and (because heavyer,) thrusting out the Air (which had prepossessed the place,) till it come to a due Equipoise. And the same (from a like reason,) must happen, if some parts of the same Liquor, do accidentally acquire (by expansion or otherwise,) a greater degree of lightness than the other parts; those lightened parts ascending, the heavier subside. As when Water, Beer, or other thin fluids, be gradually heated by a fire underneath; the lower parts, being first warmed, ascending to the top, while the colder and heavier subside; whence we find, in such cases that Bubbles do arise, and that at the top is warmer than that at the bottom. But in case what is warmed, be of a thicker consistence, so as that the parts cannot readily shift places, that at the bottom will be hotter; and in case it be heated by fire over it, there will (I suppose,) be not such bubbling (or not so much of it,) that at the top being first heated. From such consideration as this, Dr. Garden doth well observe, that some parts of the Air being thus (by rarefaction, or increasing the spring thereof, or otherwise,) become lighter than others; these heavier parts, rushing into the places of those lighter, may cause a wind as from such parts; (in like manner as, on a like occasion, a Tyde or Current would arise in water;) and other accidents of a like nature. And contrarywise, on a contrary occasion. And this I take to be very true, (though such accidents happening very variously and uncertainly, will cause such confusion of motions, and disturbance of each other, that it will be hard to reduce them to a regular adjustment.) But I add thereunto, that the Earth's diurnal motion, compounded with its annual, (if we admit that hypothesis, as most do now a-days,) the one in some parts accelerating, in others retarding the other; and its difference in different times of the year, (by reason of the obliquity of the Zodiac to the Equinoctial,) and in different times of the month (because of the Moon's different position, which is an Appendance to the Earth's motion, and doth thereby differently affect it,) and according to the different place of the Earth and Moon, as to the Aphelion or Perihelion of the one, and the Apogæum or Perigæum perigæum of the other: seem to me to be of much greater consideration, not only as to the Ebbing and Flowing of the Sea, (which I take to depend principally thereon, but to be variously modified also by the various positions of the continent on which it beats,) but as to the Winds also: especially the briezes and Trade-Winds; which at certain times of the day, or of the year, or of the month, are observed to blow constantly (or most frequently,) from such or such a Coast. And I am not sure, that the body of Earth and Water (or Terraqueous Globe,) is exactly spherical (allowing only for the small inequalities of Hills and Dales, which in a body of that greatness, are inconsiderable,) but may have somewhat of an oblong spheroid, having a longer Axis from Pole to Pole, than at the Equator. And though this cannot be much; because of the Earth's shadow in the Moons Eclipse appearing circular, and the descent of heavy bodys being always (as to sense,) in a perpendicular to the Horizon: yet, if it be but little, this (with the compound motions before mentioned,) will give the Air a considerable disturbance. To which I may add also, that we are not sure, that the Seas and continents (which are of very different comparative weights, Earth being heavyer then Water,) are so adequately adjusted the one to the other, as that its center of gravity (by which a plain passing divides it into parts æquiponderant,) is the same with its center of Magnitude (by which it is divided into parts equally great,) which if it happen to be otherwise, will (with the rest,) make the confusions of the Air's motions yet greater. From the comparative weight or lightness of the Air at different times, he deduceth also the rising or falling of Vapours in it. As if, when such Fumes or Vapours, or other the like matters, are lighter than the Air, they ought (according to the exact rules of Hydrostatics,) to ascend therein; but, when heavyer than so, to fall down. down. And this certainly (ceteris Paribus,) is to be admitted also. Onely I add thereto; that these statick principles do cheifly take place, when things are otherwise at rest and quiet. But when they are in commotion, it is many times much otherwise. And, in such cases, we must, beside the respective gravity, take into consideration, the force, impulse, or impetus, that is superadded to the respective gravity of the parts or matter. Thus, if a Bottle be shaken, the sediment at the bottom, (though heavier, and for which cause it did before subside,) will be mingled with the supernatant (finer and lighter,) Liquor. And, if a room be swept, it will (as we use to speak,) make a dust: that is, the small Earthy particles of dust, will rise and mingle with the Air: not because they be lighter then it, (for we see that, at leisure, they will subside again,) but because, by a force upon them, they be put into motion. So in a Jetty of Water (in artificial Water-Works,) the water is thrown up into the Air to a great height; not because it so becomes lighter than the Air, but because of the force impressed. (To say nothing of a Bullet, shot upward into the Air.) And Bottled Ale, when opened, flies many times upward to a great height; and sometimes blows up the Cork, to release itself from confinement. And, in the late Fire of London, not Papers only, but other things of greater weight, were mounted into the Air, and there carried aloft for many miles, and so taken up at more than Twenty miles distance. And this I take to be the cause of Fumes, Vapours, and other like matters (most of them,) which ascend in the Air, not because lighter than it, but because impelled upward out of the bowels of the Earth, or from the superficial parts of it, either by some subterranean heats or other ferments that put them into motion, and force them upward; where they remain suspended in the Air, so long as as that force continues, or the force of others sent after them on the like errand (which rather impells them farther, then gives them leave to fall,) till either such force abate, or the great weight of so many things suspended, doth overpower, not only the Air's weight, but the strength of that, that impelled them. And that there are such Fumes and other like matter projected upwards, from the Bowels of the Earth, and some of them with great violence, is undeniable, not onely from Earthquakes, and other eruptions (with great noises,) as well of Vapours, as of Burning Mountains; but even Poisonous Steams (and others,) in Mines, and Bubbling springs (where Bubbles of Air are seen to make their passage through the Water,) and other perspirations of Air or Vapour through crannies or small passages of the Earth, discoverable by Steams (whereof some will take fire at the light of a Candle,) or by the moving of Leaves and other light things laid on the mouths of such private passages, and by many other means. And to such causes I do principally attribute the Origine of Winds, and the ascent of most other things, which, from this lower World, mount into the Air. And, without this, the comparative gravity of the Air and them, would give us but a lame account of them. There is yet another notion suggested, which is also very considerable as to this affair: which is, the weakening or strengthening the spring of the Air. That Water hath (of it self,) nothing of spring or Elasticity (otherwise then by reason of some Airy parts, or other elastic bodies, which may be included within it,) is generally held, at least none considerable, such as by any experiments hitherto made, can be clearly evinced so to be. But, that the Air (such Air at least as is the common Air which we are conversant with,) is Elastic; is, I think, out of doubt: the experiments which prove its spring, being so many and evident, beyond exception. And, that this spring of the Air, is sometimes stronger; and sometimes weaker, I think is undoubted also. And, that the spring of the Air, is strengthened both by compression, and by heat, but in a different manner. If the same quantity of Air be compressed into a lesser room, the spring is certainly stronger. As is undoubtedly seen in the Wind-Gun, and other compressive Engines. And from hence it is, that Air, of the common texture with the external or open Air, if it be (retaining its own texture,) included in a close Vessel (as in the Air-Pump,) whereby it is excluded from communication with the external Air, and defended from the pressure of it, doth yet sustain Quick-Silver in a Tube (after the Torriellian manner,) at as great an height, as the open Air by its counterpois would have done. Which cannot be by its weight (which, in this cause, is but small; and not equal to that of a cylinder thereof contained to the height of the Atmosphere;) but by its spring only; which is now pressed to as great a degree as it was in the open Air (by such weight as was then incumbent on it,) or would be now, if so again exposed. (Like as that of a Fleece of Wooll would be, if confined to the same dimensions it had, when a great pile of Fleeces was incumbent on it.) But if some of this Air so included (in the Air-Pump or such other close Vessels,) be Pumped out, whereby the residue thereof hath a greater room to possess than before it had, (namely, as much as before the whole of it did possess,) the Quick-Silver will presently subside, because the spring of the Air which did before sustain it, is now (by expansion,) become weaker. Again; the same quantity of included Air in a close Vessel (so as not to communicate with the external Air,) will, by application of heat to it, have its spring strengthened, and drive its counterpois farther off, or (if need be, be,) compress it. As is to be seen in Thermoscopes of all sorts. Whether, by strengthening the Air's spring (in that discourse,) he means that of the former or the latter way, I do not well remember. Nor, whether he apply it to the increasing, or diminishing, of its intensive weight. But it may be reasonably accommodated either way. If the spring be strengthened by compression; it is manifest that the intensive gravity must be thereby increased: because the same quantity of Air, and consequently of weight, extensively taken, is now contracted into a lesser room, which therefore must be intensively heavier, (as being the same weight in a lesser bulk:) Now this may possibly, as a greater pressure or stronger spring, force up the Vapours under it with a greater impetus (according to the notion I mentioned before,) and so make them fly higher: (unless we should suppose, it may be relieved, by shortening the height of the Atmosphere;) but not so as to make them lighter; but rather the contrary, as pressing them closer: much less to make them (as the phrase is,) specifically lighter then is the Air itself (though thus compressed,) and it leaves less room to receive them between the particles of the compressed Air, as being now thrust closer together. If the spring be strengthened the other way; so as by heat it useth to be: this doth rather diminish its intensive gravity, by thrusting its parts further asunder, and so possessing a larger room. Now in case this Air be, by a close Vessel, confined so as not to expand upward; it will certainly press the harder on the stagnant Quick-Silver under, and make that in the Tube rise higher. But in case it be unconfined, as in the open Air, it may as well relieve itself upward, by making the Atmosphere, in this part so much higher. Nor is there any necessity, as to the subjacent parts; that the Atmosphere should be everywhere of the same height. But the laws of Statics, as to the subjacent parts, be equally preserved without it, (the greater altitude compensating for the levity of the parts,) as when a Portion of the Sea, is covered with a Fleet of Ships; the under parts are equally pressed partly by water, and partly by Ships, though the tops of the Ships over some parts be higher than the surface of the water over others. One-ly, in such case, the upper part of the Atmosphere, being fluid, may flow collaterally over the other parts on either side if lower. And so, at leisure, (if thus remaining, otherwise indisturbed,) reduce itself to an equal height in all parts. Like as the Sea would do, in a perfect calm, though otherwise its waves and billows are far from being in all places perfectly plain and even. But however, though the spring fortified by heat, may thus relieve itself upward, (yet because it presseth every way,) it must endeavour the like downward also, and thereby press harder on what is under it; and because it will require time to work upward gradually before the effect reach the top of the Atmosphere; and because, by such dilatation of its parts, more room is left in the intervals to receive what is forced: 'tis reasonable to believe, that, in such cases, the pressed vapours (ceteris paribus,) may rise more copiously then when the spring of the Air (for want of heat,) is less strong. The rather because the same heat which thus fortifies the spring of the Air, doth also rarify the Vapours and make them lighter: and may also fortify the subterranean heat (or whatever else it is,) that drives them up. Notwithstanding all which, we have more rains in winter; which should argue, that more Vapours do then arise to supply them. But I suspect that, in this whole business (of strengthening the spring,) there may be a fallacy put upon us. And what what we think to be done upon the open Air; is indeed done upon the Quick-Silver; or rather upon the Air latent therein. My meaning is this. We find that in very hot weather, and also in frosty weather, the Quick-Silver in the tube commonly stands very high; from whence we are apt to conclude, that therefore the outward Air presses very hard on the stagnant Quick-Silver, without the tube. Wherein I am not satisfied. For we are to consider, that, in filling the Tube with Quick-Silver before it be inverted, if great care be not used to cleanse it from Air, many airy particles will remain mixed with it; which, while their spring is weak, are easily pressed by the weight of the Quick-Silver close as hardly to be discerned otherwise than by the effect; but when, by the external heat, their spring is strengthened, they expand themselves, and cause the Quick-Silver, wherein they are, to swell in bulk, without increasing its weight; and consequently to stand higher though not to press heavier. Of this I had manifest experience, when (many years since) I did for five or six years together constantly observe the height of the Quick-Silver in the tube, and kept a register of it, and of the weather with it: and did for two or three of the first years observe, that in very hot weather the Quick-Silver commonly stood very high. But, for some of the latter years, I found it much lower in hot weather than in those former years. Of which I satisfied myself with this account; that, in those former years, the Tube having been but lately filled, the airy parts remaining in the Quick-Silver, being by the heat expanded, made the Quick-Silver to swell and stand higher: but, when the Quick-Silver, by divers years standing unmolested, had by its own weight cleansed itself, and forced that Air (which had been in it into the upper part of the glass; it now ceased to make the Quick-Silver swell higher, higher, but rather (by expanding itself in the upper part of the glass,) pressed the Quick-Silver downward. And I suspect, that others (without heeding it,) may have been in like manner imposed upon, as I was; especially while their Baroscope hath been but newly prepared, and not well cleansed of Air. And the same account, perhaps, may serve for its standing so high in frosty weather. Water, we know, though it contract with cold, yet when it comes to Frieze doth expand itself. (Which makes Ice lighter than water, and to swim on the top of it.) Now whether this be purely of itself, or (in part at least,) from the particles of Air lodged in it, may not perhaps be so easy to determine. However, if there be the like effects on Air, as on water, (namely, that it expands with Friezing;) or if, in the Quick-Silver there be lodged particles of water as well as of Air; we have, either way, an account of this Phænomenon. For, then, the small particles, whether of Air, or water, lodged in the Quick-Silver, being thus expanded by friezing, will make the Quick-Silver fwell, and so stand higher, without increasing its weight; and consequently, without arguing a greater weight of external Air pressing on the stagnant Quick-Silver.