Another collection of philosophical conferences of the French virtuosi upon questions of all sorts for the improving of natural knowledg made in the assembly of the Beaux Esprits at Paris by the most ingenious persons of that nation / render'd into English by G. Havers, Gent. & J. Davies ..., Gent.

CONFERENCE CCXV. Whether of two Bodies of different weight, the one descends faster than the other, and why? (Book 215)

OF Natural Bodies, some move from the Centre to the Cir∣cumference, as Fire; others, from the Circumference to the Centre, as the Earth; others, are in the mean between both, as Air and Water, the latter whereof inclines downwards, but both of them are principally design'd to fill the Vacuum. Whence it comes, that the Air descends as much, nay, faster to the bot∣tom of a Well when it is dry'd up, than the Water had done before; which consideration, hath given occasion to some to attribute a mean, or circular Motion to those two Elements, as they have done a direct Motion to the two first. And whereas these two kinds of Local Motion, to wit, the direct and the circu∣lar, are the Principles of the Mechanicks, the most profitable parts of the Mathematicks, and that among the said Motions, that which tends downwards, which proceeds from weight, is the most ordinary Agent, and such as is the most commonly us'd in Machins or Engins, where it is the most considerable, either for the assistance it gives to fixt and setled instruments, or for the obstruction it gives those which are moveable; thence comes that famous dispute there is, concerning the causes of Motion from above to beneath. Which since it must needs proceed from one of these three, to wit, the weight of the Body descend∣ing, and lightness of the mean through which the descent is made; or from the impulsion of the said mean: Or, last∣ly, from the attraction made by the Centre; the Question is, To which of those three Causes that Motion is to be re∣ferr'd? If it be attributed to the weight, it will follow, that

the heaviest Body shall descend soonest; if to the impulsi∣on, the celerity or slowness of the Agent shall accordingly render that Motion swift or slow; but, if only the attraction made by the Centre be the Cause of it, the lesser weight shall descend as fast, nay, faster than the greater, upon the same account as that the same piece of Loadstone more easily draws a small needle than it does a great key. Nor can Experience always as∣sist us in this case, in regard the different composure and form of heavy Bodies, as also the diversity of the means, and the variety of the Agents, whereby they are thrust forwards, will not per∣mit us to make an allowable Comparison between them. Thus a ball of Cork, which descended as fast in the Air as one of Lead, shall not do the like in the water, to the bottom whereof the Lead shall fall, but not the Cork. And again, the same Lead being put into the form of a Gondola, or other hollow vessel, shall swim on the water, which it could not before: A Cloak, folded close together into a bundle, shall have a speedy descent in both Air and Water; but let a Man fasten the same Cloak under the arm-pit, so as that it may spread into a circle, it shall so sustain him the Air, that he shall fall very gently, and receive no hurt by his fall. Hence it also comes that many Women have been sav'd, when, falling into the Water, their Clothes were spread all abroad. The same thing may also be observ'd in those frames beset with Feathers, or cover'd with Paper, which Chil∣dren call Kites, and sustain in the Air, and suffer to be carryed away with the Wind, giving them ever and anon little checks or jerks by drawing the pack-thread to them whereby they are held, imitating in that action the beating of the wings in Birds. In fine, the different manner of giving the first shock to weighty Bodies, does accordingly diversifie their Motion towards the Centre. For as the impulsion made downwards hastens its bent towards the Centre; so when it is forc'd circularly, it is retard∣ed. Whence it comes that a glass so cast down that it hath cer∣tain turns by the way, does sometimes fall to the ground with∣out breaking. But to speak absolutely, all conditions being sup∣pos'd equal, it should seem that the more weighty a Body is, the sooner it falls to the Centre: And this is made good by daily Experiences, as may be seen in the weighing of Gold and Silver in the balance, which hath a speedier and shorter cast, when the piece is much weightier or lighter, than it hath when there is but half a grain difference between both the scales.

The Second said, That the Nature of weight or heaviness was to be number'd among the occult things. Aristotle defines it to be a Quality inclining Bodies downwards and towards the Centre. Others would have it to be an Effect of density, which proceeds from the great quantity of Substance and Matter, com∣prehended and contracted in a small room. There are yet others, who would have it to be an impulsion or fastning of one Body up∣on another, in order to Motion downwards: But, to come nearer

the business, it is only the relation or report there is between a Body and its mean, and its comparison with another Body. According to this account of it, the same piece of Gold is said to be light in respect of one weight, and heavy in respect of an∣other; Wood is heavy in the Air, and light in the Water; Tin is light in comparison of Gold, though very weighty in respect of Wood. Whence it follows, that weight hath only a respective being, and such as depends on some other thing, and not on it self: The Cause of it therefore is not to be sought in it self, but else-where; as must be that of the recoiling of a Tennis-ball, which is not in the Ball, nor in the arm of him that playes, nor yet in the walls of the Tennis-Court, but resulting from all these three▪ together. And whereas Experience seems to decide the Question propos'd, 'tis fit we should refer our selves to it. Now it is certain, that of two Bodies of unequal weight, and of the same Figure and Matter, equally forc'd, or suffer'd to fall, the one will as soon come to its Centre as the other; as those may see who shall let fall at the same time from the top of a Tower, two leaden bullets, one of two pounds, and the other of a quarter of a pound, both which will come to the ground at the same in∣stant; the reason whereof is, That the stronger impulsion in the bullet of two pounds, meets with a stronger resistance of the Air to break through as it falls, than that of quarter of a pound. Whence we are to make a distinction between the greater im∣pulsion which the weightier Body makes upon another Body, and the celerity or slowness of the Motion wherewith it de∣scends; a hundred weight being heavier on the shoulders of a Porter, than one pound, but not coming sooner to the ground than it. In like manner, a stone descending so much the more swiftly the nearer it comes to its Centre, clearly shews, that it derives the force of its Motion from the Centre, as its principle; as we conclude, that the strength of a bullet is spent, and the Motion of it grows fainter, the further it is at a distance from the arm and gun from which it came, and which we hold to have been the cause and principle of it.

The Third said, That the weightiest Bodies make the more haste to their Centre, the nearer they approach it; for their weight is increas'd by their approaching of it; gravity, in the scent of weighty Bodies, increasing by the continuance of Mo∣tion, quite contrary to violent Motion, which admits of remis∣sion thereby; artifice, it seems in this point giving place to Na∣ture, so as that the latter never grows weary, nay, is infallible in all her Motions, and that such a propension of weight to the centre, is the only certain rule to draw direct lines to that centre; and which is yet the more certain, the greater the weight is. And whereas the Mind of Man judges the better of things when they are oppos'd one to the other, behold one of those little Atomes which dance up and down in the beams of the Sun striking in at a window, it is a Body sustain'd in the Air only by its smalness,

and requires a long time to make an impression in that part of the Air which is under it, which thing cannot be said of a Musket-bullet: It is therefore deducible thence, that the heaviest Bodies descend fastest to the Centre.

The Fourth said, That we are not to seek for any other reason for the speedier descent of heavy Bodies, than there is in all the other Motions of Nature, which proceed from the instinct im∣printed by her in all things, of loving their good, which is their rest and natural place, which till they have attain'd, they are in perpetual disquiet; and whereas the heavier a Body is, the more parts there are in it, concern'd in the pursuance of that good, it is not to be wonder'd if it happens to them as to di∣vers sollicitors in the same cause, who press it more earnestly than one alone would do. We may therefore say, that the same natural instinct that makes the Mulberry-tree expect till the cold weather be over before it buds; and the Halcyons till the tem∣pests be past, before they build their nests, and makes them to secure their young ones before the Rain, may much rather cause the most massie and weighty Bodies to make more haste. For these, fore-seeing that the Centre is not able to lodge all the Bodies tending thereto, endeavour to get to it as soon as they can, adding to their haste the nearer they approach it. But the most certain reason of this speedier Motion, is the general rule, that the more the Cause is increas'd, the more is the Effect augmented; whence it follows, that if weight be the Cause of Motion downwards, the greater the weight is, the more intense ought to be the Motion.