tion between the Diameter of the Equator and the Axis, which he finds to be as 230 to

229.

Herman alfo has examined into the Figure of the Earth by the Hypothesis of a Gravity proportional to the distance from the Center, and found that the Earth must be an Ellipfoid whofe Equatorial Diameter is to its Axis as 289 to 288; in which he agrees pretty

nearly with Huygens.

None of thefe Measures agree with that actually taken by Caffini and Maraldi; but if from their Observations, the most famous perhaps that ever were made, it follows that the Earth, inftead of being an oblate Spheroid towards the Poles, is an oblong, tho' this Figure feems to be repugnant to the Laws of Statics; we fhould fee this to be an abfolute Impoffibility, before we cavil with fuch Observations.

In the feveral Calculations we just now mentioned, the Earth was confidered as compounded of an Homogeneous and fluid Matter; and in this Cafe would moft certainly be an oblate Spheroid at the Poles: but fuch an Homogeneity may not be in the Matter which forms the Earth; and for that Reason it is of a different Figure.

I will not here examine into the Manner how de Mairan thought we might be fure the Earth is an oblong Spheroid at the Poles; it has been fufficiently difcuffed in the MeA a 2

moirs

moirs of the Academy*, and the Philosophical Transactions t.

CHA P. II.

A Metaphyfical Discourse on Attraction.

I Propole here to offer fome, probable Conjectures, whereby to explain how it hap pens that the Stars feem fometimes to increase and decrease in Magnitude: how new Stars feem fometimes to be lighted up in the Heavens, or old ones to go out; in a word, how a Ring like Saturn's may be formed around a Planet.

Thefe Phenomena are, as we may fay, no more than the Corollaries to the Problems whereby I demonftrate the Figure which must be affumed by a Mafs of fluid Matter revolving about an Axis, or a Stream which circulates about an Axis taken out of itself.

These Forms depend upon the Gravity of the parts of thefe Fluids, and their Centrifugal Force. Philofophers are perfectly agreed as to what concerns this last; but they

* Memoires de l'Acad. 1720. No. 386. 387. 388.

+ Philo. Tranf. 1725.

vary from each other on the Subject of Gravity.

Bodies that revolve inceffantly endeavour to escape in the Tangent of the Curve they defcribe, and this their effort is called their Centrifugal Force.

As for Gravity, fome look on it as the Effect even of the Centrifugal Force of fome Matter, which circulating about the Bodies towards which others gravitate, impels or forces them down to the Center of its Circu lation; others without diving into the Cause, reckon it to be a property proportionably inherent in all Bodies.

Tho' the Mathematical Solutions of the Problems in this Tract, bear no relation to the nature of Gravity; yet as the Applica tion I make of them to the Phenomena of Nature, depend in fome fort thereon; it may be proper to touch a little on it here, to fhew how far our Explications may extend, according to the different Ideas we may have of Gravity.

It is not for me to pronunce upon a doubt which divides the greatest Philofophers; but I may have leave to compare their Notions.

A Body in Motion, meeting with another Body has the Power to move it. Upon this Principle the Cartefians endeavour to explain every Thing, and to evince that even Gravity is but confequential thereto. Their Syftem has, in this, the advantage of Simpli

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city; but it must be confeffed that great Objections are started against it, and occur in our Researches by this System.

Sir Ifaac Newton not fatisfied with the Cartefian Doctrine, which accounts for every thing by Impulfion only, difcovers another active Principle in Nature, meaning that the Parts of Matter gravitate towards each other. This Principle laid down, he wonderfully unravels every Phenomenon; and the more we purfue him, the deeper do we dive into his Syftem, and the more does it feem to be confirmed. But befides that the foundation of this Syftem is lefs fimple, in that it supposes two Principles; a Principle whereby Bodies at a Diftance act upon each other, appears hard to admit.

The word Attraction has chafed the Minds of Men; many were afraid of seeing the Doctrine of occult Qualities revived again in Philofophy.

But to do justice to Sir Ifaac, he never thought Attraction to be explanatory of the Gravitation of Bodies towards each other ; he often takes notice that he used that Term only to exprefs an Effect, not a Cause; that he applied it only to avoid Systems and Explanations; that poffibly this Tendency might be caufed by fome fubtile Matter which might emanate from Bodies, and might be the Effect of an actual Impulfion; but be it what it would, it was certainly a primary Effect, whereby

whereby might be explained all the fubfequent Effects dependant thereon. Every regular Effect, may, tho' its cause be unknown, be the object of Mathematicians; because whatever is fufcipient of More or Lefs, is within their Sphere, be its Nature what it will; and the Uses they make of it will be to the full as demonftrative, as the Applications they may make of Objects whofe Nature. fhould be abfolutely known. If we were allowed to treat on fuch only, the Bounds of Philofophy would be ftrangely narrow.

Galileo, tho' ignorant of the Cause why Bodies gravitated towards the Earth, has nevertheless obliged us with a very fine and certain Theory upon this fame Gravitation, and explained the Phanomena thence arifing. If Bodies ftill continue to gravitate towards each other, why may we not investigate the Effects of this Gravitation, without diving into the cause of it. Our whole Bufinefs will then be to enquire whether or no it be true that Bodies have this Tendency towards each other; and if we find the thing to be a fact, let that content us for our deductions with respect to the Phenomena of Nature; and let us leave it to fublimer Philofophers to fearch into the Cause of this Tendency.

And this to me feems to be rather the best way, as I believe it in vain for us to trace up to the primary Caufes of Things; and that we are unable to comprehend in what manner Bodies act upon each other,