learn't as much of the Elements of Geometry and Mechanifm, as will qualify him to comprehend them.

I not only prov'd this variation of Gravity, from its effect upon Pendulums, but I alfo fhew'd the cause of it, and that it must be fo, upon fuppofition, that the Earth turns round its own Axis. For all Bodies that turn round an Axis, endeavour to recede from that Axis; and because at the Equator Bodies moved fwifteft, the Circles there being greatest, this Conatus or Centrifugal force would be alfo greateft; now this force at the Equinoctial acting directly against the force of Gravity, (which it does no where befides) it is evident, that upon this account Gravity muft be lefs there than any where else. Upon the account of this diminution of Gravity it must follow alfo, that the Diameter of the Equator will be greater than its Axis, or that the Matter at the Equator rifes higher than at the Poles.

The Defender fays, that this is agreed and own'd on all hands, in cafe there were no impediment to hinder the rifing or retroceffion of the middle parts; but (fays he) the Theorift did believe, that the Vortex was of a fhorter Diameter there than thro' the Poles, which hindred the rifing of the fluid. What this impediment is, or what fhould occafion this ftraitnefs of the Orb at the Equator I know not; I hope the Theorift does not fuppofe

pofe that there is a great iron hoop at those parts, a folid ring like that of Saturns, which keeps in the fluid from rifing. Yet how it fhould be elfe kept in is beyond my skill to guefs; I wish he would explain this more at length, that we might know what he means by it.

But let us fuppofe this ftraitness of the Orb at the Equator, and fee what will follow from it; [Fig. 18. Plate IX.] Let ACB represent the Earth, firft of a Spherical Figure: EFGH a fluid Orb of Air which furrounds it, which we will fuppofe ftraiter at F than at E and G; Let A B be the Axis of the Earth, because the matter of C has a greater endeavour to recede from the Center than the matter any where elfe upon the Earth; it is evident that this matter muft prefs ftronger upon the fluid immediately above it, than the matter at other parts can do upon the fluid immediately above them; and because the matter of the Earth is much more folid than the fluid Air, it is plain, that the Vis motrix or abfolute force that the matter of the Earth has to recede from the Center, will be greater than the abfolute force that the Air has to recede from its Center; it is plain upon this account, that the Air muft yeild to it; but according to the Theorift the Air cannot rife higher than F, therefore it must recede towards E and G, and leave room to the matter at C to rife up to M; by which means it

is

is evident, that (this matter thus rifing at the retroceffion of the fluid Air) the folid matter of the Earth muft fettle its felf in the form of a broad Spheroid. Thus, from the Theorifts own principles and fuppofitions, I have prov'd that the Earth must be higher at the Equator than at the Poles.

The Author tells us, that those who affirm that the Earth is in form of a broad Spheroid, will allow of no Vortices to the Planets; but then (fays he) they muft affign fome other fufficient caufe to carry the Planets in their Periodical motions with the fame velocity for innumerable Ages about their common Center; and the Secondary about the Primary; as alfo what gives them their diurnal rotation, and the different pofition of their Axes.

I thought that this Defender had been better acquainted with the hiftory of Philofophy for these twelve years paft, than it feems he is. One would think that he had done nothing but por'd upon the Theory all this time, fince he is not acquainted with what is known to every body that pretends to Philofophy now a days.

He may find feveral hundreds of people that can tell him, that there are other causes found for the Coeleftial motions than the Vortices, which will eafily explain all thofe Phanomena he has juft now mentioned. The caufes why the Planets move in Elliptical Or

bits are now discover'd; it is known why they move swifteft at their Perihelia, and floweft at their Aphelia. The cause of the proceffion of the Equinox is now no longer a mystery; and (which is for our purpose) it depends upon principles that ruin the Theoriff's Figure of the Earth, and affert the direct contrary, making it in the form of a broad Spheroid.

The motion of the Moons Apogeon forward and of its Nodes backwards, its variation, and all its other motions, are easily accounted for by the fame caufes, none of which could ever be made out by the Vortices. For by them we can't answer the first question the Defender puts, viz. What is it that carries the Planets round the Sun with the fame velocity for many Ages? Nay, fuppofing that we were altogether ignorant of any other caufe, yet it is no hard matter to prove, that the Vortices can never be the cause of the Coeleftial motions; and therefore there being no Vortex, there can be no fuch thing as a straitness in the Orbit at the Equator, which the Theorist and the Defender fuppofe. But if I should allow them both their Vortices and the ftraitnefs of their Orbs, I have already prov'd, that they will fignify nothing to their purpose.

The Defender tells us, that this reasoning about the Figure of the Earth depends upon the Theorift's Hypothefis, that the Globe of

it

it was once fluid; and from thence he pretends to confirm the Theory: For fays he) neither Figure of the Earth, oblong or oblate, can be proved from the rotation of the Earth and its Gravity, without fuppofing the Globe formed into that jhape before it came to be loaded and fiffned with Rocks and ftony Mountains; and therefore upon both Hypothefes it must be allowed, that there was fuch a time; fuch a state of the Earth when its tender Orb was capable of thefe impreffions and modifications, and that Orb must have yen above the waters not under them, nor radicated to the bottom of them; and in the last place, this concretion upon the waters (fays he) muft have been throughout all the parts of the Earth, for there is no reason why one part of the fluid fhould be covered more than another; fo (fays he) that in effect we must fuppofe, that all the watery Globe was at firft covered over with an Earthy concretion: Now this being admitted (fays he) we bave confirmed the main point of the Theory, namely, that the Abyss was at firft covered over with an Orb of earth; and if we will grant him this he will compound for the reft.

He is a little too unreafonable in expecting grants of fuch things as are altogether precarious, and affirm'd without fo much as a fhew of an argument. My business was to prove, that he had deduced a wrong conclufion from his own Hypothefes and Principles; and therefore, fuppofing that the Globe of the Earth was once fluid, I prov'd from thence,

that