vis inertia in fome fluids than in others. In quickfilver, for inftance, it is greater than in water; in water than in air; in the air than in the ether of Sir Ifaac, that remains in the receiver when the grofs air is drawn out; and which ether, by his own confeffion, has fo little of the vis inertiæ, as to give no fenfible refiftance to bodies moving in it: though resistance it must give, or there could be no motion; for that which gives no refiftance, can give no impulfe.

THIS vis inertia is made the standing argument against a plenum; whereas we now fee, that a plenum is the very cause of the vis inertia for if the parts of this aereal fluid were not continuous and contiguous quaquaverfum, bodies would neither have refistance nor motion. But the particles of the heavens being preffed one against the other, and having no lurking holes or spaces big enough to creep into, are forced to fight their way, and act against each other: and to this conflict is owing all the resistance bodies give and receive. Whilft the conflict,

in the line in which a body put into motion is carried, lafts, the body continues in motion; when peace is made between the contending parties, the body refts.

THE most plausible, as well as popular objection to a plenum, is the abfurdity to fuppofe, that, upon the motion of every little atom, the whole frame of things must be difturbed, and fet a-going. But the whole frame of things is in a continual motion, and always a-going; the largest masses of the aereal fluid moving one way, the small ones the oppofite way; the largest maffes moving foremost, and the reft in order, and proportion to their fize, towards the fun, the centre of the system; the smallest moving foremost, and the reft in proportion to their fize, towards the circumference; except where they are reftrained. And the action of movement is no more than each fli

ding or rolling upon or by another. And though this produces a motion of the parts, confiderable, where the parts of the fluid are divided very fmall, or a confiderable quantity of it is put into that condition; yet M 2 each

each part is fhifted only a fmall distance from its place, furthest near the light, and lefs and less at greater diftances; and every part finds its place of reft again in a moment, or in two or three vibrations or rebounds, (which, when great, we call echoes), and come again to an equilibrium. So that any operation can be performed, or body moved, by moving part of the aereal fluid, proportionate to the particular action or motion, without disturbing the rest, and thereby removing other bodies that were not intended to be moved.

THE fame may be faid of the other two laws. For as it is the impulfe of the air which continues a body in motion, fo it moves fafter or flower in proportion to the force with which the air is pushed away: and motion will be in the direction of the right line in which the moving force is impreffed; because the lateral air prevents the body from diverging to either fide, and makes a kind of groove or channel for it to move in; and it is continued in motion un

til the refiftance before become greater than the impulse behind.

AND fo of his third law: The atoms are preffed out from the fun, the centre, with the fame velocity as the grains are preffed in; and the firmament or airs refift, reprefs, or react against a body, as much as the body preffes or acts upon the firmament or airs: fo action and reaction are equal and contrary. But as these their laws are confeffedly confequences of the vis inertia of matter, and the vis inertia has been shewn above to be a confequence of a plenum; they must likewise depend upon it, and will be beft understood and explained when the laws of this fluid of the air fhall be deigned worthy the philofophers enquiry and attention.

THE fecond of the two great theories upon which the Principia turns, is the refiftance of mediums to the motion of bodies. This comes next under confideration; for, to use the words of Mr Fontenelle, "Sir Ifaac Newton has, as ufual, ❝ deduced

« deduced from the profoundeft geometry "the effects of this refiftance, whether a

rifing from the denfity of the medium, "the velocity of the body moved, or the

largeness of its furface;" and thence draws his conclufions, to prove that the heavenly motions cannot arife from, or be continued by the impulfe of any medium. The late ingenious Mr Cotes, in his preface, has judiciously fummed up the evidence: and as his own great skill in mathematics, and his thorough knowledge of our author, fufficiently qualified him for the task; fo his profound admiration of, and entire attachment to his fcheme, leave no room to doubt, that he has done justice to his mafter, by placing every argument in the clearest and strongest light, and giving each its full weight. I fhall therefore, to avoid any fufpicion of mifrepresentation, quote his own words.

Corpora progrediendo motum fuum fluido ambienti paulatim communicant, et communicando amittunt, amittendo autem retardantur. Eft itaque retardatio motui communi

cato