"principle is neceffary for conferving the "motion *"

THE point in queftion is, What powers or agents continue motion, after the vis impressa, that which first put the body in motion, is removed? What evidence is there produced to determine this point? His definitions and laws fuppofe fome properties inherent in matter, which move it after the vis impreffa has put it in motion: for, by definition 4. this vis is faid to consist in action alone, and not to remain in the body after action, the body perfevering in its new ftate by the vis inertia alone. The first law supposes, that if a body be once projected, it will move itself ever after in a straight line, till something ftop it, or divert its courfe. From hence they argue, that the planets move in spaces free from resistance; and, for proof of this, appeal to the abovementioned law. Now, what is this, but bringing two things in proof of each other, neither of which are proved? To this it will be answered, That the cause of gravity

Optics, English edit. 1721, p. 372.373.

and

and attraction is not here confidered: what is called attraction may be performed by impulse, or by fome other means unknown to them. This, though indeed a fpecious, yet is only a pretence, as I have proved above, given out to deceive unwary readers, to caft a mist before their eyes, and prevent their prying too clofely. For if impulse by ather be allowed, (that fubtile spirit he hints at in the end of the Principia, and to which, under the name of ather, he attributes attraction, and every thing, in his Optics), what will become of his Principia? Are not all his calculations, proportions, &c. made and suited to his imaginary power of projection, and a vacuum or spaces free from refistance, and confequently free from impulfe? (for what cannot refift, cannot impel). When the whole of his book is to prove, that bodies do not move mechani❤ cally by impulfe, is it not ridiculous and trifling to fly to impulfe, a cause of motion he has fo dogmatically rejected, and which is utterly inconfiftent with, and destructive of his scheme? For if bodies move by impulfe, the impelling medium must be in all

places

places where the bodies are, and whither they are to be moved to; and as bodies may be moved in every direction, therefore the impelling medium must be every where. It is in vain therefore to pretend, "that he has « fecured his philosophy against any hazard "of being difproved or weakened by fu"ture discoveries; that he has taken care "to give nothing for demonftration, but "what muft ever be found fuch: and ha

ving feparated from this what he owns is "not fo certain, he has opened matter for "the enquiries of future ages, which may "confirm and enlarge his doctrines, but

can never refute them." For though the ingenious Mr Maclaurin does thus endeavour to fhore up the Newtonian edifice, it only fhews, that he is conscious of its tottering condition, and that it ftands in need of a buttress. For the doctrine of motion by impulse, and its consequence a plenum, must for ever deftroy the doctrine of jection, gravity, and attraction, and their confequence a vacuum; and also the vis inertia of matter; and of course his three laws of motion, which depend on them.

pro

Befides,

Befides, if gravity and attraction are only used in general for any force by which bodies tend towards one another, whatsoever be the cause; what neceffity for a vacuum? Why must the medium of the air be rejected? a medium which all philofophers, down to our author, made the cause of motion, though they could not explain its effects, or fhew how it acted. If he is ignorant of the cause, as he has fufficiently proved himself to be; the air, for any thing he has faid to the contrary, may be the very medium he has rejected.

BUT to speak more closely to the matter, and to explain this first law of motion, on which the reft depend, I must observe, that as this system is full, and bounded, (according to the scripture-philosophy, which it is my design in these sheets to follow); and as one moiety of the matter of the heaven, the light, which is the purer and finer part, is in continual motion from the fun, the centre of the system, to the circumference; and the other moiety, the fpirit, which is the groffer part, is in as continual a motion from

the

the circumference to the centre: thefe adverfe motions lay a preffure and stress upon every particle of the matter of the heaven itfelf, or the ethereal fluid, as I may call it, and alfo upon every thing placed in it. When therefore the ethereal fluid which furrounds the body, preffes it equally on every fide, the body is at reft, and continues fo: when, by any accident, this equality of preffure is destroyed, motion enfues; and continues as long as the impulfe behind is greater than the refiftance before; or, to speak more strictly, until the impulse behind ceafes to act, and the preffure on every fide, behind and before, becomes equal. And this explains the vis inertia, to which they attribute the continuance of a body in motion; and fhews it to be an accident, and not a property of matter. For as all bodies exift in this plenum of the air, it is plain, that no one body can move without ftriking againft, and moving away another, and fo meeting with fome refiftance. The vis inertia cannot be an effential property of matter, or a vis infita, as it is called by Sir Ifaac Newton; because a body has a greater