CHAP. V.

The argument from resistance proved to be an absurdity; as it implies, that the cause of motion must resist the motion which it A plain experiment to illustrate

causes.

and confirm the author's reasoning.

HUS much, I think, may suffice to shew,

THUS

that the celebrated demonstration of a vacuum has set out wrong. It will be as easy to prove, even waving all that has been said on the vis inertiæ, that it has concluded wrong; if the relation between a moving body and a resisting medium be rightly understood.

When a pendulum is made to swing in air, water, or mercury, the resistance it meets with is greater, as the medium is denser; and as a plenum of æther, such as the mechanical philosophy requires, would be more dense than any other fluid, its resistance, they say, must be greater: no motion could possibly continue in it. But then as motion is observed to continue in the heavens, without any sensible diminution,

there

there can be no resistance in the heavenly spaces, and consequently no matter of sufficient density to occasion it.

This was Sir Isaac Newton's way of computing resistances, and the use he made of them when computed. That we may see whether this doctrine agrees with experience, let us suppose a ship, with its sails spread, to be in motion before the wind: every body must allow me, that if the wind were to keep its direction, and the ship to have an open sea, it would go quite round the globe; and for the same reason that it makes one revolution, it would make another, and so on ad infinitum. Are we to say, that the air, in which it moves, is an unresisting medium? We ought to say this, if the demonstration above mentioned really is what it pretends to be. But the truth is, a medium may, in its nature, be a resisting one, and yet in fact give no such resistance as shall be any impediment to a body moving in it. For let any person tell me, how much resistance the ship receives from the air in this case? The answer must be-less than none: the resistance here is a negative quantity; and the ship is so far from losing its motion, that it is continually receiving it,

as

as it passes through the air; yet it would be false to affirm of air in general, that it is not a resisting medium. As to the water the ship sails upon, this being not the cause of its motion, will serve to retard it; but as the continued impulse of the air behind is superior to the sum of all the following resistances, 1st, of the air before, 2dly, of the water the ship sails upon, and 3dly, of the cause of gravity which is continually acting upon it; the motion will continue notwithstanding these impediments.

Were it to be laid down as a general rule from this particular instance, that water resists motion, but air does not; neither will this coincide with experience. A cork, or any other light body, thrown upon the stream of a sluice or floodgate, will be carried off with it; and as it is common for a considerable part of the water to return again upon the stream in a curve, if it be obstructed by the banks, and have but a narrow outlet, the cork may come about with it, and complete its revolutions, so long as the cause continues to act upon it. The water gives no more resistance in this case, than the air did in the other: and thus

it will happen universally, that every fluid, where it is the cause of motion, will not be found in that case to give any resistance, be its quantity of matter great or small.

We are now prepared to return to the pendulum. If it vibrates in air, the air will retard its motion; and there is a good reason why it should do so, for air is not the cause of its motion. If in water, neither is that the cause of its motion; and it will give a greater resistance to it in proportion to its quantity of matter, that is, in proportion to the action of gravity upon it. If in quicksilver, it will meet with a still greater resistance, for the same reasons. But if there be any elementary æther, acting as the natural cause of gravity in bodies; it is just as absurd, to search for the resistance of such a fluid, from the motion of a falling body; as for that of the air, from the motion of a ship that sails by it; or for that of water, from the motion of bodies carried down by a current of it. If one philosopher may conclude, that gravity cannot be owing to any material fluid, because he has found that this fluid does not resist a gravitating body; may not another demonstrate, with equal truth,

truth, that a ship cannot sail by the action of the air upon it, because he finds, from the nicest observations, that the air does not deprive it of its motion?

Lest the reader should have suspected me of a design to prejudice him beforehand, and to inject scruples through a want of better arguments; I did not observe, at the begining of this disquisition, that Sir Isaac had drawn two opposite conclusions from a capital experiment relating to the affair now before us, but it will be proper just to mention it in this place. Having caused an empty wooden box to vibrate as a pendulum, he loaded the same box with 77 times its own weight of metal; and in this latter case found the motion to be retarded more than it ought to have been by the theory. In two different editions of his Philosophy, he imputes this to two different causes, without the least hint or apology to his readers for such an important change in his opinion. In that of 1687 he accounts for this, from an increase in the resistance of the air, occasioned by a swifter motion in the heavier pendulum: and has demonstrated elsewhere, that the resistance of a fluid to a body moving in it, must increase