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for several years. As the subject was seldom long out of my mind, I took an opportunity, when I was abroad, of trying how it would appear to two learned gentlemen with whom I conversed; who being divested of English prejudices, and strangers to my name and labours, could be influenced neither by fear nor favour. One of them admitted the truth of what I proposed; saw the reasonableness and reality of an impulsive agency; and declared he thought the time would come when it would be generally admitted. The other insisted, that the fame of Newton had given such weight to a contrary opinion, that men would never listen to this, neither could he himself be inclined to it.

Of late, some gentlemen of Cambridge, whom I have reason to honour for their learning, while I cannot but esteem them cordially for their candour and liberality, have considered the book with the attention due to the importance of its object. From their learned criticisms, I am convinced of what I was always forward enough to believe, that the Essay has not only its inaccuracies, but some errors which occasionally affect the argument, and also some positions, which are either precarious from the depth and difficulty

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of the subject, or stand in need of a farther explanation, which it is partly the business of the following work to give; and there is unfortunately added a false calculation from an error of the press, without any notice given of it. All these things I take to myself as the writer of the book; but at the same time I must say, in justice to the cause, that the remarks take the defensive part, and that I find nothing to invalidate the leading principle of the work. I rather find, that the agency, for which I pleaded twenty years ago as an adventurer in the argument, is now better understood, from the progress which has been made since that time in the experimental knowledge of the elements; and I venture to predict, that it must be more generally admitted by the learned, when it shall have had the good fortune to have been farther considered.

By some, who were less liberal in their manners and sentiments, and pronounced too hastily upon the case, without appearing to me to have taken due pains to understand it, it was given out, soon after the publication of the work, that I had only revived the arguments of Leibnitz against Dr. Clarke; and, consequently, that what I had advanced

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had been already refuted long before. But any person who compares the matter of the Essay with the controversial papers of Mr. Leibnitz, will find that I had no assistance from his arguments, and that I have no concern with the fate of his system, which savours too much of metaphysical temerity to recommend itself to my taste. The great argument for a vacuum from the vis inertia, and the resistance of fluids, urged by Dr. Clarke, received so weak an answer from that celebrated mathematician, that he was evidently perplexed, and knew not how to get over the difficulty, which he rather evades than attempts to answer *. But this great objection to the agency of impelling fluids, is taken off in the Essay, by the introduction, of a very easy distinction, illustrated with a few plain facts, which shew, that if the argument is taken in that general sense for which it seems to have been intended, it proves too much for it supposes a case to be impossible in Nature, which really happens in all the instances there produced; as the reader may see, if he will give himself the

* See the Collection of Papers between Mr. Leibnitz and Dr. Clarke, p. 127, 185, 227.

the trouble of recurring to them *. The facts I allude to are given as so many actual examples of an unresisted motion, from an impelling fluid in a resisting medium: the most palpable of which is a machine, moving on the principle of the smoke-jack, carrying two lights, which begin to move of themselves, and, consequently, retain undiminished the motion they acquire. The instrument was taken from the Entretiens Physiques of Pere Regnault, where I found it introduced for the trifling purpose of giving an alternate motion to some magnetical puppets, by means of two lighted candles in a pair of scales. The form of the instrument

was altered in such a manner as to accommodate it better to the subject to which it is applied in the Essay.

The argument to demonstrate a vacuum, is built (as mathematicians well know) on this observation; that all fiuids resist a moving body in the direct ratio of their densities. The body communicates to the fluid the motion it had received; what it communicates it loses; and a body which continues to lose its motion, must in time lose the whale,

*See Book i. c. 5.

whole. But if, on the contrary, it does not appear to lose any of its motion, it follows, that it must either move in a vacuum, or in a medium of no sensible density; which comes to the same. This argument, which appears very strong, is applicable only to such bodies as are put into an unnatural state by some violence; as when they derive their motion from a projectile force: and it would be universal, if all bodies were moved on that principle; but when bodies are moved on other principles, another law takes place. For, let an impelling fluid be the primary cause of motion in any moving body; and that body will then meet no such resistance from the same fluid as tends to deprive it of its motion; it being impossible, as implying a contradiction, that a cause of motion should resist the motion which it causes. Now, apply this distinction to the machine. The impulse of the air, rarefied by fire, against the vanes, is the cause of its revolution therefore, the resistance of the other air, in which it moves, is of no account as an obstruction in this case, whatever it may be in other cases. It is true that the machine, as it revolves, communicates motion to the air; but it receives more than it communicates,

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