to me, of which I shall speak hereafter, nothing has been advanced that affects the general plan of the work; and that it has been attentively considered by readers of capacity and candour, both at home and abroad.

Having observed very early in life, from a few obvious examples, that great effects are produced in Nature by the action of the elements on one another, I was so much encouraged by the apparent usefulness of this principle, and so soon convinced of its importance in the several chief branches of Natural Philosophy, that I determined to pursue it as far as I should have light and opportunity; and to examine, to the utmost of my ability, whether the same powers which obtain so evidently in some cases, might not be extended with advantage to others--that all philosophy might be reduced to one simple and universal law, the Natural Agency of the Elements.

But, how pleasant soever this inquiry might be in the prospect, I found, by degrees, that there were many great difficulties in the way to discourage me in the prosecution of it. For, first, it was generally believed by learned men, that a Vacuum had actually been demonstrated by our great Newton; and, consequently, that no powers were to be admit

ted

ted in Nature but such only as were consistent with that principle. It must be allowed, that this celebrated author, by the help of numerous and judicious experiments, settled all the laws of communicated motion, computed the resistance of fluids, dissected the subtile body of the light, and, with indefatigable industry and sagacity, discovered certain constant and regular effects, to which he gave the names of Attraction and Repulsion, and, by the most profound skill in Geometry, demonstrated the proportions of those effects in almost all possible cases, and shewed by experiments how the phenomena of Nature agree with his calculations. This is his philosophy, which stands upon the firm basis of Demonstration: but as to the Demonstration of a Vacuum, he left it in suspense, as we shall see hereafter: and as to the Causes of those effects above mentioned, he advised future philosophers to inquire farther into them; confessing, in his latest works, that what he calls Gravity might, for any thing he knew to the contrary, be the effect of Impulse.

He therefore did not believe that his philosophy precluded such an inquiry as that of

the Essay. But, as many of his followers, and some of his readers, had seriously taken up such a persuasion, I foresaw that my proposed investigation of Natural Causes would be at first but coldly regarded, if not absolutely condemned, as an undertaking, at best superfluous, and perhaps totally groundless and absurd. This was the first difficulty I had to encounter; and it was great enough, without the addition of a second.

But there remained another difficulty, equally formidable, and more likely to defeat all my endeavours; as rendering it next to impossible that I should ever get to the ear of the public. The Agency of the Elements, the principle which has engaged my attention, and to the study of which I had determined to dedicate the chief labours of my life, had been already taken up, and maintained in a manner neither acceptable nor satisfactory, by Mr. Hutchinson, an author of a singular cast, and under a state of final reprobation with the learned of this kingdom; whom every scribbler can deride, and every incipient in philosophy can confute. This was very much against me: for there

there wanted nothing to render all my reasonings abortive, but to represent me as a follower or favourer of Mr. Hutchinson.

To get over the first of these difficulties, I thought it was my duty to examine impartially, and then to represent as faithfully as I could, the true state of the question; and to give as much weight as should be found due to all the experiments and doctrines that were supposed to militate against my projected inquiry into the Agency of the Elements. This This gave occasion to the preparatory work I have already mentioned, which I called An Essay on the First Principles of Natural Philosophy. Its object was to consider, 1. The Mechanism of Nature, and, under this head, shew the insufficiency of the objections which had been raised against the doctrine by Dr. Clarke and other learned writers: to shew how motion might consist with a Plenum, or space filled with fluid matter and to prove yet farther, from actual observation, that the presence of resisting Matter was, under certain circumstances, not an obstruction, but even necessary in itself to preserve in bodies an undecaying motion. 2. To shew, that, so far as unmechanical causes had been proposed to answer

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the purpose of, first principles, they were neither well understood by those who had espoused them, nor capable of being explained from the nature of them: that it was not determined whether they were properly causés or effects, material powers or immaterial; for that some learned persons had inclined to one side, some to the other; and that, if considered as immaterial, neither experiment nor geometry had demonstrated them in that capacity. 3. To produce some positive proofs of a matter in the heavens, and that there is a subtile medium in the pores. of all bodies; with some examples of its action and power in producing the effects of cohesion and repulsion. 4. To shew the reasonableness of an impulsive Agency in Nature, from the general attestation of antiquity, both sacred and profane; and also from the judgment of some of the most eminent scholars among the moderns. To these four Books, an Appendix of Papers was subjoined, to carry on and illustrate some points which had been started in the course of the work.

Important as many of the questions are which came under consideration in that Essay, I never met with any thing worth my notice against it, in print or in manuscript,

for