When I shall have told the Reader, that the following Discourse was written in the year 1665, while the Authour, to avoid the great Plague that then rag’d in London, was reduc’d with many others to go into the Countrey, and frequently to pass from place to place, unaccompanied with most of his Books; it will not, I presume, be thought strange, that in the mention of some things taken from other Writers, as his memory suggested them, he did not annex in the Margent the precise places that are referr’d to. And, upon the same score, it ought not to seem strange, that he has not mention’d some late Discoveries and Books that might have been pertinently taken notice of, and would well have accommodated some parts of his Discourse; since things that may thus seem to have been omitted, are of too recent a Date to have been known to him when He writ. But if it be demanded, why then a Discourse finished so long ago, did not come abroad much sooner? I must acquaint the Reader, That ‘twas chiefly his real Concern for the welfare of the Study he seems to depreciate, that kept these Papers so long by him. For he resisted for several years the desires of Persons that have much power with him, and suppress’d the following Discourse, whilst he fear’d it might be misapply’d by some Enemies to Experimental Philosophy, that then made a noise against it, without suffering these Papers to come abroad, till the Addresses and Encomiums of many eminent Forreign Virtuosi, and their desire to be admitted into the Royal Society, had sufficiently manifested, how little its Reputation was prejudic’d, or like to be endanger’d, by the attempts of some envious or misinform’d Persons. And to this Reason must be added the Authors backwardness to venture abroad a Discourse of an unusual Nature, on which account, among others, he declin’d to have his Name prefix’d to it; though, now the Book is Printed, he finds cause to fear, that ‘twill not be long conceal’d; since he meets with some Marginal References to other Tracts of His, which (these Papers having long lain by him) he forgot to have been set down for private use, and which should not have been expos’d to publick view.