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PREF A C E.

T HE quick Sale of the first Impression

1 of this work, is a satisfactory Proof of its Utility to the Public; and the urgent Demand for it, which still continues, is the Reafon of this Second Edition. The Errors of the first have been carefully corrected, as well of the Plates as of the Letter-Press, There is also added a Table of Contents ; and a copious Index to each Volume. Some Passages have been altered for the better; but no considerable Additions have been made, in Justice to the former Purchasers. The principal Improvements I have made in these Subjects since their first Publication, have been in a New ORRERY, MICROSCOPE, and Clock; the Orrery and Clock I have already published the Description and Use of, with a Copper-Plate Print of each ; also a short Sketch of the Microscope, under the Title of the Polydynamic MICROSCOPE,

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because to each Object Lens there are given, by this new Construction, several different magnifying Powers. I propose to give a further Account of this Microscope, and a great many curious Copper-Plates of Microfcopic Objects, which, together with the abovementioned Pieces, and my Optical Elays, will make a Third Volume, that may be properly considered as supplemental to the Gentleman and Lady's PHILOSOPHY ; and will probably be the last Effort I may ever make of rendering my Lucubrations in this Way) serviceable to the Public.

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