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PLATE I.

FIG. 1. Continuity of Matter with Increase of Dimen

sion

PAGE

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2. Equal Degrees of Heat and Cold, not com-
municated in equal Times

194

5. Consumption of Air by a burning Candle 283

PLATE II.

FIG. 1. A Pyrostatical Machine for weighing the Force of Fire....

88

210

2. Essay-Box to obtain the Degrees of the Index, 296
s. Heater and its Stand, for Experiments in Vacuo, 304
4. An Heater of Iron to be applied to the Bar of
the Machine....

297

5. A Vessel for weighing the Force of Frost .. 345 6. Occasional Lever when the Vessel 5 is used.. ibid.

PLATE III.

FIG. 1. and 2. Frost on Panes of a Glass Window... 431

INTRODUCTION.

IN

N the year 1762 I published An Essay on the First Principles of Natural Philosophy, and sent it abroad for an experiment, without a Preface, and without a Dedication. In the year following an edition of the same was printed in a smaller size, in Ireland. The design of the work, as the Title sets forth, was to demonstrate the use of Natural Means, or Second Causes, in the Economy of the Material World, from Reason, Experiments, and the Testimony of Antiquity. The Inquiry itself, however imperfectly executed, seemed to be of great importance, and worthy of an impartial exa→ mination from all scholars who were competent judges of the subject. A very considerable step is made in Natural Philosophy,

VOL. IX,

B

if

if the great question is determined concerning the Nature of those Causes, or First Principles, which are established by the Creator in the Constitution of the Natural World.

The ingenious Author of one of our modern Systems of Philosophy, objected to the use of the terms First Principles, as if I had been guilty of an impropriety in the Title of my work but the expression is allowable, and seemed to convey, my meaning better than any other I could find. It was used by Dr. Desaguliers, a writer well skilled in experimental philosophy; and his words are cited in the course of my work *. “Attraction and repulsion," says he, "seem to be settled by the great Creator as FIRST PRÍNCIPLES in Nature; that is, as the first of second causes." Now, as my design in that work was to demonstrate the use of Second Causes in Nature, and to point out those which have the first place, I took the words in the same sense in which they had been used by Dr. Desaguliers, and called my treatise An Essay on the First Principles, that is, on the first of those Second Causes whereby

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Oxford Edit. P.
65.

Dublin Edit. P.

63.

whereby we endeavour to account for the operations of Nature.

Every author, when he sits down to write, wishes to give general satisfaction: but to please all readers in a subject of such deep inquiry, and concerning which there always have been different and contradictory opinions, would have been a hopeless undertaking. It was justly apprehended, at the publication of the work, that a treatise, presuming to interfere with the very foundations of Philosophy, raising questions about Causes supposed to be already established and explained as far as they could be, and proceeding on Principles new to some, and exploded by others, could never make its way without some interruption. The author, therefore, prepared himself for the worst; and gave up, at the first step, every prospect of popular encouragement-a sacrifice which every man must be ready to make, who thinks he has any thing great and useful to propose; especially in an age more addicted to the airy amusements of the fancy, than to the serious and painful disquisitions of solid learning. However, I had the satisfaction to find, after waiting for several years, that although some observations have been offered

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