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PREFACE TO FIRST EDITION OF UNITS
AND PHYSICAL CONSTANTS, 1879.

THIS Book is substantially a new edition of my “Illustrations of the C.G.S. System of Units" published in 1875 by the Physical Society of London, supplemented by an extensive collection of physical data. The title has been changed with the view of rendering it more generally intelligible.

Additional explanations have been given upon some points of theory, especially in connection with Stress and Strain, and with Coefficients of Diffusion. Under the former head, I have ventured to introduce the terms "resilience" and "coefficient of resilience," in order to avoid the multiplicity of meanings which have become attached to the word "elasticity."

A still greater innovation has been introduced in an extended use of the symbols and processes of multiplication and division, in connection with equations which express not numerical but physical equality. The advantages of this mode of procedure are illustrated by its application to the solution of the most difficult problems on units that I have been able to collect from standard text-books (chiefly from Wormell's 'Dynamics').

I am indebted to several friends for contributions of experimental data.

A Dutch translation of the first edition of this work has been made by DR. C. J. MATTHES, Secretary of the Royal Academy of Sciences of Amsterdam, and was published in that city in 1877.

Though the publication is no longer officially connected with the Physical Society, the present enlarged edition is issued with the Society's full consent and approval.

PREFACE TO THE PRESENT EDITION.

In collecting materials for this edition, I have gone carefully through the Transactions and Proceedings of the Royal Society, the Royal Society of Edinburgh, and the Physical Society of London, from 1879 onwards, besides consulting numerous papers, both English and foreign, which have been sent to me by their authors. I have also had the advantage of the co-operation of Dr. Pierre Chappuis (of the Bureau International des Poids et Mesures), who has for some years been engaged in preparing a German edition. Several items have been extracted from the very elaborate and valuable PhysikalischCheinische Tabellen of Landolt and Börnstein (Julius Springer, Berlin, 1883). Among friends to whom I am indebted for data or useful suggestions, are Prof. Barrett, Mr. J. T. Bottomley, Prof. G. C. Foster, Prof. Lodge, Prof. Newcomb, Mr. Preece, and the Astronomer-Royal,

The expository portions of the book are for the most part unchanged; but a Supplemental Section has been added (p. 34) on physical deductions from the dimensions of units; a simplification has been introduced in the discussion of adiabatic compression (p. 125); and the account of thermoelectricity (p. 172) has been re-written and enlarged.

The name “thermoelectric height” has been introduced to denote the element usually represented by the ordinates of a thermoelectric diagram.

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The preliminary “Tables for reducing other measures to C.G.S. measures " have been greatly extended, and in each case the reciprocal factors are given which serve for reducing from C.G.S. measures to other measures. Professor Miller's comparison of the kilogramme and pound is supplemented by three later comparisons officially made at the Bureau International.

A nearly complete list of the changes and additions now introduced is appended to this Preface, as it will probably be useful to possessors of the previous edition.

The adoption of the Centimetre, Gramme, and Second, as the fundamental units, by the International Congress of Electricians at Paris in 1881, led to the immediate execution of a French translation of this work, which was published at Paris by Gauthier-Villars in 1883. The German translation was commenced about the same time, but the desire to perfect its collection of physical data has caused much delay. It will be brought out by Ambrosius Barth, the publisher of Wiedemann's Annalen. A Polish edition, by Prof. J. J. Boguski, was published at Warsaw in 1885; and permission has been asked and granted for the publication of an Italian edition,

J. D. EVERETT. BELFAST, September, 1886.

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