rendering, and in several instances I have been glad to avail myself of the permission of the editors (Drs. Chappuis and Kreichgauer) to make use of their material.

A Russian translation was published at St. Petersburg in 1888, edited by Mr. P. N. Verbitsky and Captain J. Th. Gerabiatieff, and revised by Professor S. A. Ousoff. I am indebted to these gentlemen for pointing out several errors which they discovered in a searching scrutiny not only of the book itself but of the authorities quoted in it. They have, for example, corrected some numbers quoted from Péclet, by the aid of a table of errata in Péclet's book which I had overlooked.

in

The following are the principal items of new matter:1. A collection of determinations of viscosities of liquids and gases; in connection with which I have to express my deep obligations to Mr. Carl Barus, who lent me a large bundle of MSS., cluding all recent results on this subject. 2. A summary of recent investigations of the magnetic properties of iron and other substances, with the necessary expositions of theory and terminology. Under this head I have to acknowledge very special obligations to Mr. Shelford Bidwell, some six or eight pages of the new matter being substantially from his pen. I have also received valuable aid from Professor Ewing and Dr. Hopkinson.

3. A revolutionising of the introductory sections on Heat, rendered necessary by Rowland's discovery that the specific heat of water decreases by 1 per

cent. as the temperature rises from 5° to 30° C. The new sections include Rowland's results for the mechanical equivalent of heat, and for the comparison of the thermodynamic scale of temperature with the air thermometer and with a Kew standard.

4. Two sections on self-induction, mutual induction, and the theory and terminology of alternating

currents.

5. Two sections on the dimensions of electrical and magnetic quantities in terms of K and p Under this head I have to acknowledge kind assistance from Professors Rücker and Fitzgerald.

6. A table of values of the magnetic elements at a number of stations in the British Isles, selected from the results of the recent survey of Professors Rücker and Thorpe.

Information on several subjects has been brought down to date; including

Wave-lengths of light.

Mercury standards of resistance, and their correction for temperature.

Standards of light-giving power.

Emission of heat.

Conduction of heat by liquids.

Departure from Boyle's law at very high pressures. Compression of fresh water, sea water, mercury, and glass, at very high pressures.

Coefficients of diffusion of liquids and gases, with a simplified exposition.

Comparison of the mass of unit volume of water with standards of mass.

Besides the names above mentioned, I desire to return my thanks to several well-known men of science who have aided me by corrections or suggestions, all of which have been carefully considered, though in some instances I have not seen my way to carry them out.

PREFACE TO "ILLUSTRATIONS OF THE C.G.S. SYSTEM OF UNITS," 1875.

THE quantitative study of physics, and especially of the relations between different branches of physics, is every day receiving more attention.

To facilitate this study, by exemplifying the use of a system of units fitted for placing such relations in the clearest light, is the main object of the present treatise.

A complete account is given of the theory of units ab initio. The Centimetre-Gramme-Second (or C.G.S.) system is then explained; and the remainder of the work is occupied with illustrations of its application to various branches of physics. As a means to this end, the most important experimental data relating to each subject are concisely presented on one uniform scale-a luxury hitherto unknown to the scientific calculator.

I am indebted to several friends for assistance in special departments-but especially to Professor Clerk Maxwell and Professor G. C. Foster, who revised the entire manuscript of the work in its original form.

Great pains have been taken to make the work correct as a book of reference. Readers who may discover any errors will greatly oblige me by pointing them out.

EXTRACT FROM 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.

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.

EXTRACT FROM PREFACE TO THE EDITION OF 1886.

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 Physikalisch-Chemische Tabellen of Landolt and Börnstein (Julius Springer, Berlin, 1883).

A Supplemental Section has been added on physical deductions from the dimensions of units; a simplification has been introduced in the discussion of adiabatic compression; and the account of thermoelectricity has been rewritten and enlarged. The name "thermoelectric height" has been introduced to denote the element usually represented by the ordinates of a thermoelectric diagram.

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.