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TRANSLATOR'S PREFACE.

THE “TRAITÉ ÉLÉMENTAIRE DE PHYSIQUE” of Professor Deschanel, though only published in 1868, has already obtained a high reputation in France, and has been adopted by the Minister of Instruction as the text-book for Government Schools.

I did not consent to undertake the labour of translating and editing it till a careful examination had convinced me that it was better adapted to the requirements of my own class of Experimental Physics than any other work with which I was acquainted; and in executing the translation I have steadily kept this use in view, believing that I was thus adopting the surest means of meeting the wants of teachers generally.

The treatise of Professor Deschanel is remarkable for the vigour of its style, which specially commends it as a book for private reading. But its leading excellence, as compared with the best works at present in use, is the thoroughly rational character of the information which it presents. There is great danger in the present day lest science-teaching should degenerate into the accumulation of disconnected facts and unexplained formulæ, which burden the memory without cultivating the understanding. Professor Deschanel has been eminently successful in exhibiting facts in their mutual connection; and his applications of algebra are always judicious.

The peculiarly vigorous and idiomatic style of the original would be altogether unpresentable in English; and I have not hesitated in numerous instances to sacrifice exactness of translation to effective rendering, my object being to make the book as useful as possible to English readers. For the same reason I have not scrupled to suppress or modify any statement, whether historical or philosophical, which I deemed erroneous or defective. In some instances I have endea

voured to simplify the reasonings by which propositions are established or formulæ deduced.

As regards weights and measures, rough statements of quantity have generally been expressed in British units; but in many cases the numerical values given in the original, and belonging to the metrical system, have been retained, with or without their English equivalents; as it is desirable that all students of science should familiarize themselves with a system of weights and measures which affords peculiar facilities for scientific calculation, and is extensively employed by scientific men of all countries. For convenience of reference, a complete table of metrical and British equivalents has been annexed.

The additions, which have been very extensive, relate either to subjects generally considered essential in this country to a treatise on Natural Philosophy, or to topics which have in recent years occupied an important place in physical discussions, though as yet but little known to the general public.

The sections distinguished by a letter appended to a number are all new; as also are all foot-notes, except those which are signed with the Author's initial “D.”

In many instances the new matter is so interwoven with the old that it could not conveniently be indicated; and I have aimed at giving unity to the book rather than at preserving careful distinctions of authorship.

Comparison with the original will however be easy, as the numbering of the original sections has been almost invariably followed.

Additions have been made under the heads of Dynamics, Capillarity, and the Barometer. The chapter on Hydrometers has also been recast.

In the present edition the chapters relating to Consonance and Dissonance, Color, the Undulatory Theory, and Polarization, are the work of the Editor ; the chapter on Thermo-dynamics is almost entirely his. Large portions of the chapters on Conduction and on Terrestrial Temperatures have also been rewritten; and considerable additions have been made in connection with Hygrometry, the Theory of Exchanges, the Specific Heats of Gases, and the Motion of Glaciers, besides numerous changes and additions in other portions of the work.

The accurate method of treating electrical subjects which has been established by Sir William Thomson and his coadjutors, has TRANSLATOR'S PREFACE.

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not yet been adopted in France; and some of Faraday's electromagnetic work appears to be still very imperfectly appreciated by French writers. The Editor has accordingly found it necessary to recast a considerable portion, besides introducing two new chapters (XXXIX4. and XL14.) and an Appendix. Potential and lines of force are not so much as mentioned in the original.

The elements of the theory of magnetism have been based on Sir William Thomson's papers in the Philosophical Transactions ; and the description of the apparatus used in magnetic observatories has been drawn from the recently-published work of the Astronomer Royal. The account of electrical units given in the Appendix is mainly founded on the Report of the Electrical Committee of the British Association for the year 1863.

The nomenclature of units of heat which has been adopted, is borrowed from Prof. G. C. Foster's article “ Heat” in Watts's Dictionary of Chemistry.

M. Deschanel's descriptions of apparatus, of which some very elaborate examples occur in the present volume, left little to be desired in point of clearness. In no instance has it been found necessary to resort to the mere verbal rendering of unintelligible details.

The numbering of the original sections has been preserved only to the end of Chapter LX.; the last two chapters of the original having been transposed for greater convenience of treatment.

CONTENTS.

machine.--Spaces.--Velocities.-- Bourbouze's modification of Attwood's machine.--

Morin's apparatus.--Formulæ for falling bodies.--Examples.-Upward and downward

course in vacuo and in air.- Motion of projectiles.--Composition of motions.-Uniform

acceleration.—Acceleration in general.- Proportional directly to force and inversely to

mass.—Gauss's absolute unit of force.-The pound a standard of mass.—Masses which

are equal by the acceleration test, are equal by the balance test, ... pp. 40-55.

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