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ON

MAGNETISM,

GENERAL AND TERRESTRIAL.

BY

HUMPHREY LLOYD, D.D., D. C. L. ;

PROVOST OF TRINITY COLLEGE, DUBLIN,

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DUBLIN:

PRINTED AT THE UNIVERSITY PRESS,

BY M. H. GILL.

PREFACE.

THE present volume is a fragment of a work projected and partly completed many years ago, which was intended to embrace the various branches of physical science usually comprehended in the Traitès de Physique of French writers. In the long interval which has elapsed since its commencement, the sciences of heat and electricity have been fully treated by English writers, their selection being in great measure due to their connexion with chemistry; while those of light, sound, and magnetism have not received the attention, here or elsewhere, which their intrinsic importance demands. The author has already endeavoured to supply in some degree this want, by the publication of a volume on the wave-theory of light; and he now seeks to satisfy it more fully by the production of a treatise introductory to the science of terrestrial magnetism.

The researches connected with this science may be classed under two heads-namely, first, the determination of the elements of the earth's magnetic force at several points of the earth's surface, and the laws

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