Joint Author of 'Navigation and Nautical Astronomy'; Head Master of the NEW EDITION. LONDON: LONG MANS, GREEN, AND CO. 1876. All rights reserved. 2-38 PREFACE. OWING to the changes in the Examinations conducted by the Board of Trade, it has become necessary that a small manual, embracing the required information on Magnetism and Deviation of the Compass, should be brought within the capacity and means of the Student. Having been engaged as one of the oldest Science Teachers under the Science and Art Department, in instructing candidates for both its and the Board of Trade Examinations, the writer, at the solicitation of many pupils, has prepared the present little work. The notes, of which this is the enlargement, have been used in his school for the past ten years; they have now been recast, and are offered to the public to supply the present want. How far they will answer this purpose must be left for others to judge. Part I. is intended to cover the ground for the Class Examination in Science Schools; and the whole work should be mastered by candidates in Navigation at the Department Examinations, as well as by intelligent Sailors, especially those who intend to present themselves for either the Extra-Master's Certificate, or for the special endorsement on the Ordinary Master's Certificate. When this has been done, the Student is strongly recommended to study Deviation of the Compass, by F. J. Evans, R.N., F.R.S., Practical Information on the Deviation of the Compass, by J. T. Towson, F.R.G.S., The Admiralty Manual, Parts I. and II., and A Treatise on Magnetism, by G. B. Airy, M.A., F.R.S., the last two of which are especially adapted for the mathematical Student. J. M. Plymouth, July 1st, 1872 CONTENTS. ᏢᎪᎡᎢ 1. MAGNETISM. CHAPTER I. Definition-Poles, Magnetic Equator and Axis-How the Poles are found-Position which Free Magnets take-Magnetic Field-Theory of Magnetic Fluids-Coercive Force-Magnetic Induction-Attraction and Repulsion-Magnetic Substances CHAPTER II. Terrestrial Magnetism-Magnetic Meridian-Magnetic Elements-Variation or Declination-Inclination or Dip-Intensity-Secular Variation-Annual Variation-Diurnal Variation-Instruments used in obtaining Magnetic Elements-Magnetic Charts-Terrestrial Induction-Astatic Needle CHAPTER III. How Magnets are Made-Method of Single Touch- CHAPTER IV. To compare the Strength of Magnets-By the Deflection of a small Needle-By the Torsion Balance-By the Method of Oscillations-Absolute Unit of Force PAGB 1 10 25 30 |