This book makes no claim to be a biography of Lord Kelvin in the usual sense. It is an extension of an article which appeared in the _Glasgow Herald_ for December 19, 1907, and has been written at the suggestion of various friends of Lord Kelvin, in the University of Glasgow and elsewhere, who had read that article. The aim of the volume is to give an account of Lord Kelvin's life of scientific activity, and to explain to the student, and to the general reader who takes an interest in physical science and its applications, the nature of his discoveries. Only such a statement of biographical facts as seems in harmony with this purpose is attempted. But I have ventured, as an old pupil and assistant of Lord Kelvin, to sketch here and there the scene in his class-room and laboratory, and to record some of the incidents of his teaching and work. I am under obligations to the proprietors of the _Glasgow Herald_ for their freely accorded permission to make use of their article, and to Messrs. Annan, photographers, Glasgow, and Messrs. James MacLehose & Sons, Glasgow, for the illustrations which are given, and which I hope may add to the interest of the book. A. GRAY. _The University_, _Glasgow_, _May_ 20, 1908. CONTENTS CHAP. PAGE I. PARENTAGE AND EARLY EDUCATION 1 II. CLASSES AT THE UNIVERSITY OF GLASGOW. FIRST SCIENTIFIC PAPERS 13 III. UNIVERSITY OF CAMBRIDGE. SCIENTIFIC WORK AS UNDERGRADUATE 23 IV. THE MATHEMATICAL THEORY OF ELECTRICITY IN EQUILIBRIUM. ELECTRIC IMAGES. ELECTRIC INVERSION 33 V. THE CHAIR OF NATURAL PHILOSOPHY AT GLASGOW. ESTABLISHMENT OF THE FIRST PHYSICAL LABORATORY 61 VI. FRIENDSHIP WITH STOKES AND JOULE. EARLY WORK AT GLASGOW 79 VII. THE 'ACCOUNT OF CARNOT'S THEORY OF THE MOTIVE POWER OF HEAT'--TRANSITION TO THE DYNAMICAL THEORY OF HEAT 99 VIII. THERMODYNAMICS AND ABSOLUTE THERMOMETRY 114 IX. HYDRODYNAMICS--DYNAMICAL THEOREM OF MINIMUM ENERGY--VORTEX MOTION 153 X. THE ENERGY THEORY OF ELECTROLYSIS--ELECTRICAL UNITS--ELECTRICAL OSCILLATIONS 176 XI. THOMSON AND TAIT'S 'NATURAL PHILOSOPHY'--GYROSTATIC ACTION--'ELECTROSTATICS AND MAGNETISM' 194 XII. THE AGE OF THE EARTH 229 XIII. BRITISH ASSOCIATION COMMITTEE ON ELECTRICAL STANDARDS 244 XIV. THE BALTIMORE LECTURES 254 XV. SPEED OF TELEGRAPH SIGNALLING--LAYING OF SUBMARINE CABLES--TELEGRAPH INSTRUMENTS--NAVIGATIONAL INSTRUMENTS, COMPASS AND SOUNDING MACHINE 264