Dr. G. Wilson's "Life of Cavendish," which forms the first volume of the publications of the Cavendish Society; and the "Electrical Researches of the Hon. Henry Cavendish, F.R.S.," edited by the late Professor James Clerk Maxwell. "The Life of Sir Benjamin Thompson, Count Rumford," by George E. Ellis, published by the American Academy of Arts and Sciences, in connection with the complete edition of his works. "Memoir of Thomas Young," by the late Dean Peacock. Dr. Bence Jones's "Life of Faraday;" and Professor Tyndall's "Faraday as a Discoverer." "Life of James Clerk Maxwell," by Professor Lewis Campbell and William Garnett. It is hoped that the perusal of the following sketches may prove as instructive to the reader as their preparation has been to the writer. WM. GARNETT. NEWCASTLE-UPON-TYNE, _December, 1885_. CONTENTS. PAGE INTRODUCTION 1 ROBERT BOYLE 5 BENJAMIN FRANKLIN 38 HENRY CAVENDISH 125 COUNT RUMFORD 148 THOMAS YOUNG 194 MICHAEL FARADAY 237 JAMES CLERK MAXWELL 278 CONCLUSION 309 HEROES OF SCIENCE. INTRODUCTION. The dawn of true ideas respecting mechanics has been described in the volume of this series devoted to astronomers. At the time when the first of the following biographies opens there were a few men who held sound views respecting the laws of motion and the principles of hydrostatics. Considerable advance had been made in the subject of geometrical optics; the rectilinear propagation of light and the laws of reflection having been known to the Greeks and Arabians, whilst Willebrod Snellius, Professor of Mathematics at Leyden, had correctly enunciated the laws of refraction very early in the seventeenth century. Pliny mentions the action of a sphere of rock-crystal and of a glass globe filled with water in bringing light to a focus. Roger Bacon used segments of a glass sphere as lenses; and in the eleventh century Alhazen made many measurements of the angles of incidence and refraction, though he did not succeed in discovering the law. Huyghens developed to a great extent the undulatory theory; while Newton at the same time made great contributions to the subject of geometrical optics, decomposed white light by means of a prism, investigated the colours of thin plates, and some cases of diffraction, and speculated on the nature, properties, and functions of the ether, which was equally necessary to the corpuscular as to the undulatory theory of light, if any of the phenomena of interference were to be explained. The velocity of light was first measured by Roemer, in 1676. The camera obscura was invented by Baptista Porta, a wealthy Neapolitan, in 1560; and Kepler explained the action of the eye as an optical instrument, in 1604. Antonio de Dominis, Archbishop of Spalatro, discovered the fringe of colours produced by sunlight once reflected from the interior of a globe of water, and this led, in Newton's