singular law, rudely harmonizing with some of Copernicus's measures, would have failed, for want of solids, in its application to Uranus and Neptune; but it took possession of Kepler's mind, and he declared that he would not barter the glory of its invention for the whole electorate of Saxony. When Galileo's opinion of this hypothesis was requested by Kepler, he praised the ingenuity which it displayed; but when a copy of the Prodromus was presented to Tycho, he advised his young friend "first to lay a solid foundation for his views by actual observation, and by ascending from these to strive to reach the causes of things;" and there is reason to believe that, by the magic of the whole Baconian philosophy thus compressed into a nutshell, Kepler abandoned for a while his visionary speculations.

When driven by religious persecutions from the States of Styria, he accepted an invitation from Tycho to settle at Prague as his assistant. Here he was introduced to the Emperor Rodolph, and upon Tycho's death in 1601, he was appointed mathematician to the Emperor, a situation which he held during the successive reigns of Matthias and Ferdinand.

After devoting much of his time to the subjects of refraction and vision, and adding largely to our knowledge of both these branches of optics, he resumed his enquiries respecting the orbits of the planets. Possessed of the numerous and valuable observations of Tycho, he endeavoured to represent them by the hypothesis of a uniform motion in circular orbits; but in examining the orbit of Mars he found the deviations from a circle too great to be owing to errors of observation. He therefore compared the observations with various other curves, and was led to the fine discovery that Mars revolved round the sun in an elliptical orbit in one of the foci of which the sun himself was placed. By means of the same observations he computed the dimensions of the planet's orbit, and by comparing the times in which Mars passed over different parts of it, he found that they were to one another as the areas described by the lines drawn from the centre of the planet to the centre of the

sun, or, in more technical language, that the radius vector, or line joining the sun and planet, describes equal areas in equal times. These two brilliant discoveries, the first ever made in physical astronomy, were extended to all the other planets of the system, and were given to the world in his 'Commentaries on the Motions of the Planet Mars.'

Thus successful in his researches, and overjoyed with the result of them, Kepler renewed his attempts to discover the mysterious relation which he believed to exist between the mean distances of the planets from the sun. Distrusting his original hypothesis of the geometrical solids, he compared the planetary distances with the intervals of musical notes, but though he was supported in this notion by the opinions of Pythagoras, and even of Archimedes, his comparisons were fruitless, and he was about to abandon an enquiry which had more or less occupied his mind during seventeen years of his life.

After Kepler had refused to accept the mathematical chair at Bologna, which was offered to him in 1617, he seems to have resumed his speculations "on the exquisite harmonies of the celestial motions." On the 8th of March, 1618, he conceived the idea of comparing the powers of the different numbers which express the distances of the planets, with the powers of the different numbers which express their periods round the sun. He compared, for example, the squares and the cubes of the distances with the same powers of the periodic times, and he even made the comparison between the squares of the periodic times and the cubes of the distances; but having in the hurry and impatience of research, been led into an error of calculation, he rejected the last of these relations—the relation that was true—as having no existence in nature. Before a week, however, had elapsed, his mind reverted to the law which he had rejected, and, upon repeating his calculations and discovering his error, he recognised with rapture the great truth of which he had for seventeen years been in search, that the periodic times of any two planets in the system are to one

another as the cubes of their distance from the sun. This great discovery was published in 1619 in his "Harmony of the World," which was dedicated to James VI. of Scotland, and which is marked with all the peculiarities of the author. The passage which describes the feelings under which he recognised the truth of this third law is too instructive to be omitted from his history. "What sixteen years ago I urged as a thing to be sought, that for which I joined Tycho Brahe, for which I settled in Prague, for which I have devoted the best part of my life to astronomical contemplations, at length I have brought to light, and have recognised its truth beyond my most sanguine expectations. It is now eighteen months since I got the first glimpse of light, three months since the dawn; a very few days since the unveiled sun—most admirable to gaze on—burst out upon me. . . The die is cast, the book is written, to be read either now or by posterity, I care not which. It may well wait a century for a reader, as God has waited six thousand years for an interpreter of his works." As the planes of the orbits of all the planets, as well as the line of their apsides passed through the sun, Kepler could not fail to suspect that some power resided in that luminary by which the motions of the planets were produced, and he went so far as to conjecture that this power diminishes as the square of the distance of the body on which it was exerted; but he immediately rejects this law in favour of that of the simple distances. In the introduction to his commentaries on Mars he distinctly recognises the mutual gravitation of matter in the descent of heavy bodies to the centre of the earth, as the centre of a round body of the same nature with themselves. He main tained that two stones, situated beyond the influence of a third body, would approach like two magnets and meet at a point, each describing a space proportional to the mass of the other. He maintained, also, that the tides were occasioned by the moon's attraction, and that the lunar inequalities were owing to the joint action of the sun and earth. Our countryman, Dr.

Gilbert, in his celebrated book-De Magnete, published in 1600, had about the same time announced similar opinions on gravitation. He compares the earth's action upon the moon to that of a great loadstone; and in his posthumous work, which appeared half a century afterwards, he maintained that the earth and moon act upon each other like two magnets, the influence of the earth being the greater on account of its superior mass. But though these opinions were a step in celestial physics, yet the identity of the gravity which is exhibited on the earth's surface by falling bodies, with that which guided the planets in their orbits, was not revealed either to the English or the German philosopher. It required more patience and thought than either could command, and its discovery was reserved for the exercise of higher powers. The misery in which Kepler lived stands in painful contrast with his arduous labours as an author, and his noble services to science. His small pension was ever in arrears, and when he returned to Silesia to spend the remainder of his days in retirement, his pecuniary difficulties became more embarrassing than before. He was compelled to apply personally for his arrears; and, in consequence of the great fatigue which he suffered in his long journey to Ratisbon on horseback, he was seized with a fever, which carried him off on the 30th November, 1630, in the fiftyninth year of his age.

Thus perished one of the noblest of his race, a victim of poverty, and a martyr to science.

234

HARVEY-1578-1658.*

William Harvey was born at Folkestone in Kent, April 1st, 1578. His schoolboy days, i.e., from his tenth to his fifteenth year, were spent at the King's School, Canterbury. On May 13th, 1593, he entered as a student at Caius College, Cambridge, where he took his degree of B.A. in 1597 and left the university. When nineteen years of age he visited France and Germany, and in the same year, 1597, entered the university at Padua, where he remained for about five years. He obtained his diploma there in 1602. Fabricius, of Aquapendente, was then his chief anatomical professor. After taking his degree, Harvey returned to England and obtained a similar distinction at Cambridge. In 1604 he was admitted a licentiate of the College of Physicians, and three years later a fellow. He married a daughter of Dr. Lancelot Browne, physician to Queen Elizabeth, when he settled in London and commenced practice. In 1609 he became physician to St. Bartholomew's Hospital. In 1615 he was appointed Lumleian lecturer, that is, to lecture at the college on anatomy and surgery, during which he developed, and to a certain extent propounded, the discovery which has immortalized his name. In 1628, his great work, the Exercitatio anatomica de motu cordis et sanguinis, appeared. It was published at Frankfort-on-the-Main. About this time he was appointed physician-extraordinary to James I. In 1630 he attended the young Duke of Lennox, a kinsman of Charles I., to the continent, and in 1632 was made physician-in-ordinary to His Majesty. In 1636 he accompanied Thomas, Earl of

* The matter of this sketch is derived from "The works of William Harvey, M.D., etc., translated from the Latin, with a life of the author by Robert Willis, M.D.-London: printed for the Sydenham Society, 1847." "The Harveian Oration for 1874, by Charles West, M. D.-Longmans, Green & Co." "The Harveian Oration for 1884, by J. Russell Reynolds, M.D., F.R.S.-J. & A. Churchill."