the author deduced the theory of evolutes. In the fourth chapter he determined the center of oscillation of a pendulum, and consequently the length of the simple isochronous pendulum; and in the fifth chapter was estimated the measure of the centrifugal force in circular motion.
We next find Huygens devising the application of the spiral spring to clock-movements, and making pocket watches and sea chronometers possible, and then disputing for the priority of the invention with the Abbé Hautefeuille, "one of those schemers who begin everything and finish nothing."
Huygens turned his attention to the study of the properties of light and weight and of the magnet, and communicated his results to the French Academy and the Royal Society. His theory of light was the one which is now generally accepted after having slept for a hundred and fifty years. Double refraction attracted his attention, and he explained that it was occasioned by an ellipsoidal form given to the light-waves, while in ordinary refraction the waves were spherical. To account for gravity he accepted the Cartesian vortices, and supposed that those bodies which were too unwieldy to keep up with the motion of the outside circles were forced to fall back into the inner circles, where the motion was slower, thus approaching the center. Considering the phenomena of terrestrial gravity exhibited in the variations of the oscillations of the pendulum, he concluded that the earth was a spheroid and not a sphere. He accounted for magnetism in a paper which has never been published, by a theory that has not endured. He left France in 1681, some say on account of the Edict of Nantes, others because his health was bad and he needed a change. At home in Holland he constructed an automatic planetarium to represent the motions of the solar system, and in doing it discovered the theory of continuous fractions.
In the mean time a revolution was taking place in the world of mathematics, through the discovery of the differential calculus by Leibnitz, a philosopher who has said of his intercourse with Huygens, some ten years previous to this time (1672 and 1673), that it opened a new world to him and made him feel like another man. The use of the new method would have greatly facilitated the calculations Huygens was making, but he had become skilled in the old ways, imperfect as they were, and not always of universal application, and, being too old to change his method readily, continued to employ them. But, after a discussion of the merits of the new system in correspondence with Leibnitz, he came to a full appreciation of its value, which he expressed freely by saying that he observed "with surprise and admiration the extent and fruitfulness of that art; on whatever side he turned, he discovered new uses for it; and conceived it destined to infinite progress and speculation."
The "Cosmotheoros," or "Observer of the World," which was not
