and there were already six planets—Mercury, Venus, Earth, Mars, Jupiter, and Saturn; and six satellites—one for the Earth, four for Jupiter, and one for Saturn. This fancy did not, however, prevent his afterward accepting Cassini's discovery of four other satellites of Saturn, and speculating from it upon the possibility of there being still others, either between some of those already discovered, or beyond the orbits of all.
Huygens, having now attained a very high and extensive reputation, visited France and England in 1660 and 1661. He explained his method of grinding lenses to the scientific men of England, and, finding them occupied with the recently introduced air-pump, took back with him the idea of that instrument when he returned to Holland, after two years, to develop it and improve upon it. Remarking in his experiments the close adherence of two plates of polished metal in vacuo, he conceived that it was due to the same cause as that which, operating at still closer quarters, produces cohesion. At about the same period he developed a rule for estimating the height of a place by the local pressure, and reciprocally, for calculating the pressure at a given place from its elevation above the sea. He was made a member of the Royal Society of London, and communicated to it the solution of the law of impact of bodies, at which Descartes had made an unsuccessful attempt. His own solution involved the laws of motion, and of action and reaction, in the main as they are now understood, and contained the germ of the law of the conservation of forces.
In 1665 he accepted an invitation from Colbert to go to Paris and reside in the Bibliothèque Royale. There he wrote his treatises on dioptrics and the law of percussion, in a literary style which won from Newton the remark that it more nearly approached the style of the ancients than that of any other modern author. Subsequently he composed the greatest of his works, the "Horologium Oscillatorium," which was published in 1673, and has been pronounced, with the exception of Newton's "Principia," the finest work on the exact sciences of the seventeenth century. In the dedication of this work to King Louis XIV, he revealed the dominant characteristic of his mind, making it the great object of all his researches to find out useful things, to promote the knowledge of nature, and add to the comforts of living. "I shall not waste any time, great king," he said, "in demonstrating to you the usefulness of these things, for my automatons (clocks) placed in your apartments will impress you every day with the regularity of their indications and the consequences they promise you in the progress of astronomy and navigation." The first chapter of this work was devoted to the description of pendulum-clocks; the second chapter embodied a study of the motion of a grave body moving along a given curve, in which was established the tautochronism of motion in a cycloid. In the third chapter, concerning the evolution and dimension of linear curves, was introduced the idea from which
