SKETCH OF HUYGENS. 837 



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 uj)on the possibility of there being 

 still others, either between some of those already discovei'ed, or be- 

 yond the orbits of all. 



Huygens, having now attained a very high and extensive reputa- 

 tion, visited France and England in 1660 and 1661. He explained his 

 method of grinding lenses to the scientific men of England, and, find- 

 ing 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 mem- 

 ber of the Royal Society of London, and communicated to it the solu- 

 tion 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 Biblioth^que 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 com- 

 posed the greatest of his works, the " Horologium Oscillatorium," 

 which was published in 1673, and has been pronounced, with the ex- 

 ception 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, mak- 

 ing 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 regu- 

 larity 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 



