838 THE POPULAR SCIENCE MONTHLY. 



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 chap- 

 ter was estimated the measure of the centrifugal force in circular mo- 

 tion. 



We next find ITuygens devising the application of the spiral 

 spring to clock-movements, and making pocket watches and sea chro- 

 nometers possible, and then disputing for the priority of the invention 

 with the Abb6 Hautef euille, " one of those schemers who begin every- 

 thing 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 hun- 

 dred 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 sup- 

 posed 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 cir- 

 cles, where the motion was slower, thus approaching the center. Con- 

 sidering the phenomena of terrestrial gravity exhibited in the varia- 

 tions 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 en- 

 dured. 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 rep- 

 resent 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 Leib- 

 nitz, 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 prog- 

 ress and speculation." 



The " Cosmotheoros," or " Observer of the World," which was not 



