230 Kansas Academy of Science. 



THE FOUOAULT EXPERIMENT. 



By J. T. LovEWELL, Topeka. 



A MONG the scientific achievements of the French philosopher, 

 ■^-^ Foucault, there is none that made him so widely known as the 

 pendulum experiment, whereby he rendered visible the rotation of 

 the earth. This experiment was published in 1851 and was at once 

 repeated by many scientific men in Europe and America. Arago 

 performed it at the University of Paris, and it was exhibited in the 

 Pantheon, with a pendulum 220 feet long. 



In our own country it was set up in Bunker Hill monument, in 

 the capitol, at Washington, and in various state capitols. 



Use was made of these lofty edifices, which had requisite solid- 

 ity, and allowed a longer pendulum than can often be installed in 

 smaller structures. While long pendulums are desirable in this 

 experiment, they are by no means essential to show the main fact 

 of the rotation of the earth. Foucault's original experiment was 

 made with a pendulum only six and one-half feet long, and with 

 this he demonstrated the earth's rotation with entire success. 



The idea of Foucault's pendulum is involved in the law of in- 

 ertia, and it had occurred to others long before his time, but he 

 was the first to bring it to a demonstration. The vibration is 

 caused by force of gravity, always directed toward the center of 

 the earth, and is entirely independent of the motion of the point 

 of suspension. The absolute motion of the pendulum in space is 

 very complicated, and involves both the annual and diurnal rota- 

 tions of the earth, but we are here concerned only with the relative 

 change in direction, with lines on the earth, and this is, as we shall 

 see, at the poles, fifteen degrees per hour. By considering the 

 pendulum mounted on a prolongation of the earth's axis, its plane 

 of oscillation will cut across the meridians and will make a com- 

 plete circuit in twenty-four hours. 



At the north pole this apparent rotation would be like the hands 

 of a watch, from left to right, while at the south pole this motion 

 would appear in the contrary direction. At the equator there 

 would be no rotation, while in other latitudes the motion would be 

 the rate at the poles multiplied by the sine of the latitude. This 

 may be shown in various ways by mathematical demonstration, 

 and may be simply illustrated by the use of a globe, and drawing a 

 line in the plane of oscillation, and comparing its direction with 



