26 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



be the height of the point of departure. In a word, the oscillations 

 of the pendulum are always isochronous, and this isochronism is 

 independent of the amplitude of oscillation. Another difficulty 

 presents itself, inasmuch as the length of the pendulum varies with 

 the temperature, increasing when the temperature increases, and 

 lessening when the temperature lessens. We shall see further on, in 

 the Book devoted to the applications of heat, how these difficulties 

 have been surmounted. We may here conclude by emphasizing the 

 extreme importance of Huygens' discovery consequent on Galileo's 

 observations. From this period a little more than two centuries 

 ago clock-making has be.come an art of such precision as to render 

 most valuable service to all the physical sciences, and especially 

 to astronomy. 



IV. THE MOVEMENT OF KOTATION OF THE EARTH AND APPARENT 

 DEVIATION OF THE PENDULUM. 



We mentioned in the Forces of Nature some of the applica- 

 tions of the properties and laws of the pendulum to the physics of 

 the globe. It remains for us here to say a few words about an 

 experiment which, some years ago, took great hold on the public. 

 We speak of the experimental proof of the rotation of the earth by 

 the deviation of a pendulum, a proof thought out and realized by 

 Foucault. The experiment of which we speak is based on a principle 

 of mechanics, which, applied to the rotation of a spheroid like the 

 earth, may be summed up in these three propositions : ' 



1. A pendulum placed at one of the poles of the earth, its point 

 of suspension being in the axis of terrestrial rotation, will oscillate 

 so that the plane of its successive oscillations would maintain a 

 constant direction in space. Then an observer placed at that spot, 

 finding himself drawn round by the rotation of the earth, without 

 being conscious of his own movement, would believe he saw the 

 pendulum oscillate in variable planes coinciding successively with all 

 the meridians. After a sidereal day, that is, after twenty- three hours 

 fifty-six minutes of mean time, the plane of oscillation of the pen- 

 'dulum would appear to him to have gone through a complete revolu- 

 tion, and in a direction opposite to that of the rotation of the earth. 



