64 THE WONDERFUL CENTURY. CHAP. vm. 



experiments on the measurement of the velocity of light 

 has also discovered a method by which the rotation of the 

 earth on its axis can be experimentally demonstrated. 

 When a heavy body is in free motion in any direction, it 

 requires force to change the direction; and if no such 

 force is applied, it will continue its motion in the same 

 straight line or in the same plane. If a heavy pendulum 

 is suspended from the axis of a horizontal wheel by a 

 very long, thin wire, and if, when swinging in a fixed 

 line across the room, the wheel is slowly turned, either 

 the wire will twist a little or the ball forming the weight 

 of the pendulum will revolve, but the plane in which the 

 weight swings will not be altered. On the same prin- 

 ciple, any pendulum freely swinging near the ^"orth 

 Pole will not change the direction of its swing, although 

 its point of support revolves in twenty-four hours with 

 the earth's surface to which it is attached. On trying 

 the experiment with a heavy weight suspended from the 

 dome of the Pantheon in Paris and carefully set swing- 

 ing, the plane of oscillation of the weight was found ap- 

 parently to change at a uniform rate, and always in the 

 same direction, which was opposite to that of the earth's 

 rotation; proving that the surface of the earth moved 

 round while the plane of oscillation remained fixed in 

 space. This experiment can be tried in any place free 

 from currents of air, such as a cellar. It only requires 

 a heavy weight, say of 28 Ibs., to be suspended by a 

 string just strong enough to bear it. The weight must 

 be drawn three or four feet away from the vertical line 

 and fastened by a thread, so as to be set swinging by 

 burning the thread without giving it any lateral motion. 

 In an hour the line of swing will be found to have 



