448 UNIVERSITY OF VIRGINIA PUBLICATIONS 



Both of these objects, the projection of. the Foucault movement and 

 its great amphfication, were secured by the use of a very delicate lever of 

 straw well balanced on its fulcrum, which was a fine needle thrust per- 

 pendicularly through the straw and resting its point in a minute glass cup. 

 The lever's length was not less than half the entire swing of the wire at the 

 selected point. It was mounted so as to move in a horizontal plane on the 

 side of the pendulum swing toward which the Foucault motion was ex- 

 pected. It was so placed that the wire in its vibration touched the ex- 

 tremity of the long arm. At its next swing in the same direction, it would 



move the lever sUghtly, and so from swing to swing always in the clock- 

 wise sense in our northern hemisphere. To the straw at the fulcrum was 

 cemented a small plane mirror of thin silvered glass. This reflected a 

 pencil of light from a lantern to a properly placed screen in front of the 

 class. The spot of light on the screen exhibited by its displacement to the 

 entire assembly the fact of the change of vibration-plane. 



To amplify and measure this change, the following adjustment was 

 made. In figure 3 in a horizontal plane, let be the point through which 

 the wire passes when the pendulum is at rest. Place the straw lever so 



