456 UNIVERSITY OF VIRGINIA PUBLICATIONS 



match is applied. The segment of string, attached to the ball by a hook 

 pointed downward, then drops to the bottom of the box leaving the pendu- 

 lum perfectly free. 



The lantern is placed at a distance from the mirror comparable with 

 that of the screen. It is well to be able to move the lantern about in order 

 to adjust, while the pendulum is working, the zero position of the spot on 

 the screen. The apparatus can be set up in about 15 minutes and give an 

 accuracy of 2 per cent, corresponding to less than 1 degree in latitude here. 

 Further pains would yield greater accuracy than this. 



A Foucault pendulum for class room use is said to have been exhibited 

 at the Paris Exposition of 1900. It was a rigid pendulum about 2 meters 

 long suspended from crossed knife edges. The motion was followed by 

 the projection, through a microscope objective (focus, say, 2 inches) parallel 

 to the vibration plane, of a fine wire affixed to the bottom of the pendulum. 

 This wire came in focus momentarily, at the end of each swing and thus 

 the lateral displacement could be traced upon the screen. This form is 

 in use at the Case School of Applied Sciences and is said to be satisfactory. 



In the Physikalische Zeitschrift several years ago there was described 

 a tripod arrangement similar in principle to that described here." The 

 author, however, actuates it by a point under the bob and so it is subject 

 to the difficulties which Professor Smith has so beautifully avoided in his 

 apparatus. 



