23] 



COMPARISON OF THEORY AND EXPERIMENT. 



51 



is similar to Fig. 

 15, the angle sub- 

 tended at the 

 center by two 

 successive points 

 of tangency being 

 greater than a 

 right angle, as we 

 shall see. 



Figs. 14, 15 a, 

 15b, 15c, 16a, 

 are reproductions 

 of photographs of 

 actual swings of 

 a pendulum. A 

 brass ball was 

 swung by a string 

 attached to a 

 screw - eye, and 

 carried a small in- 

 candescent lamp. 

 On the floor 

 below, and at one 

 side were placed 

 cameras with open 

 shutters, in a dark 

 room. When the 

 ball was swung, 

 the light was 

 turned on for a 

 sufficient number 

 of swings, and the 

 path registered on 

 the photographic 

 plate. On the 

 photograph Fig. 

 15c, the maximum 



and minimum 

 radii were mea- 

 sured, from which 

 could be calcu- 

 lated the roots a, 

 /J, and thence y. 

 Then from equa- 



Fig. 15 c. 



Fig. 15 d. 



