248 Mr. Scrymgeour's Experiments to determine the Influence 



cessively lengthened it, even beyond its proper length, and tried 

 the vibrations at every small interval; but the experiments 

 failed to give a nearer approximation to isochronism than in 

 the former case. In short, the vibrations thus obtained did 

 not appear to be more isochronous than those which were ob- 

 tained by employing a knife-edge. 



Being disappointed in not finding the vibrations equal in 

 time, I began to doubt the property of the cycloid in render- 

 ing the vibrations of the pendulum isochronous. For my own 

 satisfaction on this point, I therefore instituted the following 

 experiments. I described a circle with beam compasses (the 

 distance of the points of which was double the diameter of the 

 circle with which the cycloid had been described), so close by 

 the central portion of the cycloid on the same sheet of brass, 

 as to be within ^ of an inch of that curve. To an extent of 

 about 9° or 10° on each side of the middle of the curve, there 

 was no perceptible difference between it and the circumference 

 of the circle to the same extent. 



My next step was to construct metallic cycloidal cheeks of 

 the proper size for the seconds pendulum, and to fix them in 

 a vertical position, so that their extreme points were at the 

 distance of the base of the curve from each other; while the 

 middle portions of the curve inverted served as cheeks for the 

 vibrations of the pendulum, in the usual way prescribed in 

 philosophical treatises on the subject. A ball was then sus- 

 pended between the cheeks, by means of a piece of small gold 

 w r ire flattened to about ji^ of an inch in thickness. As soon 

 as the ball was made to vibrate, it was obvious that the cheeks 

 would have no effect in equalizing the vibrations, even when 

 the extent of vibration was 9° or 10° beyond the point of rest. 



Subsequently, it occurred to me to try the experiment by 

 employing the extreme portions of the curve as cheeks, and 

 fixing them for this purpose in the manner above mentioned. 

 The cycloidal cheeks formed in this manner were found to 

 have a considerable effect in equalizing the times of the vibra- 

 tions; for the short vibrations of 1°*5 were not above 8 or 

 10 seconds faster in 24? hours than the longer vibrations of 4?° 

 or 5° ; and had all been as perfect as theory requires, the vi- 

 brations might have been isochronous. 



In ascertaining the proportions of the times of the long and 

 short vibrations, the detached pendulum was always adjusted 

 to the clock pendulum, so as to make them vibrate alike. The 

 fixture for the detached pendulum was a little in front of the 

 clock ; and the two pendulums were observed by viewing them 

 from such a point that they were both seen in a line when at 

 the extent of their vibrations ; the experiment being always 



