292 



Professor John H. Poynting 



[Feb. 23, 



In our experiment, the couple, if it existed, would very soon 

 establish its vibration, which would always be there aud would go 

 through all its values in 115 seconds. An observer, watcliiug the 

 wheel at the top of the revolving axis, gave the time signals every 

 11*5 seconds, regulating the speed, if necessary, and an observer at 

 the telescope gave the scale reading at every signal, that is, 10 times 

 during the period. The values were arranged in 10 columns, each 

 horizontal line giving the readings of a period. The experiment was 

 carried on for about 2j? hours at a time, covering, say, 80 periods. 

 On adding up the columns, the maxima and minima of the couple 

 eifect would always fall in the same two columns, and so the addition 

 would give 80 times the swing, while the maxima and minima of the 



':■„ 



\ 



\ 



Fig. 11. — Eesults of superposition of lengths of curves in Fig. 10 

 equal to the period of the regular one. 



natural swings due to disturbances would fall in different columns, 

 and so, in the long run, neutralise each other. The results of 

 different days' work might, of course, be added together. 



There always was a small outstanding effect such as would be 

 produced by a quadrantal couple, but its effect was not always in the 

 same columns, and the net result of about 350 period observations 

 was that there was no 115 second vibration of more than 1 second of 

 arc, while the disturbances were sometimes 50 times as great. 



The semicircular couple required the turning sphere to revolve 

 in 115 seconds. Here, want of symmetry in the apparatus would 

 come in with the same effect as the couple sought, and the out- 

 standing result was, accordingly, a little larger. 



