224 Royal Society : — Messrs. Stewart and Tait on the 



being the mean of 6 observations), we obtain the following effect : — 

 Tor dry hydrogen.... (WW ^8 



and hence we find residual effect —31. 



From all these results, allowing for errors of experiment, we may 

 suppose that the residual effect is not much altered by a chamois- 

 leather blind. 



We ought to mention that the numbers of this article, similarly to 

 those of art. 38, represent the mean of two tappings, the one taken 

 immediately after rotation, and the other after the interval of one 

 minute. 



40. It was suggested to us by Prof. Helmholtz that it v/ould be 

 desirable to ascertain whether any difference was produced in the 

 results by loading the disk on one side ; for if these results were 

 due to vibration, it might be supposed that they would be affected 

 by this means. 



It will be seen by art. 39 that the residual effect obtained from 

 a disk covered with chamois leather is approximately the same as 

 that from an uncovered disk ; this would appear to us to be against 

 the vibration hypothesis. 



In the following experiment the disk was covered with a chamois- 

 leather blind with a segment cut out as underneath — 



A mean of two sets of experiments (consisting of 16 separate 

 observations, and embracing results immediately after rotation, as 

 well as results one minute after) gave as the result of tapping in 

 A with t^ dry hydrogen 47, while a mean of 24 similar observations 

 with the same atmosphere gave as the result of tapping an uncovered 

 disk 53. Allowing for errors of experiment, we may conclude that 

 this arrangement does not much influence the results. 



41. The disk was next treated in the following; manner : — 



