1873.] Disk by rapid Rotation in vacuo. 313 



Thus the proportion between the two effects is greatly altered by the 

 blind ; for while the hydrogen effect is not much diminished, the other is 

 reduced to little more than one half. 



38. A chamois-leather blind was next used, which lay close to the disk 

 during rotation. Tor all these experiments the time of vibration of the 

 galvanometer-needle was 20 to 30 seconds, while the speed was the same 

 as before. Also the disk was tapped immediately after rotation, and 

 then again at the interval of one minute • and in order to obtain as many 

 individual results as possible, the mean of these two tappings has been 

 taken in the following comparison : — 



Without blind. With blind. 



o £ i , i p-Q f mean of 76 indivi- ) KO f mean of 22 indivi- 



* of M ro S*=" • 53 t dual observations. } 52 ( dual observations. 



3 h dro en-f-- 6 - air 217 \ meai1 °^ ^ iudivi- 1 f mean of 16 indivi- 

 ."?u y 10 g en +2o air \ dual observations. J { dual observations. 



Here also we find that the proportion between the two effects is greatly 

 altered by the blind; so that while the hydrogen-effect is not much 

 stopped, the other is diminished very considerably. 



39. A set of experiments of 8 observations, each similar to those of 

 arts. 33, 34, 35, were made with the blind on for the two gases, dry 

 hydrogen and dry air. From these we obtain the following : — 



Pressure of Heat-indication for 



residual gas, 



Blind on 



in inches. Dry hydrogen. Dry air. 



83 178 

 52 82 



v Jo 



From which we deduce : — 



Eesidual effect, blind on .... 36-5 34 



For dry hydrogen .... *f / 2 8 0; 



V2U. 



Again, from certain experiments with the blind off (each result being 

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



:) 



y so 



and hence we find residual effect =31. 



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

 pose 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 immediatly 

 afber rotation, and the other after the interval of one minute. 



40. It was suggested to us by Prof. Helmholtz that it would be desi- 

 rable 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. 



