Heating of a Disk by rapid Rotation in vacuo. 225 



It was covered with a chamois leather blind tied into holes drilled 

 in the disk, and having the above shape. The mean of 3 sets of 

 experiments similar to those in art. 40, and consisting of 18 separate 

 observations, gave as follows for an atmosphere of -fa dry hydrogen, 

 tapping in A 54, tapping in B 56, which two results are approxi- 

 mately equal to one another, the mean being 55. 



The mean of 3 sets of experiments, consisting of 24 separate 

 observations for an atmosphere of -£$ hydrogen, but with an unco- 

 vered disk, gave 53 as the heat-result. All these experiments 

 apparently combine to prove that the result is not due to vibration. 



42. When the atmosphere was -fo hyd. + -^ - air, we also found 

 A and B approximately the same, A giving 181 while B gave 182 ; 

 we have therefore not hesitated to combine together the results of 

 tapping in this atmosphere, whether at A, or at B, or with an 

 uncovered disk, in the comparison given in art. 38. 



43. Our next experiments were made with the view of testing 

 whether or not the two effects, the residual and the gas-effect, were 

 resident in the same particles of the disk ; and for this purpose the 

 experiments made immediately after rotation were compared with 

 those made one minute afterwards. 



The experiments available for this purpose are so numerous that 

 they can bear splitting into two portions, in each of which the same 

 result is seen. 



Thus we have for an atmosphere of ^ dry hydrogen, and as the 

 mean of 30 individual comparisons, 



Effect at first : Effect one minute after : : 1*30 : 1 ; 



also, as the mean of 22 individual comparisons, we obtain the pro- 

 portion of 1*19 : 1, while as the mean of the whole we obtain 

 1-25 : 1. 



Treating in a similar manner the observations made with an atmo- 

 sphere of ^j- hyd. + 2% air, we obtain 



As the mean of 25 comparisons 1-47 : 1, 



As the mean of 21 comparisons 1*41 : 1, 



while as the mean of the whole we obtain 1-44 : 1. 



We therefore conclude that the residual effect is less diminished 

 during the interval of one minute than the gas-effect. 



44. We next made experiments with two aluminium disks -05 

 and -025 of an inch in thickness respectively. These disks were 

 covered on both sides with a coating of lampblack applied by nega- 

 tive photographic varnish. 



In both disks we found that with a vacuum of -fa hyd. the effect 

 one minute after rotation was greater than the effect immediately 

 after rotation. Thus we have : — 



F of4^b^vatio™s) n } EffeCtatfirst : Effect one minute after :: -85 : 1; 



For thin disk (mean " 

 of 2 experiments, I ^ . do : : -86 : 1. 



embracing 7 obser- 

 vations) 

 Phil. Mag. S. 4. Vol. 47. No. 311. March 1874. Q 



