Heating of a Disk by rapid Rotation in vacuo. 223 



The results are thus much more simple on the hypothesis of two 

 effects, one of these being independent of the residual gas, than on 

 the hypothesis of only one effect. 



37. It was next endeavoured to ascertain whether these Wo 

 effects were differently influenced by a blind. 



A linen blind was first used before the thin ebonite disk, which 

 it cloaked during rotation, falling down afterwards. During rota- 

 tion the blind did not appear to touch the disk. A delicate galva- 

 nometer was used for this experiment ; and the time of vibration of 

 the needle was 2 seconds. The rotation consisted of 30 turns of the 

 handle in 40 seconds, as before. The following result was obtained 

 (each number representing the mean of two observations) : — 



Without blind. With blind. 



-fa of dry hydrogen 165 127 



-fo hydrogen-^ air 477 248 



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. For 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 im- 

 mediately after rotation, and then again at the interval of one mi- 

 nute ; 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. 



, » -, i -, ro f mean of 76 indivi- 1 co f mean of 22 indivi- 



& of dry hydrogen . 53 | dua] observations . J 52 j dual obserTation9 _ 



mean of 16 indivi- 

 dual observations. 



3 , i _l 6 * oi 7 f mean of 68 indivi- \-iac) 



ju j & "r"2"(5 i dual observations. J 



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, 



in inches. Dry hydrogen. Dry air. 



Blindon j* 83 178 



I* 52 82 



From which we deduce : — 



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



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



