134 Rev. 0. Fisher on the Residual Effect of a Former 



therefore, could be employed in calculating the coefficients 

 of reflexion for a normal angle of incidence ; these coefficients 

 indicate the relation of the whole charge reflected by the 

 reflector to that impinging upon it, without giving the dis- 

 tribution of the reflected rays in the hemisphere constructed 

 over the reflector surface. 



On p. 56 I say, indeed, that small rotations of the reflector 

 do not produce a great difference in the amount of charge 

 conveyed into the cylinder. But it is evident there that this 

 applies only to very small angles (about 5°), which are barely 

 perceptible with the apparatus described. So small a varia- 

 tion in the position of the anticathode could not influence 

 very much the amount of reflected rays, because the position 

 was that of maximum reflexion. 



I did not examine at that time either the variation of the 

 reflexion with the angle of incidence, or the distribution of 

 the reflected rays in space, because more complicated apparatus 

 (rotating cylinder &c.) — such as was employed by Mr. Swin- 

 ton — would have been necessary. However, I have always 

 emphasized the fact (p. 60) that my researches were made 

 for a normal angle of incidence, and nowhere have I asserted 

 that the orientation of the reflector does not affect the amount 

 of charge conveyed to the cylinder. My method, therefore, 

 was not less sensitive than that employed by Mr. Swinton. 

 I even think my measurements more exact because I worked 

 with an influence-machine, while Mr. Swinton generated the 

 cathode rays by a Ruhmkorff inductor, although the radiation 

 thus produced is not homogeneous but contains cathode rays 

 of different potentials of discharge. 



Yours obediently, 



H. Starke. 



Berlin : Physical 

 Laboratory of the University. 



VIII. On the Residual Effect of a Former Glacial Epoch upon 

 Underground Temperature. By the Rev. 0. Fisher, M.A., 

 F.G.S.* 



INHERE can be no doubt that a glacial epoch must have 

 left traces of its effects upon underground temperature, 

 and it appears to be worth while to examine the nature and 

 amount of these effects, and to inquire whether there is any 

 probability of obtaining from observations in deep wells and 

 mines an estimate of the lapse of time since the ice disappeared 

 from the land. 



* Communicated by the Author* 



