52 Mr. W. Sutherland on Thermal 



Thus on the negative side at a distance great in comparison 

 with the wave-length we get, as in (99), (111), (112), 



/^»*=— aF-7-(r' ' —> (135) 



4ttV ~ 3tt r \ r 2 ' r 2 ' r 2 )' ' [ ' 



On the positive side these values are to be reversed, and 

 addition made of 



h Q = e ikx -e- ikx , @ = ±7rY(e ikx + e - ikx ), . (137) 



representing the plane waves incident and reflected. 



The solution for h in (135) may be compared with that 

 obtained (27), (28) in a former paper *, where, however, the 

 primary waves were supposed to travel in the positive, 

 instead of, as here, in the negative direction. It had at first 

 been supposed that the solution for cf> there given might be 

 applied directly to h, which satisfies the condition (imposed 

 upon <£) of vanishing upon the faces of the screen. If this 

 were admitted, as also # = throughout, the value of h would 

 follow by (95). The argument was, however, felt to be in- 

 sufficient on account of the discontinuities which occur at the 

 edge of the aperture, and the value now obtained, though of 

 the same form, is doubly as great. 



Terliug Place, Witham. 



VI. Thermal Transpiration and Radiometer Motion. 

 To the Editors of the Philosophical Magazine. 



Gentlemen, 

 TN the Phil. Mag. for Feb. 1897, Prof Osborne Reynolds, 

 _L in commenting upon my paper on Thermal Transpiration 

 and Radiometer Motion, remarks freely on errors into which 

 I have fallen therein. I fancy that most readers of my paper 

 will recognize that the particular errors mentioned by Prof. 

 Reynolds are rather the result of his own misinterpretation 

 than of my blundering ; but still, as he has taken six pages 

 of the Phil. Mag. in which to lay these errors to my charge, 

 I should like to point out briefly how the errors are his own. 



First : — On page 143 Prof. Reynolds writes : — " while 

 Mr. Sutherland expressly excludes the action of these walls 



* " On the Passage of Waves through Apertures in Plane Screens, and 

 Allied Problems," Phil. Mag. vol. xliii. p. 264 (1897). 



