and Radiometer Motion. 143 



After most flattering expressions respecting the importance 

 and the effect of this paper, followed by severe comments on 

 its mathematical form, Mr. Sutherland says: — 



" But what appears to me to be the fatal objection to 

 Reynolds's mathematical method, is that it takes the mindaway 

 from definite physical concepts of the actual operation of the 

 causes of thermal transpiration and radiometer motion ; and 

 the object of the present paper is to construct a theory of 

 these that will fall into line with the current kinetic theory of 

 gases and keep the physics of the phenomena to the fore." 



After this Mr. Sutherland proceeds to describe the founda- 

 tion on which he has reconstructed the theory. 



From his references to my paper, together with the some- 

 what confusing and sketchy preamble to the definite expression 

 of his foundation, it may well appear that, except for the 

 mathematical form in which it is expressed, this foundation is 

 really the same as that set forth in my paper. It seems 

 therefore to me to be important to point out that there is not 

 the least connexion between these foundations. 



According to my showing the sole cause of thermal trans- 

 piration is the lateral action of those bounding surfaces which 

 are parallel to the direction in which heat is being conducted; 

 while Mr. Sutherland expressly excludes the action of these 

 walls (or the walls themselves) from any part in thermal trans- 

 piration, which he rests solely on the action of direct conduction 

 of heat. 



It may seem that the results obtained in the application of 

 theories so differently founded to any particular case must be 

 different ; but this depends on the object sought. 



If, as in my paper, the object is to determine the nature of 

 the action, which results in transpiration, from the kinetic 

 theory of gases the results from the different theories will be 

 different ; but if, as is Mr. Sutherland's object, it is merely 

 required to express the results of transpiration " in line with 

 the current kinetic theory of gases/ 7 then, if the transpiration 

 he has assumed is the same as that obtained by the true 

 theory, the results will to a certain extent correspond. 



There is thus danger of the similarity of the results so 

 obtained being taken as a verification of the soundness of the 

 foundation on which Mr. Sutherland has built his theory, and 

 of confusion being added to the inherent difficulties of the 

 subject. 



It has therefore seemed to me to be important not only to 

 point out the difference of the foundations, but also to show 

 clearly that the foundation on which Mr. Sutherland has con- 

 structed his theory has no real existence whatever ; being the 



