ARISING FROM INEQUALITIES OF TEMPERATURE. 703 



APPENDIX. 



(Added May, 1879.) 



In the paper as sent in to the Royal Society, I made no attempt to 

 express the conditions which must be satisfied by a gas in contact with a 

 solid body, for I thought it very unlikely that any equations I could write 

 down would be a satisfactory representation of the actual conditions, especially 

 as it is almost certain that the stratum of gas nearest to a solid body is in 

 a very different condition from the rest of the gas. 



One of the referees, however, pointed out that it was desirable to make 

 the attempt, and indicated several hypothetical forms of surfaces which might 

 be tried. I have therefore added the following calculations, which are carried 

 to the same degree of approximation as those for the interior of the gas. 



It will be seen that the equations I have arrived at express both the 

 fact that the gas may slide over the surface with a finite velocity, the 

 previous investigations of which have been already mentioned;""" and the fact 

 that this velocity and the corresponding tangential stress are affected by 

 inequalities of temperature at the surface of the solid, which give rise to a 

 force tending to' make the gas slide along the surface from colder to hotter 

 places. 



This phenomenon, to which Professor Osborne Reynolds has given the 

 name of Thermal Transpiration, was discovered entirely by him. He was the 

 first to point out that a phenomenon of this kind was a necessary consequence 

 of the Kinetic Theory of Gases, and he also subjected certain actual phenomena, 

 of a somewhat different kind, indeed, to measurement, and reduced his measure- 

 ments by a method admirably adapted to throw light on the relations between 

 gases and solids. 



It was not till after I had read Professor Reynolds' paper that I began 

 to reconsider the surface conditions of a gas, so that what I have done is 

 simply to extend to the surface phenomena the method which I think most 

 suitable for treating the interior of the gas. I think that this method is, in 



* Sect. 12 of introductiou. 



