102 Rayleigh's Address to the British Association. 



that the example thus wisely set may he followed by other public 

 bodies associated with various departments of industry. 



I can do little more than refer to the interesting observations of 

 Professor Darwin, Mr. Hunt, and Mr. Forel on Ripplemark. 

 The processes concerned would seem to be of a rather intricate 

 character, and largely dependent upon fluid viscosity. It may 

 be noted, indeed, that most of the still obscure phenomena of 

 hydro-dynamics require for their elucidation a better comprehen- 

 sion of the laws of viscous motion. The subject is one which 

 offers peculiar difficulties. In some problems in which I have 

 lately been interested, a circulating motion presents itself of the 

 kind which the mathematician excludes from the first when he is 

 treating of fluids destitute altogether of viscosity. The intensity 

 of this motion proves, however, to be independent of the coefficient 

 of viscosity, so that it cannot be correctly dismissed from considera- 

 tion as a consequence of a supposition that the viscosity is infinite- 

 ly small. The apparent breach of continuity can be explained, 

 but it shows how much care is needful in dealing with the 

 subject, and how easy it is to fall into error. 



The nature of gaseous viscosity, as due to the diffusion of 

 momentum, has been made clear by the theoretical and experi- 

 mental researches of Maxwell. A flat disc moving in its own 

 plane between two parallel solid surfaces is impeded by the necessity 

 of shearing the intervening layers of gas, and the magnitude of 

 the hindrance is proportional to the velocity of the motion and to 

 the viscosity of the gas, so that under similar circumstances this 

 effect may be taken as a measure, or rather definition, of the visco- 

 sity. From the dynamical theory of gases, to the development 

 of which he contributed so much, Maxwell drew the startling 

 conclusion that the viscosity of a gas should be independent of 

 its density, — that within wide limits the resistance to the moving 

 disc should be scarcely diminished by pumping the gas, so as 

 to form a partial vacuum. Experiment fully confirmed this 

 theoretical anticipation — one of the most remarkable to be found 

 in the whole history of science, and proved that the swinging disc 

 was retarded by the gas as much when the barometer stood at 

 half an inch as when it stood at thirty inches. It was obvious, 

 of course, that the law must have a limit, that at a certain point 

 of exhaustion the gas must begin to lose its power ; and I remem- 



