14 REPORT — 1884. 



surfaces is really borne by the fluid, and the work lost is spent in 

 shearing, that is, in causing one stratum of the oil to glide over another. 

 In order to maintain its position, the fluid must possess a certain 

 •degree of viscosity, proportionate to the pressure ; and even when this 

 condition is satisfied, it would appear to be necessary that the layer 

 should be thicker on the ingoing than on the outgoing side. We may, 

 I believe, expect from Professor Stokes a further elucidation of the pro- 

 cesses involved. In the meantime, it is obvious that the results already 

 obtained are of the utmost value, and fully justify the action of the 

 Institution in devoting a part of its resources to experimental work. 

 We may hope indeed that the example thus wisely set may be 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 M. 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 comprehension 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, howevei-, to be independent of the coefficient of viscosity, so that 

 it cannot be correctly dismissed from consideration in consequence of a 

 supposition that the viscosity is infinitely 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 experimental 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 viscosity. 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 out the gas, so as to form a 

 partial vacuum. Experiment fully confirmed this theoretical anticipation, 

 — one of the most remai'kable 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 



