750 THE POPULAR SCIENCE MONTHLY, 



adequate, the friction is found to be nearly independent of the load, 

 and much smaller than is usually supposed, giving a coefficient as low 

 as Yoto-- When the layer of oil is well formed, the pressure between 

 the solid 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 

 processes involved. In the r&ean time, it is obvious that the results al- 

 ready 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 fol- 

 lowed 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 pro- 

 cesses 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 prob- 

 lems in which I have lately been interested, a circulating motion pre- 

 sents 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 can not be correctly dismissed 

 from consideration as a consequence of a supposition that the viscos- 

 ity is infinitely small. The apparent breach of continuity can be ex- 

 plained, 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 momen- 

 tum, has been made clear by the theoretical and experimental researches 

 of Maxwell. A flat disk 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 in- 

 dependent of its density — that within wide limits the resistance to 

 the moving disk should be scarcely diminished by pumping out the 

 gas, so as to form a partial vacuum. Experiment fully confirmed this 



