Rayleigh's Address to the British Association. 101 



been published by Professor 0. Reynolds, who has investigated 

 the flow of water in tubes as dependent upon the velocity of 

 motion and upon the size of the bore. The laws of motion in 

 capillary tubes, discovered experimentally by Poiseuille, are in 

 complete harmony with theory. The resistance varies as the 

 velocity, and depends in a direct manner upon the constant of 

 viscosity. But when we come to the larger pipes and higher 

 velocities with which engineers usually have to deal, the theory 

 which presupposes a regularly stratified motion evidently ceases 

 to be applicable, and the problem becomes essentially identical 

 with that of skin-friction in relation to ship propulsion. Professor 

 Reynolds has traced with much success the passage from the one 

 state of things to the other, and has proved the applicability under 

 these complicated conditions of the general laws of dynamioal 

 similarity as adapted to viscous fluids by Professor Stokes. In 

 spite of the difficulties which beset both the theoretical and 

 experimental treatment, we may hope to attain before long to a 

 better understanding of a subject which is certainly second to 

 none in scientific as well as practical interest. 



As also closely connected with the mechanics of viscous fluids 

 I must not forget to mention an important series of experiments 

 upon the friction of oiled surfaces, recently executed by Mr. Tower 

 for the Institution of Mechanical Engineers. The results go far 

 towards upsetting some ideas hitherto widely admitted. When 

 the lubrication is 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 1-1000. When a 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 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, 



