ADDUEsiS. 



13 



e in 



surface waves by which energy is carried off. At speeds which aro 

 moderate in relation to the size of the ship, the resistance is practically 

 dependent upon skin friction only. Although Professor Stokes and other 

 mathematicians had previously published calcalntions j)ointing to the 

 same couclnsion, there can bo no doubt that the view generally enter- 

 tained was very diiferent. At the first meeting of the Association which 

 I ever attended, as an intelligent listener, at IJath in lH(j !•, I v/ell remember 

 the surprise which greeted a statement by Hankine, that he regarded skin 

 friction as the only legitimate resistance to the progress of a well-designed 

 ship. Mr. Fronde's experiments have set the question at rest in a maimer 

 satisfactory to those who had little confidence in theoretical prevision. 



In speaking of an explanation as satisfactory in which skin friction is 

 accepted as the cause of resistance, I must guard myself against being 

 supposed to mean that the nature of skin friction is itself well understood. 

 Although its magnitude varies with the smoothness of the surface, we have 

 no reason to think that it would disappear at any degree of smoothness 

 consistent with an ultimate moluciilav structure. That it is connected 

 with fluid viscosity is evident enough, but the modus operandi is still 

 obscure. 



Some important work bearing upon the subject has recently been pub- 

 lished by Professor 0. lleynolds, 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, discoveix'd experimentally 

 by Poiseuille, are in complete harmony with theory. The resistance varies 

 as the velocity, and depends in a dii-ect 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 regu- 

 larly 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 dynamical 

 similarity as adapted to viscous fluids by Professor Stokes. In spite of 

 the difficulties which beset both the theoretical and experimental treat- 

 ment, 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 Insti- 

 tution 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 xinnr* 

 When the layer of oil ia well formed, the pressure between the solid 



