100 Rayleigh's Address to the British Association, 



Under the head of scientific mechanics it is principally in rela- 

 tion to fluid motion that advances may be looked for. In speak- 

 ing upon this subject I must limit myself almost entirely to 

 experimental work. Theoretical hydro-dynamics, however 

 important and interesting to the mathematician, are eminently 

 unsuited to oral exposition. All I can do to attenuate an injus- 

 tice, to which theorists are pretty well accustomed, is to refer you 

 to the admirable reports of Mr. Hicks, published under the 

 auspices of this Association. 



The important and highly practical work of the late Mr 

 Froude in relation to the propulsion of ships is doubtless known 

 to most of you. Recognizing the fallacy of views then widely 

 held as to the nature of the resistance to be overcome, he showed 

 to demonstration that, in the case of fair-shaped bodies, we have 

 to deal almost entirely with resistance dependent upon skin-friction 

 and at high speeds upon the generation of surface-waves by which 

 energy is carried off. At speeds which are moderate in relation 

 to the size of the ship, the resistance is practically dependent 

 upon skin-friction only. Although Professor Stokr fcher 



mathematicians had previously published calculations pointing to 

 the same conclusion, there can be no doubt that the view generally 

 entertained was very different. At the first meeting of the 

 Association which I ever attended, as an intelligent listener, at 

 Bath, in 1864, I well remember the surprise which greeted a 

 statement by Eankine that he regarded skin-friction as the only 

 legitimate resistance to the progress of a well-designed ship. M r. 

 Froude's experiments have set the question at rest in a manner 

 satisfactory to those who had little confidence in theoretical 

 prevision. 



In speaking of an explanation as satisfactory in if 

 friction is accepted as the cause of resistance, I must guard my 

 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 molecular structure. That it is connected with fluid 

 viscosity is evident enough, but the modus operandi is 

 obscure. 



Some important work bearing upon the subject has recently 



