Caiion Moseley on the steady Flow of a Liquid. 55 



rable experiments of MM. Darcy and Bazin. The labours of 

 the late M. Darcy appear to me, as regards scientific precision, 

 the admirable design of the instruments used in them, and the 

 admirable industry with which they were carried on, among the 

 greatest scientific labours of our age. I could not have arrived 

 at my results without them ; but these are not founded upon 

 them, but on the principles of mechanical philosophy. They 

 have served me rather as the scaffolding on which my structure 

 has been raised than as the foundation ; the scaffolding being 

 withdrawn, the structure stands by itself. 



Note referred to in page 38*. 

 I will venture to propose the following explanation of this 

 formula. 



The resistance to the flow of the liquid which is next to that in 

 contact with the surface of the pipe is supposed to be due to its ad- 

 hesion to the liquid fixed to the surface of the pipe and to the 

 impinging of its molecules on those of that liquid and on those 

 of the pipe itself. The work per unit of time of that resistance 

 is therefore equal to the work per unit of time of the shear of a 

 flov/ing liquid film over a fixed one plus the work per unit of time 

 necessary to replace that which is lost in the impact of the mole- 

 cules of the moveable film on those of the fi.xed one. 

 Let 



P = whole resistance per unit of surface to the motion of 



the one film over the other at rest. 

 V= uniform velocity of the motion of the one film over the 



other. 

 K=area of the surfaces of the films in contact. 

 /Lt^=unit of shear, being that per unit of surface of contact 



of the films. 

 PK= whole resistance of one film to the motion over it of the 

 other. 

 PKV=work per unit of time of that resistance. 

 K//,jV = work per unit of time of shear. 

 To determine the work lost per unit of time by the impact of the 

 molecules of the moving film over that of the fixed one, let it 

 be observed that this loss is proportional to the number of 

 such impacts per unit of time and the work lost in each such 

 impact, and that this last is measured by half the vis viva lost 

 in each impact. But the number of impacts per unit of time is 

 proportional to KV ; and the vis viva lost in the impact of one 

 molecule on another at rest is proportional to the square of the 



* Editor^ s Note. — This note, evidently made in connexion with the pre- 

 sent paper, was found amongst Canon Moseley's MSS. ; and it seemed fit- 

 ting to pubhsh it here. 



