316 A proposed Hydraulic Experiment. 



conditions for the walls of an expanding channel, even in two 

 dimensions. The travelling bands of which the walls would 

 be constituted should assume different velocities at different 

 parts of their course. But it is quite possible that a very- 

 rough approximation to theoretical requirements would 

 throw interesting light upon the subject, and I write in 

 the hope of pursuading some one with the necessary 

 facilities, such as are to be found in some hydraulic labo- 

 ratories, to undertake a comparatively simple experiment. 



What I propose is the observation of the flow of liquid 

 between two cylinders A, B (probably brass tubes), revolving 

 about their axes in opposite directions. The diagram will 



sufficiently explain the idea. The circumferential velocity 

 of the cylinders should not be less than that of irrotational 

 fluid in contact with the walls at the narrowest place. The 

 simple motion may be unstable ; but, as I have had occasion 

 to remark before*, the critical situation would be so quickly 

 traversed that perhaps the instability may be of little 

 consequence. If no marked difference in the character of 

 the flow could be detected by, colour streaks, whether the 

 cylinders were turning or not, the inference would be that 

 Froude's explanation is inadequate. In the contrary event 

 the question would arise whether practical advantage could 

 be taken by specially stimulating the motion of fluid near 

 the walls of expanding channels, e. y. with the aid of 

 steam jets. 



* Phil. Mag. vol. xxvi. p. 776 (1913). 



