2 Gibson, Manner of Motion of ]Vater in a Curved Pat Ji. 



the spirals would unwind and the streak be restored. But 

 the motion being slowest at the outside against the surface, 

 eddies are formed, breaking up the spirals for a certain 

 distance towards the middle, but leaving the middle 

 revolving steadily." * 



Various phenomena, of common occurrence in hydraulic 

 work seem, however, to be at direct variance with the 

 opinion expressed above. Among these may be instanced 

 the behaviour of water in a free vortex, and at the dis- 

 charge side of a sharp-edged orifice in a thin plate. In a 

 free vortex, produced by the discharge of water through 

 a central hole in the bottom of a circular vessel, the motion 

 is readily shown, by colour bands, to be steady throughout, 

 although here the motion is ever\'where greater at the 

 inside of the curved path of a filament. Again, on the 

 discharge side of a sharp-edged orifice in a thin plate, 

 during the gradual convergence of the stream tubes to the 

 vena-contracta, the filaments, other than the central one, 

 follow curved paths with the pressure gradually increasing, 

 and the velocity diminishing towards the interior of the 

 stream, and therefore as the radius of curvature increases. 

 Yet such motion is essentially steady and non-sinuous. 

 Another somewhat analogous case is found where a steady 

 jet impinges on a solid surface, or where two stead\- jets, 

 moving in the same straight line but in opposite directions, 

 meet. In each case the motion after impact is steady, 

 however high the velocity. Here again, owing to the 

 change in the direction of flow following impact, the outer 

 layers, while this change is being effected, are moving in 

 curved paths with the velocity greatest at the outer 

 (nearer to the centre of curvature) portion of the curv^e. 

 Experiments on the flow of water around bends in piping 

 also show that the velocity is greatest at the inside of 



* "Scientific Papers," vol. 2, p. 162. 



