62 Prof. 0. Beynolds on the Tivo Manners of Motion of Water. [March 28, 



and the motion always sinuous, the motion of treacle in streams 

 of such size as we see is below its critical velocity, and the motion 

 direct. But if nature had produced rivers of treacle the size of the 

 Thames, for instance, the treacle would have flowed just like water. 

 Thus, in the lava streams from a volcano, although looked at close 

 the lava has the consistence of a pudding, in the large and rapid 

 streams down the mountain sides the lava flows as freely as water. 



I have now only one circumstance left to which to ask your 

 attention. This is the effect of curvature of the stream on the 

 stability of the fluid. 



Here again we see the whole effect altered by very slight causes. 



If water be flowing in a bent channel in steady streams, the 

 question as to whether it will be stable or not turns on the variation 

 in the velocity from the inside to the outside of the stream. 



In front of the lantern is a cylinder with glass ends, so that the 

 light passes through in the direction of the axis. The disk of light 

 on the screen being the light which passes through this water, and is 

 bounded by the circular walls of the cylinder. 



By means of two tubes temporarily attached, a stream of coloured 

 water is introduced right across the cylinder extending from wall to 

 wall ; the motion is very slow, and the taps being closed, and the tubes 

 removed, the colour-band is practically stationary. The vessel is now 

 caused to revolve about its axis. At first, only the walls of the cylinder 

 move, but the colour-band shows that the water gradually takes up 

 the motion, the streak being wound off at the ends into a spiral 

 thread, but otherwise remaining still and vertical. When the spirals 

 meet in the middle, the whole water is in motion, but the motion is 

 greatest at the outside, and is therefore stable. The vessel stops, and 

 gradually stops the water, beginning at the outside. If the motion 

 remained steady, the spirals would unwind, and the streak be restored. 

 But the motion being slowest at the outside against the surface, you 

 will see eddies form, breaking up the spirals for a certain distance 

 towards the middle, but leaving the middle revolving steadily. 



Besides indicating the effect of curvature, this experiment really 

 illustrates the action of the surface of the earth on the air moving 

 over it ; the varying temperature having much the same influence as 

 the curvature of the vessel on stability. The air is unstable for a few 

 thousand feet above the surface, and the motion is sinuous, resulting 

 in the mixing of the strata, and producing the heavy cumulus clouds ; 

 but above this the influence of temperature predominates, and clouds, 

 if there are any, are of the stratus-form, like the inner spirals of 

 colour. But it was not the intention of this lecture to trace the two 

 manners of motion of fluids in the phenomena of Nature and Art, so 

 I thank you for your attention. [0. E.] 



