1883.] 



On the Motion of Water. 



91 



of oil to prevent wind waves. As the result of this observation had 

 no small influence in directing the course of this investigation, it may 

 be well to describe it first. 



10. JEddies caused by the Wind beneath the Oiled Surface of Water. — 

 A few drops of oil on the windward side of a pond during a stiff 

 breeze having spread over the pond and completely calmed the 

 surface as regards waves, the sheet of oil, if it may be so called, was 

 observed to drift before the wind, and it was then particularly 

 noticed that close to, and at a considerable distance from, the wind- 

 ward edge, the surface presented the appearance of plate glass; 

 further from the edge the surface presented that wavering appear- 

 ance which has already been likened to that of sheet glass, which 

 appearance was at the time noted as showing the existence of eddies 

 beneath the surface. 



Subsequent observation confirmed this first view. At a sufficient 

 distance from the windward edge of an oil-calmed surface there are 

 always eddies beneath the surface even when the wind is light. But 

 the distance from the edge increases rapidly as the force of the wind 

 diminishes, so that at a limited distance (10 or 20 feet) the eddies 

 will come and go with the wind. 



Without oil I was unable to perceive any indication of eddies. At 

 first I thought that the waves might prevent their appearance even if 

 they were there, but by careful observation I convinced myself that 

 they were nofc there. It is not necessary to discuss these results here, 

 although, as will appear, they have a very important bearing on the 

 cause of instability. 



11. Experiments by Means of Colour Bands in Glass Tubes. — These 

 were undertaken early in 1880 ; the final experiments were made on 

 three tubes, Nos. 1, 2, and 3. 



The diameters of these were nearly 1-inch, ^-inch, and J-inch. 

 They were all about 4 feet 6 inches long, and fitted with trumpet 

 mouthpieces, so that water might enter without disturbance. 



The water was drawn through the tubes out of a large glass 

 tank in which the tubes were immersed, arrangements being made so 

 that a streak or streaks of highly coloured water entered the tubes 

 with the clear water. 



The general results were as follows : — 



(1.) When the velocities were sufficiently low, the streak of colour 

 extended in a beautiful straight line through the tube, fig. 3. 



Fig. 3. 



