94 



Mr. 0. Reynolds. 



[Mar. 15, 



The particular purpose of this investigation was to ascertain whether 

 there was a critical velocity at which waves or sinuosities would show 

 themselves in the surface of separation. It proved a very pretty 

 experiment and completely answered its purpose. 



When one end was raised quickly by a definite amount, the opposite 

 velocities of the two liquids, which were greatest in the middle of the 

 tube, attained a certain maximum value depending on the inclination 

 given to the tube. When this was small no signs of eddies or 

 sinuosities showed themselves, but at a certain definite inclination 

 waves (nearly stationary) showed themselves, presenting all the 

 appearance of wind waves. 



These waves first made their appearance as very small waves of 

 equal lengths, the length being comparable to the diameter of the tube. 



Fig. 7. 



When by increasing the rise, the velocities of flow were increased, 

 the waves kept the same length, but became higher, and when the rise 

 was sufficient, the waves would curl and break, the one fluid winding 

 itself into the other in regular eddies. 



Whatever might be the cause, a skin formed slowly between the 

 bisulphide and the water, and this skin produced similar effects to 

 that of oil on water, the results mentioned are those which were 

 obtained before the skin showed itself. When the skin first came on 

 regular waves ceased to form, and in their place the surface was dis- 

 turbed as if by irregular eddies above and below, just as in the case of 

 the oiled surface of water. 



The experiment was not adapted to afford a definite measure of the 

 velocities at which the various phenomena occurred, but it was obvious 

 that the critical velocity at which the waves first appeared, was many 

 times smaller than the critical velocity in a tube of the same size when 

 the motion was in one direction only. It was also clear that the 

 .critical velocity was nearly if not quite independent of any existing 

 . disturbance in the liquids. So that this experiment shows — 



(1.) That there is a critical velocity in the case of opposite flow, at 

 which direct motion becomes unstable. 



(2.) That the instability came on gradually and did not depend on 

 -the magnitude of the disturbances, or in other words, that for this 

 , class of motion question 6 must be answered in the affirmative. 



It thus appeared that there was some difference in the cause of 

 instability in the two motions. 



