956 
MR. O. REYNOLDS ON THE MOTION OP WATER AND OF 
This at once suggested the idea that the condition might be one of instability for 
disturbance of a certain magnitude and stable for smaller disturbances. 
In order to test this, an open coil of wire was placed in the tube so as to create a 
definite disturbance as in Plate 72, fig. 15. 
Eddies now showed themselves at a velocity of less than half the previous critical 
velocity, and these eddies broke up the colour band, but it was difficult to say whether 
the motion was really unstable or whether the eddies were the result of the initial 
disturbance, for the colour band having once broken up and become mixed with the 
water, it was impossible to say whether the motion did not tend to become steady 
again later on in the tube. 
Subsequent observation however tended to show that the critical value of the 
velocity depended to some extent on the initial steadiness of the water. One pheno¬ 
menon in particular was very marked. 
Where there was any considerable disturbance in the water of the tank and the 
cock was opened very gradually, the state of disturbance would first show itself by the 
wavering about of the colour band in the tube ; sometimes it would be driven against 
the glass and would spread out, and all without a symptom of eddies. Then, as the 
velocity increased but was still comparatively small, eddies, and often very regular 
eddies, would show themselves along the latter part of the tube. On further opening 
the cock these eddies would disappear and the colour band would become fixed and 
steady right through the tube, which condition it would maintain until the velocity 
reached its normal critical value, and then the eddies would appear suddenly as before. 
Another phenomenon very marked in the smaller tubes, was the intermittent 
character of the disturbance. The disturbance would suddenly come on through a 
certain length of the tube and pass away and then come on again, giving the appear¬ 
ance of flashes, and these flashes would often commence successively at one point in 
the pipe. The appearance when the flashes succeeded each other rapidly was as shown 
in Plate 72, fig. 16. 
This condition of flashing was quite as marked when the water in the tank was very 
steady as when somewhat disturbed. 
Under no circumstances would the disturbance occur nearer to the trumpet than 
about 30 diameters in any of the pipes, and the flashes generally, but not always, 
commenced at about this distance. 
In the smaller tubes generally, and with the larger tube in the case of the ice-cold 
water at 40°, the first evidence of instability was an occasional flash beginning at the 
usual plaoe and passing out as a disturbed patch two or three inches long. As the 
velocity was further increased these flashes became more frequent until the disturbance 
became general. 
I did not see a way to any very crucial test as to whether the steady motion became 
unstable for a large disturbance before it did so for a small one ; but the general 
impression left on my mind was that it did in some way—as though disturbances in 
