AMPLIFICATION OF THE LAW OF POISEUILLE 39

band appeared to expand and to fill the remainder of the tube
with a colored cloud. When looked at by means of an electric
spark in a darkened room, the colored cloud resolved itself into
distinct eddies having the appearance shown in Fig. 10. By
lowering the velocity ever so slightly, the undulatory movement
would disappear, only to reappear as soon as the velocity was
increased. If the water in the tank was not steady the eddies
appeared at a lower velocity and an obstruction in the tube
caused the eddies to be produced at the obstruction at a consider-
ably lower velocity than before. " Another phenomenon which
was very marked in the smaller tubes was the intermittent char-
acter of the disturbance. The disturbance would suddenly come on
through a certain length of the tube, pass away, and then come
again, giving the appearance of flashes, and these flashes would
often commence successively at one point in the pipe.77 The ap-
pearance when the flashes succeeded each other rapidly is shown

FIG. 11.—Flashing.
in Fig. 11. "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 the funnel than about 30 diameters in any of the pipes,
and the flashes generally, but not always commenced at about
this point. In the smaller tubes generally, and with the larger
tube in-the case of ice-cold water at 4°, the first evidence of
instability was an occasional flash beginning at the usual place
and passing out as a disturbed patch 2 or 3 in. long. As the
velocity further increased these flashes became more frequent
until the disturbance became general."
Reynolds further noted that the free surface of a liquid indi-
cates the nature of the motion beneath. In linear flow, the sur-
face is like that of plate glass, in which objects are reflected without
distortion, while in sinuous flow, the surface is like that of sheet
glass. A colored liquid flowing out into a vessel of water has the
appearance of a stationary glass rod in the first regime, but as the