958 
MR. 0. REYNOLDS ON THE MOTION OF WATER AND OF 
motion, according to whether the velocity was above or below what may be called 
the real critical value. 
The necessity of initial disturbance precluded the method of colour bands, so that 
the only method left was to measure the resistance at the latter portion of the tube 
in conjunction with the discharge. 
The necessary condition was somewhat difficult to obtain. The change in the law 
of resistance could only he ascertained by a series of experiments which had to be 
carried out under similar conditions as regards temperature, kind of water, and 
condition of the pipe; and in order that the experiments might he satisfactory, it 
seemed necessary that the range of velocities should extend far on each side of the 
critical velocity. In order to best ensure these conditions, it was resolved to draw the 
water direct from the Manchester main, using the pressure in the main for forcing the 
water through the pipes. The experiments were conducted in the workshop in 
Owens College, which offered considerable facilities owing to arrangements for 
supplying and measuring the water used in experimental turbines. 
25. The apparatus .—This is shown in Plate 72, fig. 17. 
As the critical value under consideration would be considerably below that found 
for the change for steady motion into eddies, a diameter of about half an inch 
(12 millims.) was chosen for the larger pipe, and one quarter of an inch for the 
smaller, such pipes being the smallest used in the previous experiments. 
The pipes (4 and 5) were ordinary lead gas or water pipes. These, which owing to 
their construction are very uniform in diameter and when new present a bright 
metal surface inside, seemed well adapted for the purpose. 
Pipes 4 (which was a quarter-inch pipe) and 5 (which was a half-inch) were 16 
feet long, straightened by laying them in a trough formed by two inch boards at 
right angles. This trough was then fixed so that one side of the trough was 
vertical and the other horizontal, forming a horizontal ledge on which the pipes 
could rest at a distance of 7 feet from the floor; on the outflow ends of the pipes 
cocks were fitted to control the discharge, and at the inlet end the pipes were 
connected, by means of a T branch, with an indiarubber hose from the main ; this 
connexion was purposely made in such a manner as to necessitate considerable 
disturbance in the water entering the pipes from the hose. The hose was connected, 
by means of a quarter-inch cock, with a four-inch branch from the main. With this 
arrangement the pressure on the inlet to the pipes was under control of the cock 
from the main, and at the same time the discharge from the pipes was under control 
from the cocks on their ends. 
This double control was necessary owing to the varying pressure in the main, and 
after a few preliminary experiments a third and more delicate control, together with 
a pressure gauge, were added, so as to enable the observer to keep the pressure in the 
hose, i.e., on the inlets to the pipes, constant during the experiments. 
This arrangement was accomplished by two short branches between the hose and 
