38

FLUIDITY AND PLASTICITY

lower and lower temperature and the maximum appears even in
the smallest tube used. There is a minimum of efflux after
passing the maximum but then the efflux becomes again a linear
function of the temperature. Brillouin (1907) page 208, has
confirmed the experimental results of Hagen.

A clear picture of the phenomena connected with the passage
from one regime to the other has been given by Reynolds (1883).
One form of apparatus used by him is depicted in Fig. 8. It

FIG. 9.—Linear flow.

consists of a glass tube BC, with a trumpet-shaped mouthpiece
AB of wood, which was carefully shaped so that the surfaces
would be continuous from the wood to the glass. Connected
with the other end is a metal tube CD with a valve at E having
an opening of nearly 1 sq. in. The cock was controlled by a long
lever so that the observer could stand at the level of the bath,
which surrounded the tube BC. The wash-bottle W contained
a colored liquid which was led to the inside of the trumpet-
shaped opening. The gage G was used for determining the level

FIG. 10.—The beginning of turbulent flow.
of water in the tank. When the valve E was gradually opened
and the color was at the same time allowed to flow out slowly, the
color was drawn out into a narrow band which was beautifully
steady haying the appearance shown in Fig. 9. Any consider-
able disturbance of the water in the tank would make itself
evident by a wavering of the color band in the tube; sometimes it
would be driven against the glass tube and would spread out,
but without any indication of eddies.
As the velocity increased however, suddenly at a point 30 or
more times the diameter of the tube from the entrance, the color