§ 1. _Properties of Fluids._--The fluids to which the laws of practical
hydraulics relate are substances the parts of which possess very great
mobility, or which offer a very small resistance to distortion
independently of inertia. Under the general heading Hydromechanics a
fluid is defined to be a substance which yields continually to the
slightest tangential stress, and hence in a fluid at rest there can be
no tangential stress. But, further, in fluids such as water, air, steam,
&c., to which the present division of the article relates, the
tangential stresses that are called into action between contiguous
portions during distortion or change of figure are always small compared
with the weight, inertia, pressure, &c., which produce the visible
motions it is the object of hydraulics to estimate. On the other hand,
while a fluid passes easily from one form to another, it opposes
considerable resistance to change of volume.
It is easily deduced from the absence or smallness of the tangential
stress that contiguous portions of fluid act on each other with a
pressure which is exactly or very nearly normal to the interface which
separates them. The stress must be a pressure, not a tension, or the
parts would separate. Further, at any point in a fluid the pressure in
all directions must be the same; or, in other words, the pressure on any
small element of surface is independent of the orientation of the
surface.
§ 2. Fluids are divided into liquids, or incompressible fluids, and
gases, or compressible fluids. Very great changes of pressure change the
volume of liquids only by a small amount, and if the pressure on them is
reduced to zero they do not sensibly dilate. In gases or compressible
fluids the volume alters sensibly for small changes of pressure, and if
the pressure is indefinitely diminished they dilate without limit.
In ordinary hydraulics, liquids are treated as absolutely
incompressible. In dealing with gases the changes of volume which
accompany changes of pressure must be taken into account.
§ 3. Viscous fluids are those in which change of form under a continued
stress proceeds gradually and increases indefinitely. A very viscous
fluid opposes great resistance to change of form in a short time, and
yet may be deformed considerably by a small stress acting for a long
period. A block of pitch is more easily splintered than indented by a
hammer, but under the action of the mere weight of its parts acting for
a long enough time it flattens out and flows like a liquid.
[Illustration: FIG. 1.]
All actual fluids are viscous. They oppose a resistance to the relative
motion of their parts. This resistance diminishes with the velocity of
the relative motion, and becomes zero in a fluid the parts of which are
relatively at rest. When the relative motion of different parts of a
fluid is small, the viscosity may be neglected without introducing
important errors. On the other hand, where there is considerable
relative motion, the viscosity may be expected to have an influence too
great to be neglected.
_Measurement of Viscosity. Coefficient of Viscosity._--Suppose the
plane ab, fig. 1 of area [omega], to move with the velocity V
relatively to the surface cd and parallel to it. Let the space between
be filled with liquid. The layers of liquid in contact with ab and cd
adhere to them. The intermediate layers all offering an equal
resistance to shearing or distortion, the rectangle of fluid abcd will
take the form of the parallelogram a´b´cd. Further, the resistance to
the motion of ab may be expressed in the form