It should be remembered that the laws relating to friction, between
_solid bodies_ operate quite differently from what they do when applied
to liquids; hence, the large mass of data relating to the general
subject of friction must be disregarded in the consideration of
hydromechanics and allied subjects.
For all fluids, whether liquids or gaseous, the resistance is
independent of the pressure between the masses in contact. This is in
accordance with the second law as stated.
The friction for all fluids (liquid or gaseous) is in proportion to the
area of the rubbing surface; this follows from the first law and as the
sectional area of a circle is the least, _pipes_ from their circular
form present the smallest resistance to the flow of water.
From the third law, in practice we desire the making of pipes as large
as possible; experiment having proved that low speeds are preferable to
moderate and still more so, as compared to high speeds in proportioning
the piping of hydraulic apparatus.
Friction of fluids is also independent of the nature of the solid
against which the stream may flow, but dependent to some extent upon
their degree of roughness.
Friction _for all fluids is also proportional to the density of the
fluid_, and related in some way to its adhesiveness.
_Water flowing through a pipe tends to drag the pipe along with it_
on account of friction; in all actual fluids there is viscosity or
internal friction, but if the relative motion is only slow enough it
makes little difference whether the fluid is viscous or not.
_Ordinary fluids will change in shape under the action of a force_,
however small, if enough time is given for the change to take place,
and the rate of change of shape is a measure of its viscosity.
_The laws of friction, both for solids and liquids, have been
established from experiments endlessly varied._ In investigating these
principles we first proceed on the supposition that the forces in
question act without any impediments. Great simplicity is attained
by first bringing the subject to this ideal standard of perfection,
and afterwards making suitable allowances for all these causes which
operate in any given case to prevent the perfect application of the law.
Several tables and other data relating to the friction of water will be
found in the other sections of this work, reference to which is made in
the Index.
_The term viscosity_ has been described in the Glossary at
the beginning of this work; a perfect fluid is incapable of
resisting—except by its weight or inertia—a change of shape. Such a
substance does not actually exist for all fluids have _viscosity_ or
internal friction.
This is defined as a resistance to a change of shape depending on
the rate at which the change is effected, _but_, as the fluids which
engineers have to deal with are water and vapors and gases, it
simplifies nearly all the calculations to assume that they have no
internal viscosity or friction.
A slow continuous change of the shape of solids or semi-solids under
the action of gravity, or external force, is also by the extension of
the term called _viscosity_, as the viscosity of ice, as observed in
the slow movement of those rivers of ice, the glaciers.
The _viscosity_ of liquids arises from the mutual attractions of the
molecules and is diminished by the effect of the wandering molecules
(C. D.). The viscosity of gases increases while that of liquids
diminishes as the temperature is raised.
The _viscosity_ of fluids presents a certain analogy with the
malleability of solids.
_Vis Viva_ is equivalent to _active or living force_; temperature is
the _Vis Viva_ of the smallest particles of a body; in bodies of the
same temperature the atoms have the same _vis viva_, the smaller mass
of the lighter atoms being compensated by their greater velocity.
This term, which is often met with in scientific treatises, was
invented by Leibnitz. It is well known that water in rapid circulation
will absorb more heat than when stagnant or moving slowly; this is
caused by _Vis Viva_ of the atoms.