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.