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. CAPILLARY ATTRACTION. _Capillary attraction is the attraction which causes the ascent of fluids in small tubes._ This word is derived from the Latin _capillus_—a hair. The tubes must be less than one-tenth of an inch in diameter in order to produce the most satisfactory results, and tubes whose bores are no larger than a hair present the phenomenon the most strikingly. But though the rise of water above its natural level, is most manifest in small tubes, it appears, in a degree, in vessels of all sizes and shapes, by a ring of water formed around the sides with a concavity upward. The following are the leading _facts_ respecting capillary attraction.