We see a body move; we know there must be a cause; that cause we call force. We see a body in motion come to rest; this effect must have had a cause; that cause we attribute to force. The forces acting in machines are distinguished into _driving_ and _resisting_ forces. That component of the force which does the work is called the "_effort_." ~26. Friction~ is usually a resisting (29) force, tending to destroy motion; but it is sometimes the means of the transmission of motion. ~27. Work~ is the result of force acting through space. When force produces motion, the result is work. _Work is measured by the product of the resistance into the space through which it is overcome._ ~28. Energy~, which is defined[6] as the capacity for doing work, is either _actual_ or _potential_. _Actual_ or _kinetic energy_ is the energy of an actually moving body, and is measured by the work which it is capable of performing while being brought to rest under the action of a retarding force. _Potential Energy_ is the capacity for doing work possessed by a body in virtue of its position, of its condition, or of its intrinsic properties. A bent bow or a coiled spring has potential energy, which becomes actual in the impulsion of the arrow or is expended in the work of the mechanism driven by the machine. A clock weight, condensed air and gunpowder are examples. This form of energy appears in every moving part of every machine and its variations often seriously affect the working of machinery. (84.) FOOTNOTES: [5] This and some of the definitions that follow are adapted from "Elements of Physics" by A. P. Gage. [6] Thurston. "Friction and Lost Work in Machinery," from which excellent work much of the next chapter is adapted. CHAPTER III. FRICTION--ITS NATURE AND THEORY. ~29. Friction.~ The relative motion of one particle or body in forced contact with another is always retarded, or prevented, by a resisting force called friction. Friction manifests itself in three ways: Between solids it is called _sliding_ and _rolling friction_; between the particles of liquids, or of gasses, when they move in contact with each other, or with other bodies, it is called _fluid friction_. Quite different laws govern these three kinds of friction, as they are quite different in character. Friction can never of itself produce or accelerate motion, being always a resisting force, acting at the surfaces of contact of the two particles, or masses, between which it occurs, and in the direction of their common tangent, resisting relative motion in whichever direction it may be attempted to produce it. The greatest loss of energy in a timepiece in which all the parts are rigid enough to prevent permanent distortion, is that occurring through friction. Another source of loss of energy is the reduction in elasticity of springs caused by a rise of temperature.