[ 5 ] It is well to bear in mind the distinction between "force" and "energy." These terms have been so popularly confused that it may be difficult always to discriminate them, but in Physics they are absolutely discriminated. We have a direct sense of "force," in our muscles, whether they be moving or at rest. A force in motion is a "power," it "does work" and transfers energy from one body to another, which is commonly though incorrectly spoken of as "generating" energy. But a force at rest—a mere statical stress, like that exerted by a pillar or a watershed—does no work, and "generates" or transfers no energy; yet the one sustains a roof which would otherwise fall, thereby screening a portion of ground from vegetation; while the other deflects a rain-drop into the Danube or the Rhine. This latter is the kind of force which constrains a stone to revolve in a circle instead of a straight line; a force like that of a groove or slot or channel or "guide."
[ 6 ] An article reprinted from the North American Review for May 1905.
[ 7 ] See Nature, vol. 70, p. 176, June 23, 1904.
[ 8 ] See an article on "Modern Views of Chemical Affinity" by the present writer in a magazine called Technics, for September 1904.