Note 626 — Chapter XVI · ON FORM AND MECHANICAL EFFICIENCY

In like manner, Clerk Maxwell could not help employing the term “skeleton” in defining the math­e­mat­i­cal conception of a “frame,” constituted by points and their interconnecting lines: in studying the equi­lib­rium of which, we consider its different points as mutually acting on each other with forces whose directions are those of the lines joining each pair of points. Hence (says Maxwell), “in order to exhibit the mechanical action of the frame in the most elementary manner, we may draw it as a skeleton, in which the different points are joined by straight lines, and we may indicate by numbers attached to these lines the tensions or compressions in the cor­re­spon­ding pieces of the frame” (Trans. R. S. E. XXVI, p. 1, 1870). It follows that the diagram so constructed represents a “diagram of forces,” in this limited sense that it is geometrical as regards the position and direction of the forces, but arithmetical as regards their magnitude. It is to just such a diagram that the animal’s skeleton tends to ap­prox­i­mate.