692 ON FORM AND MECHANICAL EFFICIENCY [ch. 



we might go even further, and study (after the fashion of the 

 anatomist) the "osteology" and "desmology" of the structure, 

 that is to say the bones which are represented by "struts," and 

 the ligaments, etc., which are represented by "ties." Further- 

 more after the methods of the comparative anatomist, we may 

 classify tlie families, genera and species of bridges according to 

 their distinctive mechanical features, which correspond to certain 

 definite conditions and functions. 



In more ways than one, the quadrupedal bridge is a remarkable 

 one; and perhaps its most remarkable peculiarity is that it is a 

 jointed and flexible bridge, remaining iii equilibrium under 

 considerable and sometimes great modifications of its curvature, 

 such as we see, for instance, when a cat humps or flattens her 

 back. The fact that flexibility is an essential feature in the 

 quadrupedal bridge, while it is the. last thing which an engineer 

 desires and the first which he seeks to provide against, will impose 

 certain important limiting conditions upon the design of the 

 skeletal fabric ; but to this matter we shall afterwards return. 

 Let us begin by considering the quadruped at rest, when he stands 

 upright and motionless upon his feet, and when his legs exercise 

 no function save only to carry the weight of the whole body. So 

 far as that function is concerned, we might now perhaps compare 

 the horse's legs with the tall and slender piers of some railway 

 bridge; but it is obvious that these jointed legs are ill-adapted 

 to receive the horizontal thrust of any arch that may be placed 

 atop of them. Hence it follows that the curved backbone of the 

 horse, which appears to cross like an arch the span between his 

 shoulders and his flanks, cannot be regarded as an arch, in the 



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 joinmg 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 com- 

 pressions in the corresponding pieces of the frame" (Trails. 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. 



