XVI] COMPARATIVE ANATOMY OF BRIDGES 989 



a great variety of conditions*. This framework is complicated as 

 we see it in the skeleton, where (as we have said) it is only, or chiefly, 

 the struts of the whole fabric which are represented ; but to under- 

 stand the mechanical structure in detail, we should have to follow 

 out the still more complex arrangement of the ties, as represented 

 by the muscles and ligaments, and we should also require much 

 detailed information as to the weights of the various parts and as 

 to the other forces concerned. Without these latter data we can 

 only treat the question in a preUminary and imperfect way. But, 

 to take once again a small and simplified part of a big problem, 

 let us think of a quadruped (for instance, a horse) in a standing 

 posture, and see whether the methods and terminology of the 

 engineer may not help us, as they did in regard to the minute 

 structure of the single bone. And let us note in passing that the 

 "standing posture," whether on two legs or on four, is no very 

 common thing; but is (so to speak), with all its correlated anatomy, 

 a privilege of the few. 



Standing four-square upon its fore-legs and hind-legs, with the 

 weight of the body suspended between, the quadruped at once 

 suggests to us the analogy of a bridge, carried by its two piers. 

 And if it occurs to us, as naturalists, that we never look at a 

 standing quadruped without contemplating a bridge, so, conversely, 

 a similar idea has occurred to the engineer ; for Professor Fidler, . 

 in this Treatise on Bridge-Construction, deals with the chief descrip- 

 tive part of his subject under the heading of "The Comparative 

 Anatomy of Bridges"]"." The designation is most just, for in 

 studying the various types of bridge we are studying a series of 

 well-planned skeletons %] and (at the cost of a little pedantry) 



* Our problem is analogous to Thomas Young's problem of the best disposition 

 of the timbers in a wooden ship {Phil. Trans. 1814, p. 303). He was not long of 

 finding that the forces which act upon the fabric are very numerous and very 

 variable, and that the best mode of resisting them, or best structural arrangement 

 for ultimate strength, becomes an immensely complicated problem. 



f By a bolder metaphor Fontenelle said of Newton that he had "fait I'anatomie 

 de la lumiere." 



X 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 rnutually 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 



