994 ON FORM AND MECHANICAL EFFICIENCY [ch. 



But here we pass from the, statical problem to the dynamical, 

 from the horse at rest to the horse in motion, from the observed 

 fact that weight lies mainly over the fore-legs to the question of 

 what advantage is gained by such a distribution of the load. Taking 

 the hind-legs as the main propulsive agency, as we may now safely 

 do, the moment of propulsion is about the hind-hooves; then (as we 

 see in Fig. 471) we may take the weight, W = A sin a, and the 



W H 



propulsive force, f= A cos a, and -y = y, WL = fH being the 



balanced condition. From the statical point of view the load must 

 balance over the fore-legs; from the dynamical point of view it 

 might well lie even farther forward. And when the jockey crouches 



Fig. 471. 



over the horse's neck, and when Tod Sloan introduced the " American 

 seat," both shew a remarkable, though perhaps unconscious, insight 

 into the dynamical proposition. 



Our next problem is to discover, in a rough and approximate 

 way, some of the structural details which the balanced load upon 

 the double cantilever will impress upon the fabric. 



Working by the methods of graphic statics, the engineer's task is, 

 in theory, one of great simplicity. He begins by drawing in outline 

 the structure which he desires to erect; he calculates the stresses 

 and bending-moments necessitated by the dimensions and load on 

 the structure; he draws a new diagram representing these forces, 

 and he designs and builds his fabric on the lines of this statical 

 diagram. He does, in short, precisely what we have seen nature 

 doing in the case of the bone. For if we had begun, as it were. 



