xvi] THE SKELETON AS A BRIDGE 995 



by blocking out the femur roughly, and considering its position 

 and dimensions, its means of support and the load which it has to 

 bear, we could have proceeded at once to draw the system of 

 stress-lines which must occupy that field of force : and to precisely 



B 



Fig. 472. A, Span of proposed bridge. B, Stress diagram, or diagram 

 of bending-moments*. 



those stress-lines has Nature kept in the building of the bone, down 

 to the minute arrangement of its trabecular 



The essential function of a bridge is to stretch across a certain 

 span, and carry a certain definite load; and this being so, the chief 



XfJwTX 



Fig. 473. The bridge construnted, as a parabolic girder. 



problem in the designing of a bridge is to provide due resistance 

 to the "bending-moments" which result from the load. These 

 bending-moments will vary from point to point along the girder, 



* This and the following diagrams are borrowed and adapted from Professor 

 Fidler's Bridge Construction. We may reflect with advantage on Clerk Maxwell's 

 saying that "the use of diagrams is a particular instance of that method of symbols 

 which is so powerful an aid in the advancement of science" ; and on his explanation 

 that "a diagram differs from a picture in this respect that in a diagram no attempt 

 is made to represent those factors of the actual material system which, are not the 

 special objects of our study." 



