XVI] THE COMPARATIVE ANATOMY OF BRIDGES 693 



engineer's sense of the word. It resembles an arch m form, but 

 not in function, for it cannot act as an arch unless it be held back 

 at each end (as every arch is held back) by abutments capable of 

 resisting the horizontal thrust; and these necessary abutments 

 are not present in the structure. But in various ways the 

 engineer can modify his superstructure so as to supply the place 

 of these external reactions, which in the simple arch are obviously 

 indispensable. Thus, for example, we may begin by inserting a 

 straight steel tie, AB (Fig. 339), uniting the ends of the curved rib 

 AaB ; and this tie will supply the place of the external reactions, 

 converting the structure into a "tied arch," such as we may see 

 in the roofs of many railway-stations. Or we may go on to fill 

 in the space between arch and tie by a "web-system," converting 

 it into what the engineer describes as a "parabolic bowstring 



girder" (Fig. 339 h). In either case, the structure becomes an 

 independent "detached girder," supported at each end but not 

 otherwise fixed, and consisting essentially of an upper compression- 

 member, AaB, and a lower tension-member, AB. But again, in 

 the skeleton of the quadruped, the necessary tie, AB, is not to he 

 found; and it follows that these comparatively simple types of 

 bridge do not correspond to, nor do they help us to understand, 

 the type of bridge which nature has designed in the skeleton of 

 the quadruped. Nevertheless if we try to look, as an engineer 

 would look, at the actual design of the animal skeleton and the 

 actual distribution of its load, we find that the one is most admir- 

 ably adapted to the other, according to the strict principles of 

 engineering construction. The structure is not an arch, nor a 

 tied arch, nor a bowstring girder : but it is strictly and beautifully 



