992 ON FORM AND MECHANICAL EFFICIENCY [ch. 



girder," supported at each end but not otherwise fixed, and con- 

 sisting 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, of the simple bow-girder is not to 

 be 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 admirably 

 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 comparable 

 to the main girder of a double-armed cantilever bridge. 



Fig. 470. A two-armed cantilever of the Forth Bridge. Thick lines, com- 

 pression-members (bones); thin lines, tension -members (ligaments). 



Obviously, in our quadrupedal bridge, the superstructure does 

 not terminate (as it did in our former diagram) at the two points 

 of support, but it extends beyond them, carrying the head at one 

 end and sometimes a heavy tail at the other, upon projecting arms 

 or "cantilevers." 



In a typical cantilever bridge, such as the Forth Bridge (Fig. 470), 

 a certain simplification is introduced. For each pier carries, in this 

 case, its own double-armed cantilever, linked by a short connecting 

 girder to the next, but so jointed to it that no weight is transmitted 

 from one cantilever to another. The bridge in short is cut into 

 separate sections, practically independent of one another; at the 

 joints a certain amount of bending is not precluded, but shearing 

 strain is evaded; and each pier carries only its own load. By 

 this arrangement the engineer finds that design and construction 

 are alike simplified and faciUtated. In the horse or the ox, it is 



