694 ON FORM AND MECHANICAL EFFICIENCY [ch. 



comparable to the main girder of a double-armed cantilever 

 bridge. 



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 at each end, carrying the 

 head at one end and the tail at the other, upon a pair of projecting 

 arms or "cantilevers" (Fig. 346). 



In a typical cantilever bridge, such as the Forth Bridge 

 (Fig. 345), 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 facilitated. In 

 the case of the horse, it is obvious that the two piers of the bridge, 

 that is to say the fore-legs and the hind-legs, do not bear (as they 

 do in the Forth Bridge) separate and independent loads, but the 

 whole system forms a continuous structure. In this case, the 

 calculation of the loads will be a little more difficult and the 

 corresponding design of the structure a little more complicated. 

 We shall accordingly simplify our problem very considerably if, 

 to begin with, we look upon the quadrupedal skeleton as con- 

 stituted of two separate systems, that is to say of two balanced 

 cantilevers, one supported on the fore-legs and the other on the 

 hind ; and we may deal afterwards with the fact that these two 

 cantilevers are not independent, but are bound up in one common 

 field of force and plan of construction. 



In the horse it is plain that the two cantilever systems into 

 which we may thus analyse the quadrupedal bridge are unequal 

 in magnitude and importance. The fore-part of the animal is 

 much bulkier than its hind quarters, and the fact that the fore-legs 

 carry, as they so evidently do, a greater weight than the hind-legs 

 has long been known and is easily proved ; we have only to walk 

 a horse onto a weigh-bridge, weigh first his fore-legs and then his 

 hind-legs, to discover that what we may call his front half weighs 



