XVI] COMPARATIVE ANATOMY OF BRIDGES 991 



bridge, while it is the last thing which an engineer desires and 

 the first which he seeks to provide against, will impose certain 

 important limiting conditions upon the design of the skeletal fabric. 

 But let us begin by considering the quadruped at rest, when he 

 stands upright and motionless upon his feet, and when his legs 

 exercise no function save only to carry the weight of the whole 

 body. So far as that function is concerned, we might now perhaps 

 compare the horse's legs with the tall and slender piers of some 

 railway bridge; but it is obvious that these jointed legs are ill- 

 adapted to receive the horizontal thrust of any arch that may be 

 placed atop of them. Hence it follows that the curved backbone 

 of the horse, which appears to cross like an arch the span between 

 his shoulders and his flanks, cannot be regarded as an arch, in the 



Fig. 469. a, tied arch; b, bowstring girder. 



engineer's sense of the word. It resembles an arch in 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 ahutinents 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 indis- 

 pensable. Thus, for example, we may begin by inserting a straight 

 steel tie, AB (Fig. 469), 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. 4696). 

 In either case, the structure becomes an . independent "detached 



