408 PROGRESS OF BRIDGE-BUILDING. CHAP. XVIII. 



of the best point for crossing a river or a valley. He 

 must take such ground as lay in the line of his railway, 

 be it bog, or mud, or shifting sand. Navigable rivers 

 and crowded thoroughfares had to be crossed without 

 interruption to the existing traffic, sometimes by bridges 

 at right angles to the river or road, sometimes by arches 

 more or less oblique. In many cases great difficulty 

 arose from the limited nature of the headway ; but, as 

 the level of the original road must generally be pre- 

 served, and that of the railway was in a measure fixed 

 and determined, it was necessary to modify the form and 

 structure of the bridge, in almost every case, in order to 

 comply with the public requirements. Novel conditions 

 were met by fresh inventions, and difficulties of the most 

 unusual character were one after another successfully 

 surmounted. In executing these extraordinary works, 

 iron has been throughout the sheet-anchor of the engi- 

 neer. In its various forms of cast and wrought iron, it 

 offered a valuable resource, where rapidity of execution, 

 great strength, and cheapness of construction in the first 

 instance, were elements of prime importance ; and by its 

 skilful use, the railway architect was enabled to achieve 

 results which thirty years ago would scarcely have been 

 thought possible. 



In many of the early cast-iron bridges the old form of 

 the arch was adopted, the stability of the structure 

 depending wholly on compression, the only novel feature 

 being the use of iron instead of stone. But in a large 

 proportion of cases, the arch, with the railroad over it, 

 was found inapplicable in consequence of the limited 

 headway which it provided. Hence it early occurred to 

 George Stephenson, when constructing the Liverpool 

 and Manchester Eailway, to adopt the simple cast-iron 

 beam for the crossing of several roads and canals along 

 that line this beam resembling in some measure the 

 lintel of the early temples the pressure on the abut- 

 ments being purely vertical. One of the earliest 



