RAILWAY CONSTRUCTION. 95 



very rapidly as the span increases, and for openings of more 

 than 60 or 70 feet girder-work becomes much cheaper than 

 arching. 



Figs. 92 and 93 are examples of viaducts having piers of 

 masonry, with girders to carry the roadway. In the one case 

 the roadway is carried on the bottom flange of the girders, and 

 in the other on the top. The latter arrangement affords greater 

 facility for securely bracing the girders together, while for the 

 former it is claimed that the girders form a massive parapet, 

 which would serve as a protection in the event of an engine or 

 vehicles leaving the rails. 



In the early days of railways, many large viaducts were con- 

 structed having masonry piers, and timber trusses to carry the 

 roadway. Much ingenuity was displayed in designing the 

 trusses, and in the introduction of cast-iron joint-shoes and 

 wrought-iron bracings. Many of these wooden superstructures 

 served well for several years, but they were always exposed to 

 the imminent risk of destruction from fire, and however care- 

 fully the logs may have been selected, the decay of the timber 

 was only a question of time. The deterioration of one piece was 

 equivalent to the weakening of the entire truss, and the renewal 

 of any part was both difficult and costly. The shrinkage of the 

 timber, and the working at the joints, caused the trusses to 

 deflect considerably under a passing load, and although the 

 actual strength of the structure may not have been much im- 

 paired, the creaking and depression had anything but a reassur- 

 ing effect. Timber superstructures for anything but small 

 spans are rarely adopted now, except for temporary works, or on 

 lines abroad, where the transport on girder-work would be very 

 costly, and where good timber is very cheap and abundant. 

 Even in the latter case the wooden superstructure is generally 

 looked upon as a temporary expedient, to be replaced at no very 

 remote date with iron or steel girders, when the materials can 

 be conveyed over the entire completed line. 



Figs. 94, 95, and 96 are sketches of three types of timber 

 trusses as constructed in viaducts of several spans. 



There are many localities, especially abroad, where suitable 

 stone is most difficult to obtain, and very expensive to work and 

 convey. In such cases it is compulsory to use as little of it as 

 possible, and to resort to iron or steel both for the girders and a 

 large portion of the piers. The piers may be made of cast-iron, 



