The Viaduct Works' Handbook

limiting the amount of traffic and speed of passing vehicles to an inconvenient degree ; and, as compared with girder bridges, there is no economy in cost, taking all the chains and anchorage into consideration. The suspension principle, however, as regards appear- ance, can he made to give a much more pleasing effect than the rigid girder, and, moreover, presents means of passing moderate loads at reduced speeds across spaces it might be impracticable to span with any other kind of bridge, and it is possible some means may yet be devised to make such a system more rigid. Wires laid up together in straight lines, and bound together into ropes or cables, have been employed, as is well known, in lieu of chains of bars commonly adopted. Wires thus laid up have this advantage over cables of twisted wire, that they will elongate only, under a given strain, to the extent common to iron so strained, while the twisted wire cable will stretch, yielding and contracting more or less according to the tightness or slackness with which the wires may have been originally twisted together. This yielding or stretching presents a difficulty in the way of securing the proper level of the roadway of a suspension bridge. From amongst the suspension bridges made at Crumlin, we select one of which we give an engraving. It is a small foot-bridge, and has some points worthy of notice. The suspending chains radiate from the top of the tower, intersecting the platform at various points at which they are attached, instead of forming a continuous curve as in the ordinary way, thus adding greatly to the stiffness of the platform. This has been found to answer tolerably well for small spans. Other means are now engaging the attention of engineers, in which the girder and suspension bridge are combined, and some bold examples have recently been brought to notice, and upon their success may depend future progress of the suspension principle; and now that the production of steel of reliable quality, and double the tenacity of iron, manufactured by the Bessemer process, has become so well known, it is difficult to say how far it may affect the future of suspension bridges, particularly those of very large spans, where the weight of the chains alone is so im- portant an element in the calculation of the strains they have to sustain. Similar reasoning may, of course, apply to bridges of other kinds, but, perhaps, not with the same force, for in none is the