Why, then, are massive tubular bridges not more generally built? Because they led to another and very natural development in bridge-building, a development whereby great strength for long spans is gained, with, however, a marked saving both in labour and in material. That development was the lattice bridge. CHAPTER III. LATTICE AND SUSPENSION BRIDGES. “The expense of a tubular bridge would be too great.” “But if we could get the strength without the expense.” “What mean you?” “By iron lattice work we could, I think, gain the stiffness and support needed, without such great cost of labour and material. In other words, I propose a lattice or trellis work girder, instead of a solid sided, or a tubular girder.” “That is, you would have the sides of lattice or trellis work, instead of solid plates?” “Exactly. I would use bars of iron placed diagonally. These lattice or trellis bridges are developed from the tubular bridges, also from the loose wooden lattice bridges of America. We make a web of iron instead of a solid sheet. The same kind of structures are largely used over the wide rivers of India. Sir John MacNeill designed the first in iron, and it was built in 1843 on the Dublin and Drogheda Railway with a span of eighty-four feet. I consider they will be among the most popular bridges of the future for longish spans.” The engineer’s prediction has come true; for lattice bridges have undoubtedly been very widely adopted. We may suppose that he was advising the directors of a proposed railway, and we doubt not but that he carried the day. A fine specimen of a lattice bridge is that across the Thames near Charing Cross, for the South-Eastern Railway. It has a total length of more than a quarter of a mile—viz., 1365 feet, and six of its nine spans are 154 feet wide. Two principal girders, fourteen feet deep, are connected transversely by other girders which carry the rails and project on the other side to support a footpath. The two main girders are nearly fifty feet apart and one weighs 190 tons. The sides have upper and lower booms made of plate iron connected by perpendicular bars, between which are a couple of bars crossing each other diagonally at an angle of forty-five degrees, and fixed to the booms by bolts of five and seven inches in diameter.