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No more responsible task can fall to a structural engineer than that of designing bridges. One reason for this is that bridge design may entail the most difficult problems of construction which engineers have to solve, while the other is that the bridge holds a very important place in architecture and people expect it to assume a dignified and imposing shape. In the series of articles entitled " Great Engineers " which appeared in the pages of this Journal last year I had occasion to refer to some of the most famous bridges of the past. The works of the two Rennies, Telford, the Brunels, Robert Stephenson, Sir John Fowler and Sir John Wolfe Barry were passed under review. Not only were famous stone bridges such as Waterloo Bridge, London Bridge, Dunkeld Bridge and the Royal Border Bridge described and analysed but examples belonging to " the steel age " of bridgebuilding, such as the Forth Bridge, the Tower Bridge, Saltash Bridge and the suspension bridges at the Menai Straits and at Clifton were the subject of detailed comment. I do not propose, therefore, to traverse this same ground again, but shall devote my attention entirely to the more modern developments of bridge-building, and especially to those which exemplify the new ferro-concrete construction.
A. Trystan Edwards
In view of the increasing demand for Aluminous Cements, perhaps some particulars of the methods of the production of its principal raw material-Bauxite-may be of interest. The photographs are of various sections of the plant, which will illustrate some of the conveying methods.
When a rectangular beam is supported at both ends and loaded transversely the upper fibres are compressed and the lower extended, the stresses being greatest in the outer fibres, and proportionally less towards the middle of the depth, until a layer is reached where they both vanish. This may be shown experimentally by marking parallel vertical lines upon a beam of indiarubber as Fig. 1, and supporting it at the ends with a load on top, when the lines will be found closer together in the upper part of the beam and further apart in the lower, as Fig. 2, while at some intermediate depth their distances will be unaltered, as in line a b marking the neutral layer, or neutral
axis of the cross section, and showing that neither tension nor compression exists there. If A, B, C, D, Fig. 3, represent a cross section through the centre of a beam under transverse load, e f the maximum intensity of compression drawn to scale, and g h the maximum intensity of tension, then when these stresses are produced, the neutral axis will pass through the intersection k of lines eh, fg, and when ef and gh are equal this will also be the centre of gravity of the beam. When the stress and strain are proportional to each other, and equal in tension and compression, the horizontal lines will show by their length the intensity of the tensile and compressive stress respectively in the various layers.
Professor Henry Adams