The towers, however, were designed to sustain a heavy train and locomotive and to withstand the terrific wind of the monsoon. The pressure of such a wind on a 320-foot tower is tremendous. The bridge was completed within the specified time and bore without flinching all the severe tests to which it was put. Heavy trains--much heavier than would ordinarily be run over the viaduct--steamed slowly across the great steel trestle while the railroad engineers examined with utmost care every section that would be likely to show weakness. But the designers had planned well, the steel-workers had done their full duty, and the American bridgemen had seen to it that every rivet was properly headed and every bolt screwed tight--and no fault could be found. The bridge engineer's work is very diversified, since no two bridges are alike. At one time he might be ordered to span a stream in the midst of a populous country where every aid is at hand, and his next commission might be the building of a difficult bridge in a foreign wilderness far beyond the edge of civilisation. Bridge-building is really divided into four parts, and each part requires a different kind of knowledge and experience. First, the designer has to have the imagination to see the bridge as it will be when it is completed, and then he must be able to lay it out on paper section by section, estimating the size of the parts necessary for the stress they will have to bear, the weight of the load they will have to carry, the effect of the wind, the contraction and expansion of cold and heat, and vibration; all these things must be thought of and considered in planning every part and determining the size of each. Also he must know what kind of material to use that is best fitted to stand each strain, whether to use steel that is rigid or that which is so flexible that it can be tied in a knot. On the designer depends the price asked for the work, and so it is his business to invent, for each bridge is a separate problem in invention, a bridge that will carry the required weight with the least expenditure of material and labour and at the same time be strong enough to carry very much greater loads than it is ever likely to be called upon to sustain. The designer is often the constructor as well, and he is always a man of great practical experience. He has in his time stepped out on a foot-wide girder over a rushing stream, directing his men, and he has floundered in the mud of a river bottom in a caisson far below the surface of the stream, while the compressed air kept the ooze from flowing in and drowning him and his workmen.