Engineering Wonders of the World
Volume I

■■■■■■■■■■■■ TRANSPORTER BRIDGES. the suspenders and the stiffening girders, a curious thing happened, which illustrated the theory of interference of wave action. A strong gale set up a vibration in the cable, which was supported at the ends only, the vertical wave being about ten feet high at in- tervals of four seconds. In the following week a much stronger gale sprang up from the same quarter, but there was no movement of the cables. In the first case the gusts of wind— which are not continuous—must have syn- chronized with the natural vibration period of the cable, but, in the second case, have worked antagonistically. An important question is the method of running the car. In the case of all the French bridges and the Newport Bridge the car trolley is hauled backwards and for- Propulsion warjs en(ness rOpes. This, of the x1 . / , , Car the method of working travelling and other cranes, tramways, etc., has been abandoned in favour of direct traction, and would appear to be the better system both theoretically and practic- ally. The continuous stretching of the cables must cause considerable trouble, even if appli- ances are adopted for taking up the slack ; and 299 the cables have to be supported and released as the car travels along, which involves the use of devices that are all liable to get out of order. There is certainly a slight saving in the weight of the car and trolley to be propelled, as the motors and gear- ing are ashore ; but this advantage is slight as compared with its accompanying disadvantages. The “ cradling ” of the cables described above is a distinct advantage, for it gives increased stability to the structure laterally, and en- ables it to withstand wind pressure. A strong gale arose during the construction of the Run- corn Bridge, when only about one half of the stiffening girders had been built out from the towers, and were unsupported laterally by guys. It did no damage, however, the girders moving horizontally a few inches only. But for the “ cradling,” serious damage, and even total collapse, would no doubt have overtaken the half-finished structure. There are several schemes on hand for erect- ing transporter bridges in different parts of the country, and there is no doubt that many more will be designed when people begin to appreciate their many advantages. Fig. 15.—A CABLE ANCHORAGE.