the greatest value inn situations where electric power is not available, or not obtainable on reasonable terms.

Movable bridges may be considered as consisting of three main elements, the first comprising the actual bridge or carrying structure, fixed in position during normal traffic, and which must be removed to permit of transverse traffic; the second comprising means to support the carrying structure together with the counterweights required to balance it; and the third comprising means to operate or impart motion to the carrying structure, its counterweights, and such parts of the supporting structure as must move therewith—the whole constituting a complete machine.

Of this machine it is convenient to classify those parts which require to be made in the machine shop as "machinery," in contradistinction to those fabricated in the bridge shop and classified as "structural"; and, as machine-shop work is much more costly than structural work, economics requires that the design should aim at a preponderance of parts which may be finished in the fabricating shop and at as complete a separation in manufacture between the machinery and structural parts as possible, and also that it should permit of simple and easily-adjustable connections in the field. The vertical-lift bridge meets this requirement much better than does either the swing or the bascule.

The carrying structure will consist essentially of structural steelwork and its adjuncts, such as flooring, tracks, and the like, and will comprise no machinery; the supporting structure will generally consist essentially of structural steelwork carrying such supporting-machinery parts as trunnions, rollers, hangers, sheaves, and the like; while the operating machinery will consist of actuating-or-retarding-machinery parts, such as gears, screws, rams, brakes, buffers, and similar component elements.

The supporting machinery parts are dependent for size and strength purely on the weights carried by them, hence their cost is a function of the weight of the movable masses; and, as they necessarily have slow and limited motions, and as they work under very heavy loads, they are essentially costly and must be designed for the highest permissible unit-bearing-pressures, in order to obtain the smallest practicable diameters. In general, in movable bridges the lengths of bearings are limited by space conditions, and increased bearing surface can only be obtained by enlarged diameters; and, as the weights of bearings, trunnions, etc., increase as the square of their diameters, and their costs nearly in the same proportion, the economic importance of high unit-bearing-pressures will be readily understood, consequently for these parts one is justified in adopting the best grades of bearing metals and the most complete provision for lubrication. The latter involves not only suitable oil-or-grease-grooves, pressure lubricators, and lubricant, all arranged so the said lubricant will surely reach the surfaces under pressure, but also requires convenient and safe means of access,  such  as  steps,  ladders,  platforms,  and railings,  as  well  as