forty-five seconds and lower again in the same time. A full description of the Tower Bridge is given in the Proceedings of the Institution of Civil Engineers, Vol. CXXVII.

A good example of the rolling bascule bridge is the Van Buren Street Bridge at Chicago, Ill., which structure is described in Engineering News of Feb. 21, 1895. It consists of two leaves, each about seventy feet long, ending in a cylindrical surface that rolls on a plane provided with teeth which gear into the roller to prevent slipping. When the bridge is closed the short end of each arm is anchored down to the masonry so as to permit of its acting as a cantilever.

Close alongside of this structure is the Metropolitan Elevated Railroad Company's four-track bridge, which is also of the rolling bascule type. This is divided into two similar double-track bridges placed close together and operated separately, so that, in case of accident to one bridge, the railroad traffic may be diverted to the other, while the injured span is raised out of the way of the river traffic.

It is seldom advisable to use a centre pier to rest the leaves of the bascule upon, on account of the obstruction which it would offer to navigation.

A submerged pier to receive the ends of the posts of hanging hinged bents has never been used, nor is it at all likely that it ever will be, owing to the difficulties that would be encountered in operation, such as those from ice, drifting sand, changing currents, etc., all of which would tend to prevent the column-feet from taking proper bearing on the pier.

Suspending the ends of the leaves from an overhead span, or tying them back to the tops of towers, is a perfectly feasible method, but is expensive and without advantage.

There is a bascule bridge in Chicago which is counterweighted by four masses of cast iron in carriages that run upon curved surfaces on the approaches, the curves being so figured that the varying load at the channel end of the leaf is at all times balanced by the varying tension on the cables which hold the counterweights.

There is a similar structure on Michigan Avenue in Buffalo,