(4.) When two lines of different directions bisect one another, a special arrangement permits the passengers from one line to pass to the other by means of what is called a "tangent" station, without the trains of one line crossing the tracks of another, the purpose of which arrangement is to avoid those accidents that would inevitably occur through the crossing of a track by the trains of a transverse line. (5.) The rolling stock is arranged in a manner that allows the entrance and exit of the passengers to be effected with great promptness. In ordinary avenues, comprising a roadway and two sidewalks, the elevated railway is placed in the axis of the roadway at a sufficient height to prevent it interfering with the passage of carriages, say 14 3/4 feet above the surface, while in boulevards or avenues of great width, having _contre-allees_[1] bordered by a double row of trees, it is installed in one of the _contre-allees_. [Footnote 1: Paths parallel with the public walks.] In the first case (Fig. 1), the viaduct is wholly metallic, while in the second it comprises masonry arches surmounted by a metallic superstructure. The viaduct is formed of independent spans supported by metallic piers that rest upon masonry foundations (Fig. 2). [Illustration: FIG. 1.--PROJECT FOR A PARISIAN ELEVATED RAILWAY.] [Illustration: FIG. 2.--LONGITUDINAL ELEVATION.] The line will have three kinds of stations, intermediate, "tangent," and terminal ones. It is at the latter that the two superposed lines are connected by the circular inclined plane. The waiting platforms of the intermediate stations will be formed simply by the widening of the span corresponding to the station. Access to these platforms will be had by stairs running up from the edge of the sidewalk. The passengers will make their exit by means of corresponding stairs on the opposite side. (Figs 3 and 4.) [Illustration: FIG. 3.--A STATION.] [Illustration: FIG. 4.--TRANSVERSE SECTION OF STATION.] The tangent stations are placed at the meeting point of two lines, which, instead of crossing each other, are bent inward like an X, the two parts of which will be tangent to the central point. Through such arrangements the running of the trains will be continuous, and a traveler reaching one of these stations will be able, upon changing train, to take at his option any one of the three other directions.