near the surface, it will be best to surround the hub with concrete, and protect it with a substantial cover of some kind to prevent displacement.

If driving or carting is to be carried on in the vicinity of the hub, the latter should be fenced in by four stout posts sunk into the ground on the corners of a square of seven or eight feet on a side, the posts projecting high enough above the ground to strike a wagon-box.

In locating all triangulation-hubs it is essential to place them so that the operations of construction will not obstruct the view of the transitman.

If there is a possibility that any of the hubs will be disturbed by the operations of construction or in any other manner, such hubs should be carefully "tied in" by reference points located some distance away. This should be done as soon as the base-line is measured.

There should be two base-lines, one on each side of the river and both on the same side of the bridge, or both should be on the same side of the river with one above and the other below the bridge. Usually it will be found satisfactory to locate all piers from one point on each base-line, and for that reason the ends of the base-line should be chosen so that, if possible, all the piers can be seen therefrom. If this be impracticable, or if some of the deflections would for any reason be too small, it will be necessary to put in and use intermediate hubs on the base-lines.

Base-lines, whenever it is practicable, should be run approximately at right angles to the longitudinal axis of the bridge; but this is by no means essential, and it is folly to try to make the intersection exactly at right angles, except in the following case, which represents an ideal system of triangulation that can rarely be utilized, on account of the existing conditions of shores, and obstructions both natural and artificial.

The said ideal system consists in running four base-lines, as shown in Fig. 8, all exactly at right angles to the centre line of the bridge, and laying off thereon distances equal to those from the base-line to pier centres, so that all lines of