sight will intersect the centre line at angles of exactly forty-five degrees.

The advantage of this system lies in the fact that all the piers are located by direct sight without having to measure the angle, the only angles requiring measurement being the four right angles between the base-lines and the centre line of bridge, and the four other angles required for determining and checking the distance between base-lines along the bridge tangent.

The lengths of base-lines for ordinary systems of triangulation will generally be regulated by local conditions. They

should usually be about its long as the total length of bridge, or, when there is a base-line on each side of the river, as long its the perpendicular distance between opposite baselines; but, if necessary, they may be made as short as seven tenths of same. Too short base-lines will give too sharp intersections, and therefore sometimes too great variations from correctness; nevertheless, sharp intersections can be employed at times by taking extra pains with the work and by employing an extra intersection as a check, in ease that any discrepancy occurs.