375. The principal matters to be attended to in the design of a wooden bridge are - 1st, the situation; 2ndly, the waterway that ought to be left for the river; 3rdly, the number and span of the openings; and,. fourthly, the arrangement and proportion of the several parts. Each of these is chiefly determined by local circumstances.

The principal object in erecting a bridge is to obtain a more easy and ready communication across a stream or valley or between the opposite, banks of a river; and, as a rule, the situation chosen ought to be that which is most convenient for the use of the public. Sometimes, however, it happens that the most convenient situation is not the best adapted for the erection of a bridge. In this case the advantages and disadvantages of other situations should be carefully considered, and the site determined so that the means of communication may be as direct as possible, and the access to the bridge convenient.

In the case of a river or stream the choice will be facilitated by making a correct plan of the course, showing the

* Haupt's ' Theory of Bridge Construction.' banks on each side, and of the reads that are connected with the bridge. This plan should be sufficiently extensive to give a correct idea of the natural features of the surface and of the changes which the bed and course of the river may have undergone.

Longitudinal and cross sections of the water-course should be made, showing the bed of the river, the form of the opposite banks, the nature of the subsoil, the depth of water at different seasons of the year, and the line of the highest and lowest water-marks, which should be drawn from information to be procured from the oldest inhabitants of the neighbourhood.

The nature of the bed of the river should be carefully examined, particularly on the site of the abutments or piers, by boring, driving in a rod of iron, or other means, and to a sufficient depth to be certain of the quality of the ground.

It would also be desirable to show on the section the declivity of the bed of the river for a considerable distance above and below the situation of the intended bridge, and also to mark on it the velocity of the stream at different periods. The bridge should, if possible, cross the stream at right angles; and it is an advantage, when the course of the river is nearly straight for a considerable distance above the bridge; and when there is a contraction in the channel at a little distance below it, as it renders the effects of floods less dangerous.