right angles to the centre line should be taken. These will be found useful in showing what effect a deviation to the right or left of the surveyed line would have. Cross-levels should be taken at the intersection of all roads and railroads to show to what extent, if any, these levels will have to be altered to suit the levels of the proposed road.

574. Profile. A profile is a longitudinal section of the route, made from the levels. Its horizontal scale should be the same as that of the map; the vertical scale should be such as will show with distinctness the inequalities of the ground.

Fig. 44 shows the manner in which a profile is drawn and the nature of the information to be given upon it.

575. Bridge Sites.-The question of choosing the site of bridges is an important one. If the selection is not. restricted to a particular point, the river should be examined for a considerable distance above and below what would be the most convenient point for crossing; and if a better site is found, the line of the road must be made subordinate to it. If several practicable crossings exist, they must be carefully compared in order to select the one most advantageous. The following are controlling conditions: (1) Good character of the river-bed, affording a firm foundation. If rock is present near the surface of the river-bed, the foundation will be easy of execution and stability and economy will be insured.

(2) Stability of the river-banks, thus securing a permanent concentration of the waters in the same bed. (3) The axis of the bridge should be at right angles to the direction of the current. (4) Bends in the river are not suitable localities and should be avoided if possible. A straight reach above the bridge should be secured if possible.

576. Principles to be observed in making the Final Selection. In making the final selection the following principles should be observed as far as practicable.

The

(a) To follow that route which affords the easiest grades. easiest grade for a given road will depend upon the kind of covering adopted for its surface

(b) To connect the places by the shortest and most direct route commensurate with easy grades.

(c) To avoid all unnecessary ascents and descents. When a road is encumbered with useless ascents, the wasteful expenditure of power is considerable.

(d) To give the centre line such a position, with reference to the natural surface of the ground, that the cost of construction shall be reduced to the smallest possible amount.

(e) To cross all obstacles (where structures are necessary) as nearly as possible at right angles. The cost of skew structures increases nearly as the square of the secant of the obliquity.

(f) To cross ridges through the lowest pass which occurs.

(g) To cross either under or over railroads; for grade crossings mean danger to every user of the highway. Guards and gates frequently fail to afford protection, and the daily press is filled with accounts of accidents more or less serious; and while statistics fail to give total casualties, the aggregate must be great.

577. Examples of Cases to be Treated.-In laying out the line of a road, there are three cases which may have to be treated, and each of these is exemplified in the contour map Fig. 42, page 278. First, the two places to be connected, as the towns A and B on the plan, may be both situated in the same valley, and upon the same side of it; that is, they are not separated from each other by the main stream which drains the valley. This is the simplest case. Secondly, although both in the same valley, the two places may be on opposite sides of the valley, as at A and C, being separated by the main river. Thirdly, they may be situated in different valleys, sep

arated by an intervening ridge of ground more or less elevated, as at A and D. In laying out an extensive line of road, it frequently happens that all these cases have to be dealt with; frequently, perhaps, during its course.

The most perfect road is that of which the course is perfectly straight and the surface practically level; and, all other things being the same, that is the best road which answers nearest to this description.

Now in the first case, that of the two towns situated on the same side of the main valley, there are two methods which may be pursued in forming a communication between them. A road following the direct line between them, shown by the thick dotted line AB, may be made, or a line may be adopted which will gradually and equally incline from one town to the other, supposing them to be at different levels, or which should keep, if they are on the same level, at that level throughout its entire course, following all the sinuosities and curves which the irregular formation of the country may render necessary for the fulfilment of these conditions. According to the first method, a level or uniformly inclined road might be made from one to the other; this line would cross all the valleys and streams which run down to the main river, thus necessitating deep cuttings, heavy embankments, and numerous bridges; or these expensive works might be avoided by following the sinuosities of the valley. When the sides of the main valley are pierced by numerous ravines with projecting spurs and ridges intervening, instead of following the sinuosities, it will be found better to make a nearly straight line cutting through the projecting points in such a way that the material excavated should be just sufficient to fill the hollows.

Now, of all these, the best is the straight and uniformly inclined, or the level road, although at the same time it is the most expensive. If the importance of the traffic passing between the places is not sufficient to warrant so great an outlay, it will become a matter of consideration whether the course of the road should be kept straight, its surface being made to undulate with the natural face of the country; or whether, a level or equally inclined line being adopted, the course of the road should be made to deviate. from the direct line and follow the winding course which such a condition is supposed to necessitate.