In properly conducting this part of the business of road-making, great care is necessary. The utmost judgment of the skilful surveyor will be called into action to enable him to make the best use of the natural facilities of the country, and to overcome the obstructions that he will sometimes meet with. In passing over flat land, open main drains, cut on the field side of the fences, must communicate with the natural watercourses of the country; they should be three feet deep below the level of the bed of the road, one foot wide at bottom, and five feet wide at top. If springs rise in the site of the road, or in the slopes of deep cuttings, stone or tile drains should be made into them. In cutting, small drains, technically called mitre drains, should be formed; the angle, depending on the inclination of the road, should not exceed 1 inch in 100. They should be 9 inches wide at bottom, 12 inches at top, and 10 inches deep. According to the inclinations of a road, and the form and wetness of the country, cross-drains of good masonry should be built under the road, having their extremities carried under the road fences.
One of these should be built wherever water would lie; and when the road passes along the slope of a hill, great numbers are necessary to carry off the water that collects in the channel of the road on the side next the high ground. Various descriptions of drains are made in every situation where necessary, and the preservation of the surface of the road secured by giving it a proper convexity in its cross section, as shown in the annexed section, designed for the regulation of the surfaces and wastes between the fences of the Holyhead road.
The proper convex form is particularly essential on hills, in order that the water may have a tendency to fall from the centre to the sides. The side channels, and all the road drains, should be repaired at the approach and at the end of the winter, and daily attention given to their being free from obstruction. If roads, by a proper system of drainage, be kept dry, they will be maintained in a good state, and at proportionally less expense.