392. The roadway of the bridge should he kept as low as may be consistent with the requirements of the navigation and water-way. The ascent on each side should be as easy as the circumstances will admit of, and in no case should it exceed 1 part in 12.

Smeaton remarks of old Westminster Bridge that the ascent was originally laid out to he 1 part in 20, hut he apprehends it to he at least 1 in 12; and further observes that it is a kind of rule in laying out roads and bridges, " if the ascents do not exceed 3 inches per yard, they are no ways objectionable." *

Holborn Hill, before the erection of the Viaduct, is stated to have had a rise of 1 in 18, and that it was necessary at all times to lock the wheel of a loaded waggon. Ludgate Hill rises only 1 in 36;† and when it is possible to construct a bridge with so gentle a rise it is much more desirable. Telford mentions 1 in 24 as a convenient ascent for a bridge; ‡ Wiebeking § also names a rise of 1 in 24 as that which may be used without inconvenience; but he observes that in timber bridges the settlement is generally about 1 part in 72; that is, if a timber bridge of 144 feet span rise one foot in the middle when first framed, it will settle so as to become nearly horizontal; therefore, when it is intended that the bridge shall have an ascent of 1 in 24 when finished, it must be framed so as to have a rise of 1 in 18: for 1/18= 1/24+1/72

393. The width of a bridge depends wholly on the situation where it is to be erected. It ought to be wide in proportion to the importance of the communication, and according to the population who are likely to use it; but the width should not be greater than necessary, because it increases the expense of erection without adding to the utility.

The width between the parapets of a bridge intended for wheeled carriages may be from 18* to 50 feet, according to the situation. Where the road is at a distance from any principal town, and the traffic is slight, the width of the bridge may be from 18 to 20 feet: in more frequented places, from 20 to 22 feet. Near towns and on great public roads, from 25 to 30 feet, and in or near large cities from 30 to 50 feet.

* Smeaton's ' Reports,' vol. iii., p. 226.

† 'Ency. Brit.,' art. Bridge.

‡ ' Edinburgh Ency.,' art. Bridge.

§ ' Traite' d'une Partie essentielle, etc.,' p. 125.

In private roads and parks they are made from 12 to 20 feet in width, and foot bridges from 5 to 8 feet.

394. The roadways of bridges are formed in various ways; one of the most usual is to use pitcher paving laid upon gravel; sometimes broken stone or gravel is used, and some prefer sheeting or planking with timber.

The planking in small bridges is often laid immediately upon the principal beams, which, in such cases, are placed about 2 feet apart, as in Fig. 118, but it is better as regards durability to lay cross joisting for supporting the planking; these joists should also be about 2 feet apart, and the planking laid upon them, which may be from 3 to 4 inches thick. The cross joists admit the air to circulate more freely round the principal timbers, which renders them less likely to decay.

Where bridges are intended for wheel carriages there should be a separate footpath, which may be paved with flagstones. Footpaths are made from 3 feet to 6 feet wide according to the number of the passengers. The carriageway may be paved upon a bed of gravel of about 12 inches in depth. The paving to rise in a curve across the road. The gravel should contain a considerable portion of tempered clay, so as to bind it firmly together; but if there be too much clay, it will shrink and crack in drying. Belidor states, that paved bridges are the most durable.

* Smeaton says that it is found by experience that 18 feet clear width admits of carriages passing with ease, freedom, and safety (' Reports,' vol. iii., p. 51).

If the roadway be gravelled only, the gravel should be from 12 to 18 inches deep in the middle and from 9 to 14 inches deep at the sides, according to the traffic over the bridge. Whether the roadway be paved or gravelled, the means of conveying off the water should be provided.

As the moisture which passes through the gravel soon rots the planking, it is supposed to be better to lay an additional thickness of planking, and no gravel or paving. In that case the upper planking should lay across the bridge, to prevent the feet of horses from slipping. It would be easy to renew such a roadway, but it is not clear that it possesses any other advantage.

Parapets or balustrades are made from 3.5 feet to 6 feet in height above the footpath; 4 feet is enough for protection. The railing is stayed by braces on the outside. Iron-railing is sometimes used.

The railing should, however, partake of the character of the surrounding scenery. In towns, upright or diagonal bars and other ornamental railing may be used; but in all cases where trees and cottages form the most striking features of the surrounding landscape, simple horizontal rails with posts are preferable. Nothing is more formal and stiff than ornamental or upright railing, and nothing more picturesque than the simple continuous lines of horizontal rails.

Wooden bridges are often covered with a roof. The bridges of Schaffhausen, Wettingen, Kandel, Mellingen, and various others were roofed. The roof does not appear to be of much service in protecting the bridge, and in later bridges it has been generally omitted. The planking of the roadway might be protected very much by a coat of pitch, tar, and sand. A composition of this kind was used for covering the planking under the roadway of the iron bridge over the Wear, at Sunderland.