OF PARAPETS.
The whole work having been brought up to the level of the cordon, or cornice, and that having also been set, the parapets are to be constructed. They are made from three feet six inches to six feet in height above the footpaths or roadway : four feet four inches is quite sufficient for protection and decoration, and is not so high as to obstruct the view. Parapets of the best fin ished bridges, consist of a plinth, dado, and coping. In their large bridges, the French make the thickness of the parapet two feet ; in Britain, the dado or middle member is made only from 10 to 12 inches in thickness, and the plinth so much more as to leave an offset of about an inch on each side. if the plinth has mouldings on the upper edges, the thickness is made somewhat more. The coping is made from six to nine inches in thickness, and has projections on each side. The top is most generally made to slope each way from the mid dle, sometimes in straight lines, and sometimes circu lar; and there are instances of the slope being made in one inclination, from the inside to the outside. Some times the edges are plain, and often moulded : when they are plain, a cavetto or small hollow is cut in the projecting part of the bed, to prevent the water, which falls on the top, from running down the face of the dado. In or near to large cities and large towns, or near to the dwellings of the wealthy, instead of the dado being all made solid, hallusters are introduced ; and these oc cupy a larger or smaller space, according to the fancy of the designer. Sometimes there are half ballusters on the outside for appearance, the inside being solid. There are situations which require this. The north bridge in Edinburgh being exposed to violent gusts of wind, the open ballusters were found inconvenient, and the spaces between were closed along the inside.
All the stones for the parapets should be of the best quality the neighbourhood affords : they should be work ed and set very correctly. The ballusters are frequently
turned in a lathe, and have spaces cut in the plinth and coping to receive their top and bottom ends: the coping must be secured in the end joints, by dovetails, cramps, or cast iron dowells. The latter mode is the best : the dowells are four inches in length, and about one inch square : they are jet into the middle of the end joints as the stones are set ; the rest of the joints, especially the lower side, is made up very closely with lime mortar, or British cement, and a small perpendicular channel is cut in each stone, which, when joined, communicates from the top to the dowell : down this melted lead is poured, which fills up the space round the dowel!, and also the small channel ; or British cement may be used instead of lead.
The outline of the cornice and parapet should be a curve for the whole length of the bridge, which abutting on each shore, conveys a stronger idea of security, than when the top is a horizontal line ; but the real advantage is the road way being kept constantly clean and dry.
In bridges when the parapets are made solid, and where proper stones can be procured, it is advisable to make each parapet of one row of stones, about three feet six inches in height, and diminishing from 12 or 14 inches at the bottom, to 8 or 9 inches at the top, which is made convex: each joint should be well secured by iron dowells.
Where parapets are made of rubble masonry, from 18 to 24 inches in thickness is required, to admit of two stones in breadth. These should be carefully bonded together, and coped, either with a course of squared stones dowelled together, or otherwise with stones about nine inches in depth set on edge. These parapets should have their top and coping curved clown to the ground at each end ; and be there secured by a stone of considerable size, fixed firmly under the surface.