The following is a specification for building a retaining wall on part of the Holyhead road, in North Wales (Plate IV. fig. 3.):

"On sloping ground there must be a retaining wall along the upper side of the road eighteen inches wide at top; its foundation to be laid at least four inches below the bottom of the side drains, and is to be carried up, so as to intersect the slope of the bank, when falling at the rate of two horizontal to one perpendicular, and the slope is to be formed in this manner for at least one yard from the back of the wall, swarded turf or stone pavement. The face is to have a curved batter, at the rate of one inch and a half in every foot from the top: the back may be in the shape of a rough dyke wall; but every one of the back stones are to be regularly connected with the body of the wall, and not to depend upon the earth behind them."

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If a retaining wall be built of brick, the thickness at top should be one brick, or nine inches, and it should increase in breadth by onsets of half a brick at every eight courses to the level of the road, below which the thickness for the stepping of the foundation should increase half a brick at every four courses to the bottom. All

walls of this description should batter in a curve line on the face at the rate of one inch in every foot.

BREAST WALLS.

These walls are necessary for supporting earth or other materials when used for forming a road; they should be built in the same way as retaining walls, and should increase, from one foot six inches in breadth at top, at the same rate as has been described for retaining walls.

These walls should have a strong coping of large stones, set on edge in mortar of the best description.

The following is a specification of a breast wall built across a very deep hollow, along an old road in North Wales on the Holyhead road (Plate IV. fig. 4.) :

"Across the hollow there is to be a breast wall built, in good lime and sand mortar, along the foot of the lower slope of the present road, or thirty feet distant from the retaining wall. This breast wall is to be two feet and a half thick at top, and to increase in thickness downwards at the rate of two inches and a half for every foot of depth, by a regular batter on the

outside. There is to be a four-feet parapet wall on the top, two feet thick at the bottom, and eighteen inches at the top."

FENCE WALLS.

These walls may be built without mortar, if the stones are flat bedded.

As their stability depends upon the workmanship, great care should be taken to have the stones properly selected, and laid in a correct and regular manner.

A coping should be made on the top of these walls; it should be of large stones set on edge, and laid in good mortar.

When walls are used for fences on embankments, they should always be built with mortar, or otherwise the shaking of the road will in most cases loosen and throw them down.

The following is a specification for stone fences on the Holyhead road :—

"The stone dykes are to be four feet six inches high above the side drain. They are to be, when placed on breast walls, two feet thick at the bottom, and sixteen inches at the top. And where there is no breast wall below them they are to be two feet six inches thick at the foundation, and one foot six inches at the top."

CROSS DRAINS.

Cross drains should be built of good masonry eighteen inches in the clear. (Plate IV. fig. 5.)

They should be continued under the fences into the ditches on each side of the road. When made of stone masonry the side walls should be sixteen inches thick, faced on both sides, eighteen inches high at the upper end, and twenty-three inches at the lower end. The top of the walls to be level, and the bottom of the drain to have an inclination of one inch in every ten feet. The stones at top on which the covers are to be laid are to project about two inches and a half into the open space on each side, leaving about thirteen inches clear between them; the covers to be stone not less than four inches thick and twenty-seven inches long; they should be neatly jointed and closely laid together, and properly bonded on the side walls; a concave pavement of stones, not less than five inches deep, should be laid between the side walls. The body of the building should be placed so low as to admit of six inches of earth to be laid between the cover of the drain and the bottom course of the road materials, without elevating the surface of the road.

The ends of the cross drain must be secured with a strong pavement, four feet three inches by two feet three inches; the paving-stones below the discharging end should be of large stones, sunk so deep as to secure the whole from being injured by the current of water.

When a cross drain is connected with a watercourse, the upper end should be secured with wing walls, at least five feet in length, and there should be the same walls at the lower end. These wing walls should be covered with two rows of swarded turf, the lower one with the swarded side down, and the upper one with the swarded side up.

The following is a specification of a cross drain, five feet diameter, built on the Holyhead road:

"The arch to be hammer-dressed coursed work, and the rest of good sound rubble-work. It is to be in length the full breadth of the road and dikes. The faces to range with the faces of the breast walls, and the dikes to be continued over them. The breast walls, for ten feet from the face of each abutment, are to be built with mortar, and to finish by a pilaster projecting four inches, to be three feet wide at the level of the roadway, and increase in breadth downwards, by a batter of three fourths of an