Instead of making a revetment in the form of a simple wall, it is customary to build buttresses or countcrforts at intervals from each other on the aide next to the supported earth ; consequently the thickness of the wall itself may be rather lees than that which would result from the above equation. In order to determine it, if we assume, for example, that the distance from the centre of one counter fort to that of the next is fifteen feet, the arcs of a horizontal section fifteen feet long and taken at the mean height of the wall, if the face has a slope, together with the area of the like section through the two half-counterforte, may be equal to the area of a section of the simple wall (of the same length) as determined by the above investigation ; then deducting one-fifth of that quantity for the two half-counterforta, the remainder divided by 15 will give the required breadth of a horizontal section at the part between the counterforta. It should be observed, however, that the thickness of a brick revetment which is to resist the fire of a battering-train should not be lees than seven or eight feet. it is usual to make the depth of a counterfort equal to the mean breadth of the wall; and to give it greater thickness at the part which joins the wall than at the other extremity.
Counterforts serve in part as props to keep the wall from inclining in consequence of partial compressions in the cement ; but chiefly by extending the breadth of the base at Intervals they increase, at the places where they are formed, the length of the arm of the lever by which the weight of the wall resists the lateral pressure of the earth.
The usual connection of the bricks or stones in the wall with those of the counterforta allows this advantage to be extended In nearly an equal degree to the parts which are situated between the countssrforts. Bat in order that the connection may be more complete, it has been recommended to connect the tails of every two eounterforts by a wall curved ou the plan, and haring the convex aide towards the earth which is to be Again, the nearest aides of every two counter forts have occasionally been connected by two or more arches, one above another • by which mesons the mass of retained earth is in part Irv/sported, and the lateral pressure of the whole is diminished.
Revetment. in which the counterforte are connected in either of those ways are said to be counterarchal ; and it is recommended that arches of the latter kind should be formed in the mass of the parapet above the cordon of the scarp. It is also recommended that the masonry of the arches In the rampart should be but slightly connected with that of the revetment wall ; since the greater part of the rampart and parapet will then remain supported by the arches even after the revetment has been demolished by the artillery of the besiegers. When the escarp revetment is carried completely up to the superior slope of the parapet, leaving no exterior slope, it is termed a fa rem-twill. When there is an exterior elope left It is termed a desi-terefaste.