WALLS iN GENERAL:-A wall should be able to resist a given force, acting upon it either with uniform pres sure over the whole surface, or pressing partially upon certain portions. It should be capable of sustaining the pressure of vaults or roof acting along a continued abut ment; or upon points, as groin vaulting ; also the power of the wind acting uniformly upon the whole surface ; and all these are to be ascertained by calculations, which the properly qualified architect will be capable of mak ing. He may also be assisted by referring to the theory of arches in the article BRIDGE.
There are many sorts of walls: We shall first insert the substance of what has been said on the subject by Vitruvius and Palladio, and then explain the modes prac tised in the present clay.
The wall which Vitruvius names uncertain, is one where the stones are laid in their natural shape and dimen sions, so that the size and figure of the courses are vari ous, as at A, (see Plate CLXXXIX. Fig. 1.) Perrault calls those inserted, when the stones are of a determined size, and placed in a regular order like brick-work. In this sort of work the stones should be longer and shorter alternately, or their position varied, so as to break joint, tie together, and render the whole firm and close. The Greeks made their walls frequently in the manner of brick walls, with a hard stone, having the outer face of a square form, that is, of equal depth and breadth, such as B, Fig. 2; or with stones of iiTegular size, as C, Fig. 3. The third sort were termed involved, as D, Fig. 1, where the stones were even in the front, but laid irregularly. Walls were termed cramped, when the interior was composed of rubble, or broken or pounded cement, and the outsides of squared stones tied together by iron, as at E, Fig. 4 ; or where the squared outside stones were occasionally connected by long stones that went through the whole thickness of the wall, as at F, Fig. 4 ; and sometimes these stones went only about two thirds of the thickness, having a stone overlapping it from the other side in the next course. The term network, or reticulated, was
applied to work, where the stones were square in their outer faces, but laid in courses inclining to the horizon at an angle of 45", as G, Fig. 5. This mode was much practised at Rome, but is not calculated to afford sta bility.
The precepts of Palladio are as follow : Net or reti culated works (A, Fig. 6.) he condemns as being unsta ble ; but if it is to be used, he proposes to construct buttresses at the angles BB, and place transversely or longwise six courses of stones or bricks laid horizon tally at the bottom CC, and in the middle three, as DD, wherever the net or reticulated work is raised six feet.
Brickwork in walls of considerable magnitude, he di rects to be constructed as at EE, Fig. 7, the rubbish lying concealed in the middle as at FF. I. the bottom to have six courses of bricks of a larger siz, than ordi nary, then to build up about three feet in height with the common-sized bricks, and upon these to bind the wall again with three courses of the larger bricks. Spe cimens of this mode are found in the Pantheon and baths of Dioclesian at Rome.
A third sort of walls is composed of rough pebbles out of a river, or taken from a rock, and laid irregularly in cement, as at I, Fig. 8. This should be bound by three courses of bricks at every two feet in height, as K. This kind of work is seen in the walls of Turin in Pied mont.
The fourth sort is termed uncertain, as L, Fig. 9 ; a specimen of which remains at Preneste, twenty miles from Rome.
The fifth mode is composed with squared stones, as at AI, Fig. 10. It is named pseudioodomum, and is to be seen in the temple of Augustus at Rome.
The sixth, as at Sirmion upon the lake of Garda. They are formed by making two sides of stakes and planks, as N, Fig. 11, between which stones and mortar are thrown in promiscuously; when these have set or con solidated, the wooden frames are removed, and the sides of the wall appear as at O. This is termed formaceous.