MASONRY.

343. MASONRY is the art of raising structures, in stone, brick, and mortar.

344. Masonry is classified either from the nature of the material, as stone masonry, brick masonry, and mixed, or that which is composed of stone and brick; or from the manner in which the material is prepared, as cut stone or ashlar masonry, rubble stone or rough masonry, and hammered stone masonry; or, finally, from the form of the material, as regular masonry, and irregular masonry.

345. Cut Stone. Masonry of cut stone, when carefully made, is stronger and more solid than that of any other class; but, owing to the labor required in dressing, or preparing the stone, it is also the most expensive. It is, therefore, mostly restricted to those works where a certain architectural effect is to be produced by the regularity of the masses, or where great strength is indispensable.

346. Before explaining the means to be used to obtain the greatest strength in cut stone, it will be necessary to give a few definitions to render the subject clearer.

In a wall of masonry, the term face is usually applied to the front of the wall, and the term back to the inside; the stone which forms the front, is termed the facing; that of the back, the backing; and the interior, the filling. If the front, or back of the wall, has a uniform slope from the top to the bottom, this slope is termed the batter, or bâtir.

The term course is applied to each horizontal layer of stone in the wall if the stones of each layer are of equal thickness throughout, it is termed regular coursing; if the thicknesses are unequal, the term random, or irregular coursing, is applied. The divisions between the stones, in the courses, are termed the joints; the upper surface of the stones of each course is also, sometimes, termed the bed, or build.

The arrangement of the different stones of each course, or of contiguous courses, is termed the bond.

347. The strength of a mass of cut stone masonry will depend on the size of the blocks in each course; on the accuracy of the dressing; and on the bond used.

348. The size of the blocks varies with the kind of stone, and the nature of the quarry. From some quarries the stone may be obtained of any required dimensions; others, owing to some peculiarity in the formation of the stone, only furnish blocks of small

size.

Again, the strength of some stones is so great as to admit of their being used in blocks of any size, without danger to the stability of the structure, arising from their breaking; others can only be used with safety, when the length, breadth, and thickness of the block bear certain relations to each other. No fixed rule can be laid down on this point: that usually followed by builders, is to make, with ordinary stone, the breadth at least equal to the thickness, and seldom greater than twice this dimension, and to limit the length to within three times the thickness. When the breadth or the length is considerable, in comparison with the thickness, there is danger that the block may break, if any unequal settling, or unequal pressure should take place. As to the absolute dimensions, the thickness is generally not less than one foot, nor greater than two; stones of this thickness, with the relative dimensions just laid down, will weigh from 1000 to 8000 pounds, allowing, on an average, 160 pounds to the cubic foot. With these dimensions, therefore, the weight of each block will require a very considerable power, both of machinery and men, to set it on its bed.

349. For the coping and top courses of a wall, the same objections do not apply to excess in length: but this excess may, on the contrary, prove favorable; because the number of top joints being thus diminished, the mass beneath the coping will be better protected, being exposed only at the joints, which cannot be made water-tight, owing to the mortar being crushed by the expansion of the blocks in warm weather, and, when they contract, being washed out by the rain.

350. The closeness with which the blocks fit is solely dependent on the accuracy with which the surfaces in contact, are wrought or dressed; if this part of the work is done in a slovenly manner, the mass will not only present open joints from any inequality in the settling; but, from the courses not fitting accurately on their beds, the blocks will be liable to crack from the unequal pressure on the different points of the block.

351. The surfaces of one set of joints should, as a prime condition, be perpendicular to the direction of the pressure by this arrangement, there will be no tendency in any of the blocks to slip. In a vertical wall, for example, the pressure being downward, the surfaces of one set of joints, which are the beds, must be horizontal. The surfaces of the other set must be perpendicular to these, and, at the same time, perpendicular to the face, or to the back of the wall, according to the position of the stones in the mass; two essential points will thus be attained; the angles of the blocks, at the top and bottom of the course, and at the face or back, will be right angles, and the block will therefore be as strong as the nature of the stone will admit. The principles

here applied to a vertical wall, are applicable in all cases, whatever may be the direction of the pressure and the form of the exterior surfaces, whether plane or curved.

352. A modification of this principle, however, may in some. cases be requisite, arising from the strength of the stone. It is laid down as a rule, drawn from the experience of builders, that no stone work with angles less than 60° will offer sufficient strength and durability to resist accidents, and the effects of the weather. If, therefore, the batter of a wall should be greater than 60°, which is about 7 perpendicular to 4 base, the horizontal joints (Fig. 6) must not be carried out in the same plane, to the

B

Fig. 6-Represents the arrangement of stone with abutting, or elbow joints for very inclined surfaces.

A, face of the block.

c, elbow joint.

B, buttress block, termed a newell stone.

face or back, but be broken off at right angles to it, so as to form a small abutting joint of about 4 inches in thickness. the batter of walls is seldom so great as this, except in some cases of sustaining walls for the side slopes of earthen embankments, this modification in the joints will not often occur; for, in a greater batter, it will generally be more economical, and the construction will be stronger, to place the stones of the exterior in offsets, the exterior stone of one course, being placed within the exterior one of the course below it, so as to give the required. general direction of the batter. The arrangement with offsets has the farther advantage in its favor of not allowing the rain water to lodge in the joint, if the offset be slightly bevelled off.

353. Workmen, unless narrowly watched, seldom take the pains necessary to dress the beds and joints accurately; on the contrary, to obtain what are termed close joints, they dress the ioints

with accuracy a few inches only from the outward surface, and then chip away the stone towards the back, or tail, (Fig. 7,) so

that, when the block is set, it will be in contact with the adjacent stones, only throughout this very small extent of bearing surface. This practice is objectionable under every point of view; for, in the first place, it gives an extent of bearing surface, which, being generally inadequate to resist the pressure thrown on it, causes the block to splinter off at the joint; and in the second place, to give the block its proper set, it has to be propped beneath by small bits of stone, or wooden wedges, an operation termed pinning-up, or under-pinning, and these props, causing the pressure on the block to be thrown on a few points of the lower surface, instead of being equally diffused over it, expose the stone to crack.

354. When the facing is of cut stone, and the backing of rubble, the method of thinning off the block may be allowed for the purpose of forming a better bond between the rubble and ashlar; but, even in this case, the block should be dressed true on each joint, to at least one foot back from the face. If there exists any cause, which would give a tendency to an outward thrust from the back, then, instead of thinning off all the blocks towards the tail, it will be preferable to leave the tails of some thicker than the parts which are dressed.

355. Various methods are used by builders for the bond of cut stone. The system, termed headers and stretchers, in which the vertical joints of the blocks of each course alternate with the vertical joints of the courses above and below it, or as it is termed break joints with them, is the most simple, and offers, in most cases, all requisite solidity. In this system, (Fig. 8,) the blocks of each course are laid alternately with their greatest and least dimensions to the face of the wall; those which present the longest dimen

sion along the face, are termed stretchers; the others, headers If the header reaches from the face to the back of the wall, it is

termed a through; if it only reaches part of the distance, it is termed a binder. The vertical joints of one course are either just over the middle of the blocks of the next course below, or else, at least four inches on one side or the other of the vertical joints of that course; and the headers of one course rest as nearly as practicable on the middle of the stretchers of the course beneath. If the backing is of rubble, and the facing of cut stone, a system of throughs or binders, similar to what has just been explained, must be used..

By the arrangement here described, the facing and backing of each course are well connected; and, if any unequal settling takes place, the vertical joints cannot open, as would be the case were they in a continued line from the top to the bottom of the mass; as each block of one course confines the ends of the two blocks on which it rests in the course beneath.

356. In masses of cut stone exposed to violent shocks, as those of which light-houses, and sea-walls in very exposed positions are formed, the blocks of each course require to be not only very firmly united with each other, but also with the courses above and below them. To effect this, various means have been used. The beds of one course are sometimes arranged with projections (Fig. 9,) which fit into corresponding indentations of the next course. Iron cramps in the form of the letter S, or in any other

shape that will answer the purpose of giving them a firm hold on the blocks, are let into the top of two blocks of the same course at a vertical joint, and are firmly set with melted lead, or with bolts, so as to confine the two blocks together. Holes are, in some cases, drilled through several courses, and the blocks of these courses are connected by strong iron bolts fitted to the holes.

The most noted examples of these methods of strengthening the bond of cut stone, are to be found in the works of the Romans