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receive a polish or not. Some marbles are easily wrought,some are very hard, and other kinds resist the tools alto-gether.
Mortar is another principal material used in cementing thestones of a building. The reader who wishes to obtain a fullknowledge in this department of masonry, may consult thearticles Cement , Mortak.
Wherever it is intended to build upon, the ground must betried with an iron crow, or with a rammer: if found to shake,it must be pierced with a borer, such as is used by well-dig-gers ; and if the ground generally prove to be firm, the looseor soft parts, if not very deep, must be excavated until a solidbed appears.
If the ground prove soft in several places to a great depthunder apertures, and firm upon the site of the piers, invertedarches must be turned under the apertures, so that if thefoundation sink, the arches may resist the re-action of theground ; and then the whole wall will sink uniformly, ordescend in one body. And even where the ground is of auniform texture, it is always eligible to turn inverted archesunder apertures, wherever there is a part of a wall carried upfrom the foundation to the sill of that aperture: it is fromneglect in this circumstance, that the sills of windows in theground-stories of buildings are frequently broken ; indeed, itis seldom or never otherwise.
Arches adequate to this purpose should rather be of a para-bolic form than circular, the figure of the parabola beingbetter adapted to preserve an equilibrium than the arc of acircle, which is of uniform curvature. If unfortunately the softparts of the ground prove to be the site of the piers, and,consequently, the hard places under the apertures, piers shouldbe built under the apertures, and arches suspended betweenthe piers, with their concave side towards the trench, asusual.
The use of concrete has, however, made a great change inthe art of preparing foundations, the general practice nowj being to excavate the trenches for the footings of the walls,i and put in a bed of concrete, on which is laid the masonry,j i For more information upon this subject, see the articlesj | Foundation, Concrete.
1 t In walling, the bedding joints have most commonly an hori-zontal position in the face of the work, and this dispositionought always to take place when the top of the wall termi-j nates in an horizontal plane or line. In bridge building, andI in the masonry of fence-walls upon inclined surfaces, the bed-j ding-joints on the face sometimes follow the upper surface ofthe wall or terminating line.
The footings of stone-walls ought to be constructed of largestones, which, if not naturally nearly square, should be reducedby the hammer to that form, and to an equal thickness in thesame course ; for if the beds of the stones in the foundationtaper, the superstructure will be apt to give way, by restingupon mere angles or points with inclined beds instead ofhorizontal. All the vertical joints of any upper courseshould break joint, that is, they should fall upon the solidpart of the stones in the lower course, and not upon thejoints.
When the walls of the superstructure are thin, the stoneswhich compose the foundation may be so disposed, that theirlength may reach across each course, from one side of thewall to the other. In thicker walls, where the difficulty isgreater in procuring stones of sufficient length to reach acrossthe foundation, every second stone in the course may be awhole stone in the breadth, and each interval may consist oftwo stones of equal breadth, that is, placing header andstretcher alternately. But when those stones cannot be hadconveniently, from one side of the wall, lay a header and
stretcher alternately, and from the other side lay anoth^series of stone in the same manner, so that the length ^each header may be two-thirds, and the breadth of e ® ,stretcher one-third of the breadth of the wall, and so t*>the back of each header may come in contact with the b#of an opposite stretcher, and the side of that header com®contact with the side of the header adjoining the said stretch 'In broad foundations, where stones cannot be procured i ® rlength equal to two-thirds of the breadth of the foundati® ’build the work so that the upright joints of any course m 9 lfall on the middle of the length of the stones in the co® rbelow, and so that the backs of each stone in anymay fall upon the solid of a stone or stones incourse below. . j,
The foundation should consist of several courses, of "'h'each superior course should be of less breadth than the i®^
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rior one, say four inches on each side in ordinary cases, a °
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the upper course project four inches on each side of theThe number of courses must be regulated by the weigh*,the wall, and by the size of the stones of which the fo® ntion consists. ,,
A wall, which is built of unhewn stone, is called a rub"wall, whether with or without mortar. Rubble work lS .two kinds, coursed and uncoursed. Coursed rubble is * ,of which the stones are gauged and dressed by the ham® 1
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and thrown into different heaps, each heap containingof the same thickness ; then the masonry is laid in couor horizontal rows, which may be of different thicknes !The uncoursed rubble is that where the stones are laid P rmiscuously in the wall, without any attention to placing t® j sin rows. The only preparation which the stones underg 0 ’ ^that of knocking off the sharp angles with the thick e®®the scabbling hammer. <j
Walls are most commonly built with an ashlar facing; 8backed with brick or rubble work. Brick backings are ® gmon in London , where brick is cheaper; and stone bac* ,,-in the north of England and in Scotland , where stone is P 1 Qttiful. Walls faced with ashlar, and backed with briuncoursed rubble, are liable to become convex on the ° utS ( 6 rfrom the greater number of joints, and from thequantity of mortar placed in each joint, as the shrink 1 ®;;the mortar will be in proportion to the quantity; and^
the facing and backing are of the same kind, and builtequal care, even though both sides were uncoursed ru®^ 9which is the worst of all walling. Where the outside 0 ^wall is an ashlar facing, and the inside coursed rubble; ^courses of the backing should be as high as possible, a®with thin beds of mortar. In Scotland , where stone abo®^ ^and where perhaps as good ashlar facings are construct®any in Great Britain, the backing of their walls most emonly consists of uncoursed rubble, built with very little c jIn the north of England, where the ashlar facings of
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are done with less neatness, they are much more particithe coursing of their backings. Coursed rubble and .backings are favourable for the insertion of bond tim ^but in good masonry, wooden bonds should never be i®tinued lengths, as in case of fire or rot, the wood will p eand the masonry, being reduced by the breadth of j* 16 te J,ber, will be liable to bend at the place where it was i® 9 , e g 0 (When it is necessary to have wall timber for the fasten 1 ®® ^battens for lath and plaster, the pieces of timber ought ebuilt with the fibres of the wood perpendicular to the s® ^of the wall, or otherwise in unconnected short piece 0 ’exceeding nine inches in length.
In an ashlar facing, the stones generally run from t'eight to thirty inches in length, twelve inches in heig® 1 ’