Chap. XXIV.

MASONRY.

1425

Fig. 2408.

other ; this has in modern times been applied to circular walls, ana found inefficient, onaccount of its cost and difficulty of execution, from the effects of unequal settlements in thebuilding.

In the theatre of Marcellus at Rome the stones of the piers are prevented from slidingoff their beds by sinking a portion of one into the other: the bed of each stone is dividedinto four parts by two right lineswhich cross in the centre at rightangles, and abut against each otherin the middle of their faces; thestones are placed upon each other,so that every one is united to twoothers, by means of the projectingparts of the upper stone, which keysinto the sinking or mortise of thetwo lower, to which it is adapted.

Placing , and the Form to hegiven to wrought Stone, for Walls,

Abutments , Piers, §-e-As every

solid body has a tendency to de-scend in a vertical or perpendiculardirection, it is evident that it canonly be perfectly supported on ahorizontal or level plan; thus theform best adapted to wroughtstone, when applied to walls orpiers, is that of a parallele-piped or perpendicular prism, ora solid placed on a horizontalplane, and terminated by verticalsurfaces. The stones being laidon each other in union and levelcourses, the whole effort of the weight will fall on their base, and tend to consolidatethem, so that the pressure of each stone on the other will increase tlieir stability, and if thewhole be well constructed, it will be almost as solid as if in one piece. As it is the effectof weight which unites the stones together, it is evident that the larger they are, thegreater will be their stability, and the more solid their union; but their beds must be welldressed, and brought to a level, that they may have an equal bearing ; for if of considerabledimensions they are more subject to break in parts where they have no bearing. The effectwhich causes the rupture produces also a general derangement of the construction,which renders it defective, certain points bearing a considerable weight, under which theybreak, whilst others arc not in contact; the solidity and perfection of construction inwrought stone, which should be independent of any mortar or cement, consists in thestones being placed immediately over each other, so that they touch throughout theirwhole surface, both in their beds and joints, as was done by the ancients, and it is in theprecision with which irregular constructions in large stones are executed, that their soliditydepends, the joints being so well keyed, that their stability is often greater than that ofsquared stones. It must be observed that in construction of wrought squared stones theperpendicular joints do not add to the stability, whilst in those with irregular stones,being inclined in an opposite direction, the stability is increased by the manner in whichthe stones are keyed one into the other.

Of the Dimensions of Stones. — In many edifices, both ancient and modern, it has beenobserved that the stones used were too thin, viz. that they had not sufficient thickness inproportion to their length, and that in consequence they broke under the weight. Theseaccidents arose from the stones not resting equally throughout the whole surface of theirbeds, cither because these surfaces were not exactly dressed or levelled, or because someunequal settlement took place, which deranged the lower stones; the greater the thicknessgiven to stones relative to their length, the greater is the power of resisting this effect,which it is often very difficult to foresee or prevent. For works which have very greatweights to carry, such as walls and points of support, cubes are the strongest, but theyhave less stability, and do not form sufficient bond; those in which the length is muchgreater than the height have more bond, but less strength to carry the weight; accordingto the experiments made on stone, the length may be fixed at from twice to thrice theirheight, and their width from once to twice, supposing the stone of a moderate hardness.When stones are very hard, more than a foot thick, and wrought on all sides, their lengthmay be from four to five times their height, and their width from two to three times;larger dimensions increase the expense without adding to the utility.

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