which have been preserved to our time, and in two celebrated modern structures, the Eddy-stone and Bell-rock light-houses in Great Britain. (Fig. 10.)

Fig. 10-Represents the manner of arranging stones of the same course by dove-tail joints and joggling, taken from a horizontal section of the masonry of the Bell-rock light-house.

357. The manner of dressing stone belongs to the stonecutter's art, but the engineer should not be inattentive either to the accuracy with which the dressing is performed, or the means employed to effect it. The tools chiefly used by the workman are the chisel, axe, and hammer for knotting. The usual manner of dressing a surface, is to cut draughts around and across the stone with the chisel, and then to use the chisel, the axe with a serrated edge, or the knotting hammer, to work down the intermediate portions into the same surface with the draughts. In performing this last operation, the chisel and axe should alone be used for soft stones, as the grooves on the surface of the hammer are liable to become choked by a soft material, and the stone may in consequence be materially injured by the repeated blows of the workman. In hard stones this need not be apprehended.

In large blocks which require to be raised by machinery, a hole, of the shape of an inverted truncated wedge, is cut to receive

P

Fig. 11-Represents a perspective
view A of a block of stone with
draughts around the edges of its
faces, and the intermediate space
axed, or knotted, and its tackling
for hoisting: also the common
iron lewis B with its tackling.
a, draughts around edge of block.
b, knotted part between draughts.
c, iron bolts with eyes let into oblique
holes cut in the block.

d and e, chain and rope tackling.
n, n, side pieces of the lewis.
Bo, centre piece of lewis with eye
fastened to n, n by a bolt.

p, iron ring for attaching tackling.

a small iron instrument termed a lewis, (Fig. 11,) to which the rope is attached for suspending the block; or else two holes arc

cut obliquely into the block to receive bolts with eyes for the same purpose.

When a block of cut stone is to be laid, the first point to be attended to, is to examine the dressing, which is done by placing the block on its bed, and seeing that the joints fit close, and the face is in its proper plane. If it be found that the fit is not accurate, the inaccuracies are marked, and the requisite changes made. The bed of the course, on which the block is to be laid, is then thoroughly cleansed from dust, &c., and well moistened, a bed of thin mortar is laid evenly over it, and the block, the lower surface of which is first cleansed and moistened, is laid on the mortar-bed, and well settled by striking it with a wooden mallet. When the block is laid against another of the same course, the joint between them is prepared with mortar in the same manner as the bed.

358. Rubble Stone Masonry. With good mortar, rubble work, when carefully executed, possesses all the strength and durability required in structures of an ordinary character; and it is much less expensive than cut stone.

359. The stone used for this work should be prepared simply by knocking off all the sharp, weak angles of the block; it is then cleansed from dust, &c., and moistened, before placing it on its bed. This bed is prepared by spreading over the top of the lower course an ample quantity of good ordinary-tempered mortar, into which the stone is firmly imbedded. The interstices between the larger masses of stone are filled in, by thrusting small fragments, or chippings of stone, into the mortar. Finally, the whole course may be carefully grouted before another is commenced, in order to fill up any voids left between the full mortar and stone.

360. To connect the parts well together, and to strengthen the weak points, throughs or binders should be used in all the courses; and the angles should be constructed of cut or hammered stone. In heavy walls of rubble masonry, the precaution, moreover, should be observed, to lay the stones on their quarry-bed; that is, to give them the same position, in the mass of masonry, that they had in the quarry; as stone is found to offer more resistance to pressure in a direction perpendicular to the carry-bed, than in any other. The directions of the lamina in stratified stones, show the position of the quarry-bed.

361. Hammered stone, or dressed rubble, is stone roughly fashioned into regular masses with the hammer. The same precautions must be taken in laying this kind of masonry, as in the two preceding.

362. Brick Masonry. With good brick and mortar, this masonry offers great strength and durability, arising from the strong adhesion between the mortar and brick.

363. The bond used in brick work is very various, depending on the character of the structure. The most usual kinds are known as the English and Flemish. The first consists in arranging the courses alternately, entirely as headers or stretchers, the bricks through the course breaking joints. In the second the bricks are laid as headers and stretchers in each course. The first is stated to give a stronger bond than the last, the bricks of which, owing to the difficulty of preventing continuous joints, either in the same or different courses, are liable to separate, causing the face or the back to bulge outward. The Flemish bond presents the finer architectural appearance, and is therefore preferred for the fronts of edifices.

364. Timber and iron have both been used to strengthen the bond of brick masonry. Among the most remarkable examples of their uses are the well, faced in brick, forming an entrance to the Thames Tunnel, the celebrated work of Mr. Brunel, and his experimental arch of brick, a description of which is given in the Civil Engineer and Architect's Journal, No. 6, vol. I. In both these structures Mr. Brunel used pantile laths and hoop iron, in the interior of the horizontal courses, to connect two contiguous courses throughout their length. The efficacy of this method has been farther fully tested by Mr. Brunel, in experiments made on the resistance to a transversal strain of a brick beam bonded with hoop iron, accounts of which, and of experiments of a like kind, are given by Colonel Pasley in his work on Limes, Calcareous Cements, &c.

365. The mortar-bed of brick, may be either of ordinary, or thin-tempered mortar; the last, however, is the best, as it makes closer joints, and, containing more water, does not dry so rapidly as the other. As brick has great avidity for water, it would always be well not only to moisten it before laying it, but to allow it to soak in water several hours before it is used. By taking this precaution, the mortar between the joints will set more firmly than when it imparts its water to the dry brick, which it frequently does so rapidly as to render the mortar pulverulent when it has dried.

FOUNDATIONS.

366. The term foundation is used indifferently either for the lower courses of a structure of masonry, or for the artificial arrangement, of whatever character it may be, on which these courses rest. For more perspicuity, the term bed of the foundation will be used in this work when the latter is designated.

367. The strength and durability of structures of masonry depend essentially upon the bed of the foundation. In arranging this, regard must be had not only to the permanent efforts which

the bed may have to support, but to those of an accidental character. It should, in all cases, be placed so far below the surface of the soil on which it rests, that it will not be liable to be uncovered, or exposed; and its surface should not only be normal to the resultant of the efforts which it sustains, but this resultant should intersect the base of the bed so far within it, that the portion of the soil between this point of intersection and the outward edge of the base shall be broad enough to prevent its yielding from the pressure thrown on it.

368. The first preparatory step to be taken, in determining the kind of bed required, is to ascertain the nature of the subsoil on which the structure is to be raised. This may be done, in ordinary cases, by sinking a pit; but where the subsoil is composed of various strata, and the structure demands extraordinary precaution, borings must be made with the tools usually employed for this purpose.

369. With respect to foundations, soils are usually divided

into three classes:

The 1st class consists of soils which are incompressible, or, at least, so slightly compressible, as not to affect the stability of the heaviest masses laid upon them, and which, at the same time, do not yield in a lateral direction. Solid rock, some tufas, compact stony soils, hard clay which yields only to the pick, or to blasting, belong to this class.

The 2d class consists of soils which are incompressible, but require to be confined laterally, to prevent them from spreading out. Pure gravel and sand belong to this class.

The 3d class consists of all the varieties of compressible soils; under which head may be arranged ordinary clay, the common earths, and marshy soils. Some of this class are found in a more or less compact state, and are compressible only to a certain extent, as most of the varieties of clay and common earth; others are found in an almost fluid state, and yield, with facility, in every direction.

370. To prepare the bed for a foundation on rock, the thickness of the stratum of rock should first be ascertained, if there are any doubts respecting it: and if there is any reason to suppose that the stratum has not sufficient strength to bear the weight of the structure, it should be tested by a trial weight, at least twice as great as the one it will have to bear permanently. The rock is next properly prepared to receive the foundation courses, by levelling its surface, which is effected by breaking down all projecting points, and filling up cavities, either with rubble masonry or with beton, and by carefully removing any portions of the up per stratum which present indications of having been injured by the weather. The surface, prepared in this manner, should, more

over, be perpendicular to the direction of the pressure; if this is vertical, the surface should be horizontal, and so for any other direction of the pressure. Should there, however, be any difficulty in so arranging the surface as to have it normal to the resultant of the pressure, it may receive a position such that one component of the resultant shall be perpendicular to it, and the other parallel; the latter being counteracted by the friction and adhesion between the base of the bed and the surface of the rock. If, owing to a great declivity of the surface, the whole cannot be brought to the same level, the rock must be broken into steps, in order that the bottom courses of the foundation throughout, may rest on a surface perpendicular to the direction of the pressure. If fissures or cavities are met with, of so great an extent as to render the filling them with masonry too expensive, an arch must then be formed, resting on the two sides of the fissure, to support that part of the structure above it.

The slaty rocks require most care in preparing them to receive a foundation, as their top stratum will generally be found injured to a greater or less depth by the action of frost.

371. In stony earths and hard clay, the bed is prepared by digging a trench wide enough to receive the foundation, and deep enough to reach the compact soil which has not been injured by the action of frost: a trench from 4 to 6 feet, will generally be deep enough for this purpose.

372. In compact gravel, and sand, where there is no liability to lateral yielding, either from the action of rain or any other cause, the bed may be prepared as in the case of stony earths. If there is danger from lateral yielding, the part on which the foundation is to rest must be secured by confining it laterally by means of sheeting piles, or in any other way that will offer sufficient security.

373. In laying foundations on firm sand, a further precaution is sometimes resorted to, of placing a platform on the bottom of the trench, for the purpose of distributing the whole weight more uniformly over it. This, however, seems to be unnecessary; for if the bottom courses of the masonry are well settled in their bed, there is no good reason to apprehend any unequal settling from the effect of the superincumbent weight: whereas, if the wood of the platform should, by any accident, give way, it would leave a part of the foundation without any support.

When the sand under the bed is liable to injury from springs, they must be cut off, and a platform, or, still better, an area of beton should compose the bed, and this should be confined on all sides between walls of stone, or beton sunk below the bottom of the bed.

374. If, in opening a trench in sand, water is found at a slight