tion of clay or mud, or is brought from the sea-shore and contains saline particles, it must be washed in a stream of clear water till it be divested of its impurities. The necessity of the first has been clearly proved by Mr. Smeaton, who, in the course of a long and meritorious attention to his profession of an engineer, has found, that when mortar, though otherwise of the best quality, is mixed with a small proportion of unburnt clay, it never acquires that hardness which, without it, it would have attained; and, with respect to the second, it is evident, that so long as the sand contains saline particles it cannot become hard and dry. The sharper and coarser the sand is the better for the mortar, and the less the quantity of lime to be used; and sand being the cheapest of the ingredients which compose the mortar, it is more profitable to the maker. The exact proportions of lime and sand are still undetermined; but in general no more lime is required than is just sufficient to surround the particles of the sand, or sufficient to preserve the necessary degree of plasticity. Mortar, in which sand forms the greater portion requires less water in its preparation, and consequently is sooner set. It is also harder and less liable to shrink in drying, because the lime, while drying, has a greater tendency to shrink than sand, which retains its original magnitude. The general proportions given by the London builders is 1 cwt., or 37 bushels, of lime to 2 loads of sand; but if proper measures be taken to procure the best burnt lime and the best sand, and in tempering the materials, a greater portion of sand may be used. There is scarcely any mortar that has the lime well calcined, and the composition well beaten, but that will be found to-require two parts of sand to one part of unslacked lime; and it is worthy of observation, that the more the mortar is beaten the less proportion of lime suffices. Many experiments have been made with a view to obtain the most useful proportion of the ingredients, and among the rest Dr. Higgins has given the following: Lime newly slacked one part, He also found that one-fourth of the lime of bone-ashes greatly improved the mortar, by giving it tenacity, and rendering it less liable to crack in the drying. It is best to slack the lime in small quantities as required for use, about a bushel at a time, in order to secure to the mortar such of its qualities as would evaporate were it allowed to remain slacked for a length of time. But if the mortar be slacked for any considerable time previous to being used, it should be kept covered up, and when wanted be re-beaten. If care be taken to secure it from the action of the atmosphere, it may thus remain covered up for a considerable period without its strength being in the least affected; and, indeed, some advantages are gained, for it sets sooner, is less liable to crack in the drying, and is harder when dry. Grout, which is a cement containing a larger proportion of water than the common mortar, is used to run into the narrow interstices and irregular courses of rubble-stone walls; and as it is required to concrete in the course of a day, it is composed of mortar that has been a long time made and thoroughly beaten. Mortar, composed of pure lime, sand, and water, may be employed in the linings of reservoirs and aqueducts, provided a sufficient time is allowed for it to dry before the water is let in; but if a sufficient time is not allowed, and the water is admitted while the mortar is wet, it will soon fall to pieces. There are, however, certain ingredients which may be put into the common mortar to make it set immediately under the water; or, if the quicklime composing the mortar contain in itself a certain portion of burnt clay, it will possess this property. For further information on this head the reader is referred to the sub-head-Plastering. BRICKS. The earth best adapted for the manufacture of brick is of a clayey loam, neither containing too much argillaceous mat ter, which causes it to shrink in the drying, nor too much sand, which has a tendency to render the ware both heavy and brittle. It should be dug two or three years before it is wrought, that it may, by an exposure to the action of the atmosphere, lose the extraneous matter of which it is possessed when first drawn from its bed; or, at least, should be allowed to remain one winter, that the frost may mellow and pulverize it sufficiently to facilitate the operation of tempering. As the quality of the brick is greatly dependent upon the tempering of the clay, great care should be taken to have this part of the process well done. Formerly the manner of performing it consisted in throwing the clay into shallow pits, and subjecting it to the tread of men and oxen; but this method has of late been superseded by the clay or pug mill, which is a very eligible, though simple machine. The clay or pug mill consists of a large vertical cone, having strong knives with a spiral arrangement and inclina- ( tion fixed on its internal surface. Passing through the centre, and terminating in a pivot at the bottom, is a strong perpendicular shaft with similar radiating knives, so that the knives by the revolution of the shaft, cut, separate, and purify the clay, till it be reduced to a homogeneous paste, which passes through an orifice at the bottom into a receiver placed for that purpose. The clay is taken from the receiver to the moulder's bench, and is, either by a lad or a woman, cut into pieces somewhat larger than the mould, and passed on to the moulder, who works it into a mould, previously dipped in sand, and strikes off the superfluous parts with a flat smooth piece of wood. In this country the mould used is about ten inches in length, and five inches in breadth, and the bricks when burnt are about nine inches long, four and a half inches broad, and two and a half inches thick. The degree of shringing, however, is various, according to the temper and purity of the clay, and the degree of heat attained in the burning. A handy moulder is calculated to mould from about 5000 to 7000 per day. From the moulder's bench the bricks are carried to the hack, and arranged somewhat diagonally, one above the other, and two edgewise across, with a passage between the heads of each for the admission of air, till they be eight bricks in height. They are then left to dry. The time they take ere they rerequire shifting depends entirely upon. the weather, which when fine will be but a few days: they are then turned and re-set wider apart, and in six or eight days are ready for the clamp or kiln. Clamps are generally used in the vicinity of London. They are made of the bricks to be burnt, and are commonly of an oblong form. The foundation is made either with the driest of the bricks just made, or with the commonest kind of brick, called place bricks. The bricks to be burnt are arranged tier upon tier as high as the clamp is intended to be, and a stratum of breeze or cinders to the depth of two or three inches is strewed between each layer of bricks, and the whole is finally covered with a thick stratum of breeze. At the west end of the clamp a perpendicular fire-place of about three feet in height is constructed, and flues are formed by arching the bricks over so as to leave a space of about a brick in width. The flues run straight through the clamp, and are filled with a mixture of coals, wood, and breeze, which are pressed closely together. If the bricks are required to be burnt off quickly, which can be accomplished in the space of from twenty to thirty days according to the state of the weather, the flues must not exceed six feet distance apart; but if there is no urgent demand, the flues need not be nearer than nine feet, and the clamp may be allowed to burn slowly. Coke has been recommended as a more suitable fuel for bricks than either coal or wood, as the dimensions of the flues and the stratum of the fuel are not required to be so great, which, since the measurement of the clamp has been restricted to certain limits by the interference of the legislature, is a point of some consideration; besides, the heat arising from the coke is more uniform and more intense than what is produced by the other materials, so that the burning of the bricks is more likely to be perfect throughout. The saving which is thus produced may be calculated at about 32 per cent. Kilns are also in common use, and are in many respects preferable to the clamp, as less waste arises, less fuel is consumed, and the bricks are sooner burnt. A kiln will burn about 20,000 bricks at a time. The walls of a kiln are about a brick and a half thick, and incline inwards towards the top, so that the area of the upper part is not more than 114 square feet. The bricks are set on flat arches, with holes left between them, resembling lattice-work; and, when the kiln is completed, are they covered with pieces of broken bricks and tiles, and some wood is kindled and put in to dry them gradually. When sufficiently dried, which is known by the smoke changing from a dark to a light transparent colour, the mouths of the kiln are stopped with pieces of brick, called shinlog, piled one upon another, and closed over with wet brick-earth. The shinlogs are carried so high as just to leave room for one faggot to be thrust into the kiln at a time, and when the brush-wood, furze, heath, faggots, &c. are put in, the fire is kindled, and the burning of the kiln commences. The fire is kept up till the arches assume a white appearance, and the flames appear through the top of the kiln; upon which the fire is allowed to slacken, and the kiln to cool by degrees. This process of alternately heating and slacking the kiln is continued till the bricks are thoroughly burnt, which, in general, is in the space of forty-eight hours. The practice of steeping bricks in water after they have been burned, and then burning them again, has the effect of considerably improving the quality. Bricks are of several kinds, the most usual of which are marls, stocks, and place bricks; but there is little difference in the mode of manufacturing them, except that great care is taken in preparing and tempering the marls. The finest marls, called firsts, are selected for the arches of doorways, &c. and are rubbed to their proper form and dimensions and the next best, called seconds, for the principal fronts. The colour, a light yellow, added to the smooth texture, and superior durability of the marls, give them the precedence of the other descriptions of brick. Grey stocks are somewhat like the seconds, but of inferior quality. Place bricks, sometimes called pickings, sandal, or samel bricks, are such as from being the outermost in the clamp or kiln, have not been thoroughly burned, and are, in consequence, soft, of uneven texture, and of a red colour. There are also burrs or clinkers, arising from the bricks being too violently burned, and sometimes several bricks are found run together in the kiln. They derive their colour from the nature of the soil of which they are composed, which, in general, is very pure. The best kind are used as cutting bricks, and are called red rubbers. In old buildings they are very frequently to be seen ground to a fine smooth surface, and set in putty instead of mortar, as ornaments over arches, windows, door-ways, &c. ; but though there are many beautiful specimens of red brick-work, yet these bricks cannot be judiciously used for the front walls of buildings. This objection arises from the colour being too heavy, and from its conveying to the mind, in the summer months, an unpleasant idea of heat; to which may be added, that as the fronts of the buildings have a greater or less proportion of stone and painted wood-work, the contrast in the colours is altogether injudicious. The colour of grey stocks, on the contrary, assimilates so much with the stones and paint, that they have obtained, in and near London, universal preference. At the village of Hedgerley, near Windsor, red bricks are made which will stand the greatest heat: they are called Windsor bricks. Bricks used for paving, are generally about an inch and a half in breadth; and, beside these, there are paving tiles, |