42 soft sand-stone, called by the French molasse, which is strongly impregnated with the tar. In this process, the tar is disengaged and rises to the surface of the water, or adheres to the sides of the vessel, and the earthy matter remains at the bottom. analysis of a rich specimen of the Seyssel bituminous sand-stone gave the following results : An Bituminous oil Quartzy grains .690 .204 1.000 145. The bituminous lime-stone which, when reduced to a powdered state, is mixed with the mineral tar, is known at the localities mentioned by the name of asphaltum, an appellation which is now usually given to the mastic. This lime-stone occurs in the secondary formations, and is found to contain various proportions of bitumen, varying mostly from 3 to 15 per cent., with the other ordinary minerals, as argile, &c., which are met with in this formation. 146. The bituminous mastic is prepared from these two materials by heating the mineral tar in cast-iron or sheet-iron boilers, and stirring in the proper proportion of the powdered limestone. This operation, although very simple in its kind, requires great attention and skill on the part of the workmen in managing the fire, as the mastic may be injured by too low, or too high a degree of heat. The best plan appears to be, to apply a brisk fire until the boiling liquid commences to give out a thin whitish vapor. The fire is then moderated and kept at a uniform state, and the powdered stone is gradually added, and mixed in with the tar by stirring the two well together. When the temperature has been raised too high, the heated mass gives out a yellowish or brownish vapor. In this state it should be stirred rapidly, and be removed at once from the fire. 147. The asphaltic stone may be reduced to powder, either by roasting it in vessels over a fire, or by grinding it down in the ordinary mortar-mill. For roasting, the stone is first reduced to fragments the size of an egg. These fragments are put into an ion vessel; heat is applied, and the stone is reduced to powder by stirring it and breaking it up with an iron instrument. This process is not only less economical than grinding, but the material loses a portion of its tar from evaporation, besides being liable to injury from too great a degree of heat. For grinding, the stone is first broken as for roasting. Care should be taken, during the process, to stir the mass frequently, otherwise it may form into a cake. Cold dry weather is the best season for this operation; the stone, however, should not be exposed to the weather. 148. Owing to the variable quantity of mineral tar in bituminous lime-stone, the best proportions of the tar and powdered stone for bituminous mastic, cannot be assigned beforehand. Three or four per cent. too much of tar, is said to impair both the durability and tenacity of the mastic; while too small a quantity is equally prejudicial. Generally, from eight to ten per cent. of the tar, by weight, has been found to yield a favorable result. 149. Mastics have been formed by mixing vegetable tar, pitch, and other resinous substances, with litharge, powdered brick, powdered lime-stone, &c.; but the results obtained have generally been inferior to those from bituminous mastic. 150. Mineral tar is more durable than vegetable tar, and on this account it has been used alone as a coating for other materials, but not with the same success as mastic. Employed in this way, the tar in time becomes dry and peels off; whereas, in the form of mastic, the hard matter with which it is mixed prevents the evaporation of the oily portion of the tar, and thus promotes its durability. 151. The uses to which bituminous mastic is applied are daily increasing. It has been used for paving in a variety of forms, either as a cement for large blocks of stone, or as the matrix of a concrete formed of small fragments of stone or gravel; as a pointing, it is found to be more serviceable, for some purposes, than hydraulic cement; it forms one of the best water-tight coatings for cisterns, cellars, the cappings of arches, terraces, and other similar roofings now in use; and is a good preservative agent for wood work exposed to wet or damp. GLUE. 152. The common animal glue is seldom used as a cement for any other purpose than for the work of the joiner. Although of considerable tenacity, it is weak, brittle, and readily impaired by moisture. 153. Within a few years back, a material termed marine glue, the invention of Mr. Jeffery of England, has attracted attention in England and France, in both which countries its qualities as a cement, both for stone and wood, have been tested with the most satisfactory results. This composition is said to be made by first dissolving caoutchouc in coal naphtha, in the proportion of one pound of the former to five gallons of the latter; to this solution an equal weight of shellac is added, and the composition is then placed over a fire and thoroughly mixed by stirring. 154. Owing to its insolubility in water, its remarkable tenacity and adhesion, and its powers of contraction and expansion through a very considerable range of temperature, without becoming either very soft or brittle, the marine glue promises to be not only a valuable addition to the resources of the naval architect, but to the civil engineer. BRICK. 155. This material is properly an artificial stone, formed by submitting common clay, which has undergone suitable preparation, to a temperature sufficient to convert it into a semi-vitrified state. 156. Brick may be used for nearly all the purposes to which stone is applicable; for when carefully made, its strength, hardness, and durability, are but little inferior to the more ordinary kinds of building stone. It remains unchanged under the extremes of temperature; resists the action of water; sets firmly and promptly with mortar; and being both cheaper and lighter than stone, is preferable to it for many kinds of structures, as arches, the walls of houses, &c. 157. The art of brick-making is a distinct branch of the useful arts, and does not properly belong to that of the engineer. But as the engineer is frequently obliged to prepare this material him self, the following outline of the process may prove of service. 158. The best brick earth is composed of a mixture of pure clay and sand, deprived of pebbles of every kind, but particularly of those which contain lime, and pyritous, or other metallic substances; as these substances, when in large quantities, and in the form of pebbles, act as fluxes, and destroy the shape of the brick, and weaken it by causing cavities and cracks; but in small quantities, and equally diffused throughout the earth, they assist the vitrification, and give it a more uniform character. 159. Good brick earth is frequently found in a natural state, and requires no other preparation for the purposes of the brickmaker. When he is obliged to prepare the earth by mixing the pure clay and sand, direct experiments should, in all cases, be made, to ascertain the proper proportions of the two. If the clay is in excess, the temperature required to semi-vitrify it, will cause it to warp, shrink, and crack; and, if there is an excess of sand, complete vitrification will ensue, under similar circumstances. 160. The quality of the brick depends as much on the care bestowed on its manufacture, as on the quality of the earth. The first stage of the process is to free the earth from pebbles, which is most effectually done by digging it out early in the autumn, and exposing it in small heaps to the weather during the winter. In the spring, the heaps are carefully riddled, if necessary, and the earth is then in a proper state to be kneaded or tempered. The quantity of water required in tempering, will depend on the quality of the earth; no more should be used, than will be sufficient to make the earth so plastic, as to admit of its being easily moulded by the workman. About half a cubic foot of water to one of the earth is, in most cases, a good proportion. If too much water be used, the brick will not only be very slow in drying, but it will, in most cases, crack, owing to the surface becoming completely dry, before the moisture of the interior has had time to escape; the consequence of which will be, that the brick, when burnt, will be either entirely unfit for use, or very weak. 161. Machinery is now coming into very general use in moulding brick it is superior to manual labor, not only from the labor saved, but from its yielding a better quality of brick, by giving it great density, which adds to its strength. 162. Great attention is requisite in drying the brick before it is burned. It should be placed, for this purpose, in a dry exposure, and be sheltered from the direct action of the wind and sun, in order that the moisture may be carried off slowly and uniformly from the entire surface. When this precaution is not taken, the brick will generally crack from the unequal shrinking, arising from one part drying more rapidly than the rest. 163. The burning and cooling should be done with equal care. A very moderate fire should be applied under the arches of the kiln for about twenty-four hours, to expel any remaining moisture from the raw brick; this is known to be completely effected, when the smoke from the kiln is no longer black. The fire is then increased until the bricks of the arches attain a white heat; it is then allowed to abate in some degree, in order to prevent complete vitrification; and it is alternately raised and lowered in this way, until the burning is complete, which may be ascertained by examining the bricks at the top of the kiln. The cooling should be slowly effected; otherwise the bricks will not withstand the effects of the weather. It is done by closing the mouths of the arches, and the top and sides of the kiln in the most effectual manner with moist clay and burnt brick, and allowing the kiln to remain in this state until the warmth has subsided. 164. Brick of a good quality exhibits a fine, compact, uniform texture, when broken across; gives a clear ringing sound, when struck; and is of a cherry red, or brownish color. Three varieties are found in the kiln; those which form the arches, denominated arch brick, are always vitrified in part, and present a grayish glassy appearance at one end; they are very hard, but brittle, of inferior strength, and set badly with mortar; those from the interior of the kiln, usually denominated body, hard, or cherry brick, are of the best quality; those from near the top and sides, are generally underburnt, and are denominated soft, pale, or sammel brick; they have neither sufficient strength, nor durability, for heavy masonry, nor the outside courses of walls, which are exposed to the weather. 165. The quality of good brick may be improved by soaking it for some days in water, and re-burning it. This process increases both the strength and durability, and renders the brick more suitable for hydraulic constructions, as it is found not to imbibe water so readily after having undergone it. 166. The size and form of bricks present but trifling variations. They are generally rectangular parallelopipeds, from eight to nine inches long, from four to four and a half wide, and from two to two and a quarter thick. Thin brick is generally of a better quality than thick, because it can be dried and burned more uniformly. 167. Fire-brick. This material is used for the facing of furnaces, fireplaces, &c., where a high degree of temperature is to be sustained. It is made of a very refractory kind of pure clay, that remains unchanged by a degree of heat which would vitrify and completely destroy ordinary brick. A very remarkable brick of this character has been made of agaric mineral; it remains unchanged under the highest temperature, is one of the worst conductors of heat, and so light that it will float on water. 168. Tiles. As a roof covering, tiles are, in many respects, superior to slate, or metallic coverings. They are strong and durable, and are very suitable for the covering of arches, as their great weight is not so objectionable here, as in the case of roofs formed of frames of timber. Tiles should be made of the best potter's clay, and be moulded with great care to give them the greatest density and strength. They are of very variable form and size; the worst being the flat square form, as, from the liability of the clay to warp in burning, they do not make a perfectly water-tight covering. WOOD. 169. This material holds the next rank to stone, owing to its durability and strength, and the very general use made of it in constructions. To suit it to the purposes of the engineer, the tree is felled after having attained its mature growth, and the trunk, the larger branches that spring from the trunk, and the main parts of the root, are cut into suitable dimensions, and seasoned, in which state, the term timber is applied to it. The crooked, or compass timber of the branches and roots, is mostly applied to the purposes of ship-building, for the knees and other |