from the latter, the induration in the two cases would be due to precisely the same chemical action. The materials are indeed identical in composition under this condition, with the exception that there is an excess of water, and consequently an element of weakness, in the English concrete, which does not attach to the béton. The durability of the latter in sea water, without being much discussed, has been very generally conceded.

Monolithic constructions under water cannot be executed in béton aggloméré, for the reason that the prescribed ramming in thin layers would necessarily have to be omitted, and some other mode of compacting the mixture followed. This material, however, when laid green through water, loses its distinct name and character, as well as its superior strength and hardness, and becomes common béton or concrete, with the coarser ballast omitted. Its use in this form certainly offers no advantages with regard to strength, while in point of economy the usual proportions of matrix, sand and shingle, or broken stone, is preferable.

183. Adherence of Mortar. The force with which mortars in general adhere to other materials, depends on the nature of the material, its texture, and the state of the surface to which the mortar is applied.

184. Mortar adheres most strongly to brick; and more feebly to wood than to any other material. Among stones, its adhesion to limestone is generally greatest; and to basalt. and sandstones, least. Among stones of the same class, it adheres generally better to the porous and coarse-grained, than to the compact and fine-grained. Among surfaces, it adheres more strongly to the rough than to the smooth.

185. The adhesion of common mortar to brick and stone, for the first few years, is greater than the cohesion of its own particles. The force with which hydraulic cement adheres to the same materials, is less than that of the cohesion between its own particles; and, from some recent experiments of Colonel Pasley, on this subject, it would seem that hydraulic cement adheres with nearly the same force to polished surfaces of stone as to rough surfaces.

186. From experiments made by Rondelet, on the adhesion of common mortar to stone, it appears that it required a force varying from 15 to 30 pounds on the square inch, applied perpendicular to the plane of the joint, to separate the mortar and stone after six months union; whereas only 5 pounds to the square inch was required to separate the same surfaces, when applied parallel to the plane of the joint.

From experiments made by Colonel Pasley, he concludes that the adhesive force of hydraulic cement to stone, may be taken as high as 125 pounds on the square inch, when the joint has had time to harden throughout; but, he remarks, that as in large joints the exterior part of the joint may have hardened while the interior still remains soft, it is not safe to estimate the adhesive force, in such cases, higher than from 30 to 40 pounds on the square inch.

VI.

MASTICS.

187. The term Mastic is generally applied to artificial or natural combinations of bituminous or resinous substances with other ingredients. They are converted to various uses in constructions, either as cements for other materials, or as coatings, to render them impervious to water.

188. Bituminous Mastic. The knowledge of this material dates back to an early period; but it is only within, comparatively speaking, a few years that it has come into common use in Europe and this country. The most usual form in which it is now employed, is a combination of mineral tar and powdered bituminous limestone.

189. The localities of each of these substances are very numerous; but they are chiefly brought into the market from several places in Switzerland and France, where these minerals are found in great abundance; the most noted being Val-de-Travers in Switzerland, and Seyssel in France.

190. The mineral tar is usually obtained by boiling in water a soft sandstone, 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. An analysis of a rich specimen of the Seyssel bituminous sandstone gave the following results:

191. The bituminous limestone 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 limestone 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, etc., which are met with in this formation.

192. The clay contained in asphaltic rock, as it is not impregnated, like the carbonate of lime, with the bitumen, is hurtful, causing, at times, the cracks seen in asphaltic pave

ments.

Some rocks contain an oily element, like petroleum, which, rendering the mastic made from them too fat, must first be distilled out.

193. 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.

194. 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 iron 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.

195. Owing to the variable quantity of mineral tar in

bituminous limestone, 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.

196. Mastics have been formed by mixing vegetable tar, pitch, and other resinous substances, with litharge, powdered brick, powdered limestone, etc.; but the results obtained have generally been inferior to those from bituminous mastic.

197. 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.

198. 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.

VII.

BRICK.

199. 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 semivitrified state.

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.

200. 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 may at times be obliged to prepare this material himself, the following outline of the process may prove of service.

201. 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.

202. Good brick earth is frequently found in a natural state, and requires no other preparation for the purposes of the brick-maker. 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.

203. 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.