will withstand the ordinary exposure of weather; and that, to obtain a good mortar for this purpose, either the hydraulic limes mixed with sand must be employed, or else common lime mixed either with arènes, or with a puzzolana and sand.

114. Any pure sand mixed in proper proportions with hydraulic lime, will give a good mortar for the open air; but the hardness of the mortar will be affected by the size of the grain, particularly when hydraulic lime is used. Fine sand yields the best mortar with good hydraulic lime; mixed sand with the feebly hydraulic limes; and coarse sand with fat lime.

115. The proportion which the lime should bear to the sand seems to depend, in some measure, on the manner in which the lime is slaked. M. Vicat states, that the strength of mortar made of a stiff paste of fat lime, slaked in the ordinary way, increases from 0.50 to 2.40 to one of the paste in volume; and that, when the lime is slaked by immersion, one volume of the like paste will give a mortar that increases in strength from 0.50 to 2.20 parts of sand.

For one volume of a paste of hydraulic lime, slaked in the ordinary way, the strength of the mortar increases from 0 to 1.80 parts of sand; and, when slaked by immersion, the mortar of a like paste increases in strength from 0 to 1.70 parts of lime. In every case, when the dose of sand was increased beyond these proportions, the strength of the resulting mortar was found to decrease.

116. Manipulations of Mortar. The quality of hydraulic mortar, which is to be immersed in water, is more affected by the manner in which the lime is slaked, and the ingredients mixed, than that of mortar which is to be exposed to the weather; although in both cases the increase of strength, by the best manipulations, is sufficient to make a study of them a matter of some consequence.

117. The results obtained from the ordinary method of slaking, by sprinkling, or by immersion, in the case of good hydraulic limes, are nearly the same. Spontaneous, or air-slaking, gives invariably the worst results. For common and slightly hydraulic lime, M. Vicat states that air-slaking yields the best results, and ordinary slaking the worst.

118. The ingredients of mortar are incorporated either by manual labor, or by machinery: the latter method gives results superior to the former. The machines commonly used for mixing mortar are either the ordinary pug-mill (Fig. 4) employed by brickmakers for tempering clay, or a grinding-mill, (Fig. 5.) The grinding-mill is the best machine, because it not only reduces the lumps, which are found in the most carefully burnt stone, after the slaking is apparently complete, but it brings the

lime to the state of a uniform stiff paste, which it should receive before the sand is incorporated with it. Care should be taken

Fig. 4 represents a vertical section through
the axis of a pug-mill, for mixing or
tempering mortar.-This mill consists
of a hooped vessel, of the form of a co-
nical frustum, which receives the in-
gredients, and a vertical shaft, to which
arms with teeth, resembling an ordi-
nary rake, are attached, for the purpose
of mixing the ingredients.

A, A, section of sides of the vessel.
B, vertical shaft to which the arms C are
affixed.

D, horizontal bar for giving a circular mo-
tion to the shaft B.

E, sills of timber supporting the mill.
F, wrought-iron support through which
the upper part of the shaft passes.

not to add too much water, particularly when the mortar is to be immersed in water. The mortar-mill, on this account, should be sheltered from rain; and the quantity of water with which it is

B

Fig. 5 represents a part of a mill for crushing the lime and tempering the mortar.

A, heavy wheel of timber, or cast iron.

B, horizontal bar passing through the wheel, which at one extremity is fixed to a vertical shaft, and is arranged at the other (C) with the proper gearing for a horse.

D, a circular trough, with a trapezoidal cross section which receives the ingredients to be mixed. The trough may be from 20 to 30 feet in diameter; about 18 inches wide at top, and 12 inches deep; and be built of hard brick, stone, or timber laid on a firm foundation.

supplied may vary with the state of the weather. Nothing seems to be gained by carrying the process of mixing, beyond obtaining a uniform mass of the consistence of plastic clay. Mortars of hydraulic lime are injured by long exposure to the air, and frequent turnings and mixings with a shovel or spade; those of common lime, under like circumstances, seem to be improved. Mortar, which has been set aside for a day or two, will become sensibly firmer; if not allowed to stand too long, it may be again reduced to its clayey consistence, by simply pounding it with a beetle, without any fresh addition of water.

119. Setting and Durability of Mortars. Mortar of common lime, without any addition of puzzolana, will not set in humid situations, like the foundations of edifices, until after a very long lapse of time. They set very soon when exposed to the air, or to an atmosphere of carbonic acid gas. If, after having become

hard in the open air, they are placed under water, they in time lose their cohesion and fall to pieces.

120. Common mortars, which have had time to harden, resist the action of severe frosts very well, if they are made rather poor, or with an excess of sand. The sand should be over 2.40 parts, in bulk, to one volume of the lime in paste; and coarse sand is found to give better results than fine sand.

121. Good hydraulic mortars set equally well in damp situations, and in the open air; and those which have hardened in the air will retain their hardness when immersed in water. They also resist well the action of frost, if they have had time to set before exposure to it; but, like common mortars, they require to be made with an excess of sand, to withstand well atmospheric changes.

122. The surface of a mass of hydraulic mortar, whether made of a natural hydraulic lime or otherwise, when immersed in water, becomes more or less degraded by the action of the water upon the lime, particularly in a current. When the water is stagnant, a very thin crust of carbonate of lime forms on the surface of the mass, owing to the absorption by the lime of the carbonic acid gas in the water. This crust, if the water be not agitated, will preserve the soft mortar beneath it from the farther action of the water, until it has had time to become hard, when the water will no longer act upon the lime in any perceptible degree.

123. Hydraulic mortars set with more or less promptness, according to the character of the hydraulic lime, or of the puzzolana which enters into their composition. Artificial hydraulic mortars, with an excess of lime, set more slowly than when the lime is in a just proportion to the other ingredients.

124. The quick-setting hydraulic limes are said to furnish a mortar which, in time, acquires neither as much strength nor hardness as that from the slower-setting hydraulic limes. Artificial hydraulic mortars, on the contrary, which set quickly, gain, in time, more strength and hardness than those which set slowly.

125. The time in which hydraulic mortars, immersed in water, attain their greatest hardness, is not well ascertained. Mortars made of strong hydraulic limes do not show any appreciable increase of hardness after the second year of their immersion; while the best artificial hydraulic mortars continue to harden, in a sensible degree, during the third year after their immersion.

126. Theory of Mortars. The paste of a hydrate, either of common or of hydraulic lime, when exposed to the air, absorbs carbonic acid gas from it; passes to the state of sub-carbonate of lime; without, however, rejecting the water of the hydrate, and gradually hardens. The time required for the complete satura

tion of the mass exposed, will depend on its bulk. The absorption of the gas commences at the surface and proceeds more slowly towards the centre. The hardening of mortars exposed to the atmosphere, is generally attributed to this absorption of the gas, as no chemical action of lime upon quartzose sand, which is the usual kind employed for mortars, has hitherto been detected by the most careful experiments.

127. With regard to hydraulic mortars, it is difficult to account for their hardening, except upon the effect which the silicate of lime may have upon the excess of simple hydrate of uncombined lime contained in the mass. M. Petot supposes, that the particles of silicate of lime form so many centres, around which the uncombined hydrates group themselves in a crystalline form; becoming thus sufficiently hard to resist the solvent action of water. With respect to the action of quartzose sand in hydraulic mortars, M. Petot thinks that the grains produce the same mechanical effect as the particles of the silicate of lime, in inducing the aggregation of the uncombined hydrate.

128. Concrete. This term is applied, by English architects and engineers, to a mortar of finely-pulverized quick-lime, sand, and gravel. These materials are first thoroughly mixed in a dry state, sufficient water is added to bring the mass to the ordinary consistence of mortar, and it is then rapidly worked up by a shovel, or else passed through a pug-mill. The concrete is used immediately after the materials are well incorporated, and while the mass is hot.

129. The materials for concrete are compounded in various proportions. The most approved are those in which the lime and sand are in the proper proportions to form a good mortar, and the gravel is twice the bulk of the sand. The gravel used should be clean, and any pebbles contained in it larger than an egg, should be broken up before the materials are incorporated.

130. Hot water has in some cases been used in making concrete. It causes the mass to set more rapidly, but is not otherwise of any advantage.

131. The bulk of a mass of concrete, when first made, is found to be about one fifth less than the total bulk of the dry materials. But, as the lime slakes, the mass of concrete is found to expand about three eighths of an inch in height, for every foot of the mass in depth.

132. The use of concrete is at present mostly restricted to forming a solid bed, in bad soils, for the foundations of edifices. It has also been used to form blocks of artificial stone, for the walls of buildings and other like purposes; but experience has shown, that it possesses neither the durability nor strength requi

site for structures of a permanent character, when exposed to the action of water, or of the weather.

133. Beton. The term beton is applied, by French engineers, to any mixture of hydraulic mortar with fragments of brick, stone, or gravel; and it is now also used by English engineers in the

same sense.

134. The proportions of the ingredients used for beton are variously stated by different authors. The sole object for which the gravel, or the broken stone is used, being to obtain a more economical material than a like mass of hydraulic mortar alone would yield, the quantity of broken stone should be as great as can be thoroughly united by the mortar. The smallest amount of mortar, therefore, that can be used for this purpose, will be that which will be just equal in volume to the void spaces in any given bulk of the broken stone, or gravel. The proportion which the volume occupied by the void spaces bears to any bulk of a loose material, like broken stone, or gravel, may be readily ascertained by filling a vessel of known capacity with the loose material, and pouring in as much water as the vessel will contain. The volume of water thus found, will be the same as that of the void spaces.

135. Beton made of mortar and broken stone, in which the proportions of the ingredients were ascertained by the process just detailed, has been found to give satisfactory results; but, in order to obviate any defect arising from imperfect manipulation, it is usual to add an excess of mortar above that of the void spaces.

The best and most economical beton is made of a mixture of broken stone, or brick, in fragments not larger than a hen's egg, and of coarse and fine gravel mixed in suitable proportions.

136. In making beton, the mortar is first prepared, and then incorporated with the finer gravel; the resulting mixture is spread out into a cake, 4 or 6 inches in thickness, over which the coarser gravel and broken stone are uniformly strewed and pressed down, the whole mass being finally brought to a homogeneous state with the hoe and shovel.

Beton is used for the same purposes as concrete, to which it is superior in every respect, but particularly so for foundations laid under water, or in humid localities.

137. 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.

138. Mortar adheres most strongly to brick; and more feebly to wood than to any other material. Among stones, its adhesion to lime-stone is generally greatest; and to basalt and sand-stones,