722

THEORY AND PRACTICE OF ENGINEERING.

Book II.

together: these are placed at regular distances from above the lower arches to the top,and are held by vertical iron !>olts where they join: the kiln will hold a charge of 30tons of broken cement-stones measuring 26 cubic feet to the ton, as well as the fuel re-quired for burning it. When used the bottom is covered with wood, and the coals andcement-stones are arranged in alternate layers, each about a foot in thickness : after ithas been lighted three days the lower part may be drawn, and then by constantly fillingup, this may be done every twenty-four hours : the cement-stones and coals are thrownin from the top, and every ton of cement-stone yields 21 bushels of cement powder. Inthe mill for grinding this cement, the materials are thrown by a labourer into a sievecontaining seventeen wires to an inch, which is shaken by the machinery attached to thesteam-engine, after which it is packed into casks and kept ready for use : two tons of cementpowder are ground during the day.

In France the materials selected for the manufacture of hydraulic mortar are thechalk of the country, and a clay which contains 63 parts of silica, 28 of alumina, and 7 ofoxide of iron ; after they are reduced into small lumps they are ground in a mill, thestones are placed on their edges, and followed by a strong wheel to which is attacheda set of rakes ; these are moved by a 2 horse-wheel in a circular basin 13 feet in diameter ;in the middle is the pivot of the vertical axle which supports the machinery : water is al-lowed to flow into this basin as required, and the proportions of chalk are three measuresto one of clay. When this has been subjected to the action of the mill for an hour anda half, it is drawn out in the state of a thin pulp into four or five successive reservoirs,in the last of which it stands until it has obtained its requisite consistency ; the massis then cut out into prisms, which are placed on shelves to dry, and in a short time theyare sufficiently so to be subjected to the process of calcination.

Smeaton ’s observations on water cements show that the quality of lime does not dependeither upon its hardness or its colour, for the white lias of Somersetshire , thoughapproaching to a flinty hardness, and having a chalky appearance, he says, is not equal tothe clunch lime, obtained near Lewes in Sussex, which is of great repute for all worksexecuted in water. This clunch lime is a species of chalk, not found, like the lias, in thinstrata, but in thick masses. It is considerably harder and heavier than common chalk, butyet of the lowest degree of what maybe termed stony hardness, and inclining to a yellowishash colour. When analysed, it contains three parts out of sixteen of its weight of yellowishclay, with a small quantity of sand, seemingly of the crystal kind, not quite transparent,but mixed with red spots: hence the fitness of lime for water building seems neither todepend upon the hardness of the stone, the thickness of the stratum, nor the bed or matrixin which it is found, hut in burning and falling down into a powder of a buff-colouredtinge, and in containing a considerable quantity of clay ; and he found all the water limesto agree in these particulars, among which that obtained at Dorking , he supposed, wasstronger than that made from common chalk, it containing one-seventeenth part of light-coloured clay of a yellowish tinge.

Sutton lime in Lancashire , he observed, had this buff cast, the stone itself being of a deepbrown colour, its quality as a water lime not depending upon its colour before it isburnt: this limestone was found to contain three parts out of sixteen of its weight ofbrown or red clay, and one part out of forty-two of fine brown sand, making a lime verysuperior to all others for water-building.

'l*he mortar used at the Eddystone lighthouse was composed of equal measures of puzzo-lana and blue lias lime, slaked into a powder, forming a very strong cement, and settingwell under water, though requiring considerable time to do so : in this sand was altogetheromitted, and when two bushels of slaked lime powder were added to two bushels of puzzo-lana, the whole occupied 2*32 cube feet. For the common face mortar, to two bushels oflime powder were added one bushel of puzzolana and three bushels of common sand,which occupied 4*67 cubic feet.

Smeaton also used a water-lime and minion, or siftings of the ironstone, after calcinationat the iron furnaces, which were ground in a mill before being added to the powderedlime; this minion or forged scales, he considered, when well pulverised, and sifted cleanfrom dirt and glassy slag, equivalent to as much puzzolana or tarras. Two bushels of lime,two bushels of minion, and one of sand was the proportion for face mortar ; and for hackingmortar, two of lime, £ bushel of minion, and three bushels of common sand, occupying 417cubic feet.

When common lime was mixed with minion for ordinary face mortar he employed twobushels of lime, the same quantity of minion, and two bushels of sand, making 4 75 cubicfeet, and for common lime, mixed with tarras, his proportions were for tarras mortar twobushels of powdered lime, and one of tarras or 1 *67 cubic feet; and for the backing mortar,to the same quantity of powdered lime, only £ bushel of tarras, and three bushels of sand, or3-50 cubic feet.

Puzzolana, obtained near Naples, is of a violet red colour, and of volcanic origin : itis sometimes found in coarse grains in slag, pumice, or tufa, and often of a yellow, grey, or