Among the of modern Europe, none have practised the art of building under water to so great an extent as the 1)0(.11, to whom we are indebted flw the disowery of another valuable material. admirably adapted for aquatic works: this substance is called terrus, or truss, and is nothing more than wukke, or cellular basalt. It is procured chiefly from Bockenhefin, Frankfort on the Maine, and A ndernach. whence it is transported down the Rhine, in large quantities, to Holland, and is prepared by grinding and sifting, so as to reduce it to the consistence of coarse sand; when it is mixed, in the following manner, with blue argillaceons lime from the banks of the Scheldt. They take such a quantity of quick-lime as may be judged sufficient thr a week. and spread it in a kind of bacon, in a stratum about a foot thick, and sprinkle it with water; this is covered with a stratum of terms, of about the same thickness, and thus left for two or three days; it is then beaten into a mixture. and left for two days longer; after which such portions as are wanted for daily consumption are taken from the mass, and beaten up again previous to being used.—This is the celebrated terras mortar, with which the mounds and other aquatic works, used as a defence for protecting the low lands of Holland, against the incursions of the sea, are consolidated.
The proportion of the ingredients for terras mortar, as used in Britain in the construction of our water-works, is the same as practised by the Dutch, viz., one measure of quick-lime and two of slacked, in the dry powder, mixed with one measure of terras, and well beaten together to the consistence of a paste, using as little water as possible.
Another kind, almost equally good, and considerably cheaper, is composed of two measures of slacked lime, one of terras, and three of coarse sand ; but this composition requires more labour in beating than the foregoing, and pro duces three measures and a half of excellent mortar. When the building is composed of rough stones, which leave irre gular interstices and large cavities, the joints may be filled with pebble mortar, which is thus composed : Take two mea sures of slacked argillaceous lime, half a measure of terms, or puzzolana, one of coarse sand, one of fine sand, and four of small pebbles screened and washed, and mix them toge ther. Pebble mortar was a favourite cement among the Romans, and has been used, ever since their time, in those works wherein a large quantity of mortar is required.
Terms mortar will only acquire its proper hardness under water ; for if permitted to dry by exposure to the air, it never arrives at the same degree of hardness as if the same lime had been mixed with good clean common sand, and is very friable and crumbling ; but when kept always wet, it throws out a substance something like the concretion in lime-stone caverns, called stalactite, which substance acquires a considerable hardness, and in time becomes so exuberant as to deform the face of the walls.
Although the Dutch terms has hitherto been prepared with cellular basalt, it appears, from the experiments of Norveau, that the common compact basalt, if previously calcined, will answer nearly the same purpose. Compact basalt abounds in all the districts where coal is raised, and may therefore be procured easily, and calcined with the refuse coal.
In some parts of the Low Countries, coal-ashes are sub stituted fig. terras with very good effect, of which the cendree de Tourney is a striking instanee. The deep blue argillo-ferruginous lime-stone of the Scheldt is burnt in kilns, with a slaty kind of pit-coal found in the neigh bourhood. When the calcination of the lime is completed, the pieces are taken (Hit, and a considerable q tiantit v of dust and small fragments remain at the bottom of the kiln. This refuse, consisting of coal-ash, mixed with one-fourth of lime-dust, is called the cendree, and is thus made into mortar with lime : Put a bushel of the materials into any suitable vessel, and sprinkle it with as much water as is sufficient to slack the lime; then take another bushel, and treat it in the same manner ; and so on, till the vessel is filled. In this state it remains some weeks, and may be kept for a much longer time, if covered with moist earth. A strong open trough, containing about two cubic feet, is filled about two-thirds with cement in the above state, and by means of a heavy iron pestle, suspended at the end of an elastic pole, is well beaten for about half an hour ; at the end of this time it becomes of the consistence of soft mortar, and is then laid in the shade from three to six days, according to the dryness of the air. When sufficiently dry, it is beaten again for half an hour, as betiwe ; and the oftener it is beaten, the better will be the cement ; three or four hours, however, are sufficient to reduce it to the consistence of a uniform smooth paste. After this period it becomes too stiff, on account of the evaporation of its water, as no more of this fluid is allowed to enter the composition than what was at first employed to slack the lime. The cement, thus prepared, is found in a few minutes to unite so firmly, upon brick or stone, that still water may be let in immediately upon the work, without. any inconvenience; and by keeping it dry for twenty-four hours, it has nothing farther to fear from the most rapid current.