94

HISTORY OF ENGINEERING.

Book I.

ami red earth or rough sand; winch materials were preferred on account of theirlightness. Other kinds, heavier, which do not adhere to the straw, or are dissolvedby wet, were objected to. Bricks were made in the spring or autumn, as the drying thenwas more regular; those made during the summer solstice suffered injury from the heat,the interior seldom drying regularly, and their exterior hardening rapidly cracked. Thosewere the best which had been made two years; they were hardly considered dry beforetliat time. Three kinds were used; by the Greeks called didoron, a foot long and half afoot wide, pentadoron, and tetradoron. Half bricks were made to work with them, whichenabled the artificers to break the joint, and to have a vertical joint over the middleof the brick below. Some bricks were moulded of an earth so light that they wouldswim on water, as those of Calentum in Spain, Marseilles in Gaul, and Pitane in Asia .

Of Tiles and Conduit Pipes made of Clay _The same earth used for making bricks

served for forming flat and curved tiles, and different sorts of conduit pipes. Roofs werecovered with alternate flat and curved tiles, and tubes or pipes were used to conduct waterfrom them, as well as to convey it to the fountains. In the thermae of Antoninus , waterwas supplied to the baths by cylindrical pipes, gathered in at one end sufficiently to beinserted into the adjoining one; in the baths of Titus, square pipes were used for the samepurpose. In the fountain of Egeria, long conical pipes, one end inserted in the other,are to be seen, which conducted the water from the aqueduct to the fountain.

Tiles, two feet square, with a small foot at each angle, were placed upright against thewalls, at the baths of Livia, thus leaving between the face of the tile and the wall, pressedagainst, by the four feet, a narrow space, which prevented any moisture injuring the wall;they are fixed by T cramps of iron.

The Homans built hollow walls and domes, with pots and tubes of earthenware, whichpractice was continued down to the end of the middle ages; they seem, also, as we shallhave occasion hereafter to observe, to have preferred earthenware pipes for their supply ofwater to those of metal.

The Sand , used in Roman construction, we find to have been obtained either from thepit, river, or the sea, as circumstances or convenience permitted. Several sorts wereused, as black, white, deep red, and bright red ; that which produced a grating sound,when rubbed between the fingers, is said by Vitruvius to have been preferred ; thatwhich was earthy, and which did not possess the roughness above named, was fit for thepurpose, if it merely left a stain, or a particle of earth, on a white garment, which couldbe easily brushed away. The carbunculum, or bright red sand, was dug out of mountainsof volcanic origin ; it was of a much softer nature than tufa, but more solid than thecommon earth. The property which all sand has of hardening and consolidating withlime renders it of great value in construction ; it has been observed, that the sand onthe sea-shore, nearest the action of the waves,' is the firmest and most solid, and this by theancients was accounted for upon the principle, that the larger the masses the farther theywere projected ; for the hand cannot throw small bodies to a great distance, in consequenceof their lightness. Several stones driven together on the sea-shore would also have thelighter particles of sand washed among them, and fill up their interstitial spaces, and thusform a consolidated mass. Between sabulum and arenam, as used by Vitruvius, there is aconsiderable difference : some writers observe that sabulum is a larger kind of sand, orarenam grossiorem. But arenam is not sabulum, one having the character of earth, the otherof stone. Sabulum has a flue white or yellowish grain, is found in hot climates mor*'than in cold and temperate ones, as in the deserts of Africa , where the surface of extensivetracts are agitated by the wind in the same manner as the sea.

Sea-sand was objected to for plastering, or for mixing with mortar, as it dried slowly;when dug from a pit, and exposed any length of time to the action of the weather, avegetation was encouraged, which injured its properties, and rendered it unfit for use. Riversand was always preferred on account of its grit, and was allowed to make the best mortar.

Lime, either burnt from white stone or flint, called by Vitruvius silice, was obtained,in all probability, from the same calcareous beds as the limestones of the present day, by theItalians called palombino. That which had a close and hard texture was preferred formortar, and the lighter and more porous kinds for plastering. “ When slaked for makingmortar,” Vitruvius says, 5 ‘ if pit sand be used, three parts of sand arc mixed with one oflime; if river or sea-sand, two parts of sand are given to one of lime.” Potsherds reducedto a fine powder and passed through a sieve were added : when river and sea-sand wereused in the above proportion, the mortar was considered the best. The cause of the massbecoming solid, according to our Roman authority, was that sand and water added tolime formed an artificial stone, for all stones were supposed to be compounds of theseelements ; those which contained a quantity of air were of a soft nature, those which hada large proportion of water were tough, of earth hard, and of fire brittle. Stones whichwhen burnt might make an excellent lime if pounded and added to sand, without burningdid not possess the property of adhesion, nor set hard; passing through the kiln they losttheir natural tenacity, and their pores were left open and inactive. The moisture and air