The Greeks have also invented terrace-roof[2819] pavements, and have covered their houses with them; a thing that may easily be done in the hotter climates, but a great mistake in countries where the rain is apt to become congealed. In making these pavements, the proper plan is to begin with two layers of boards, running different ways, and nailed at the extremities, to prevent them from warping. Upon this planking a rough-work must be laid, one-fourth of which consists of pounded pottery: and upon this, another bed of rough-work, two-fifths composed of lime, a foot in thickness, and well beaten down with the rammer. The nucleus[2820] is then laid down, a bed six fingers in depth; and upon that, large square stones, not less than a couple of fingers in thickness; an inclination being carefully observed, of an inch and a half to every ten feet. This done, the surface is well rubbed down with a polishing stone. The general opinion is, that oak[2821] should never be used for the planking, it being so very liable to warp; and it is considered a good plan to cover the boards with a layer of fern or chaff, that they may be the better able to resist the action of the lime. It is necessary, too, before putting down the planking, to underset it with a bed of round pebbles. Wheat-ear[2822] tesselated pavements are laid down in a similar manner. CHAP. 63.—GRÆCANIC PAVEMENTS. We must not omit here one other kind of pavement, that known as the “Græcanic.” The ground is well rammed down, and a bed of rough work, or else broken pottery, is then laid upon it. Upon the top of this, a layer of charcoal is placed, well trodden down with a mixture of sand, lime, and ashes; care being taken, by line and rule, to give it a uniform thickness of half a foot. The surface then presents the ordinary appearance of the ground; but if it is well rubbed with the polishing-stone, it will have all the appearance of a black pavement. CHAP. 64.—AT WHAT PERIOD MOSAIC PAVEMENTS WERE FIRST INVENTED. AT WHAT PERIOD ARCHED ROOFS WERE FIRST DECORATED WITH GLASS.