BOOK XII.

537

A —Lower chamber of the first furnace. B —Upper chamber. C —Vitreous mass.

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and in the third they cool the glowing glass vessels and other articles. Ofthese the first furnace must be vaulted and similar to an oven. In the upperchamber, which is six feet long, four feet wide, and two feet high, themixed materials are heated by a fierce fire of dry wood until they meltand are converted into a vitreous mass. And if they are not satisfactorilypurified from dross, they are taken out and cooled and broken into pieces;and the vitreous pieces are heated in pots in the same furnace.

The second furnace is round, ten feet in diameter and eight feet high,and on the outside, so that it may be stronger, it is encompassed by fivearches, one and one half feet thick ; it consists in like manner of twochambers, of which the lower one is vaulted and is one and one half feet thick.In front this chamber has a narrow mouth, through which the woodcan be put into the hearth, which is on the ground. At the top and in themiddle of its vault, there is a large round hole which opens to the upperchamber, so that the flames can penetrate into it. Between the arches inthe walls of the upper chamber are eight windows, so large that the big-bellied pots may be placed through them on to the floor of the chamber,around the large hole. The thickness of these pots is about two digits, theirheight the same number of feet, and the diameter of the belly one and a half