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1829.] its tray was to be placed, whilst the other end of the tube reached to and was Hush with the outside of the furnace. A loose piece of tube, similar in kind but smaller in diameter, being laid upon the bottom of the chamber, and applied at its end to the orifice of the larger one, served as a continuation of it until the inner extremity reached to and was under the cover of the glass experiment. When the furnace was hot, there was always a draft inwards through this tube; but the quantity of air admitted was regulated by a valve (70). The air, by first passing through the hot sides of the furnace, and then through the shorter ignited tube serving for connexion, was transmitted in a thoroughly heated state to the place where its presence was required, without producing any serious cooling effect; it there maintained a continually oxygenating atmosphere, and, judging from the effects, prevented the draught inwards of any vapours from the fire to the space beneath the glass covers.

56. The next point of importance, in the preparation of the glass, is the arrangement of the tray in the furnace just described. To understand this, it will be necessary to say that the glass-chamber is 25 inches long, 13 inches wide, and 8 inches deep, and that the fire being at one end, the Hue is at the other. Plates of glass 7 inches square have been made in it; but it would probably require a larger furnace to make much larger pieces.

57. The bottom of the chamber being perfectly level and clean, the gauge-board, on which the tray was formed (35),