The gas passes from the channel through the pipe, a, into the vertical conduits, b, and is afterward disengaged through the tuyeres into the chamber. In order that the gas may be equally applied for preliminary heating or smoking, a small smoking furnace, S, has been added to the apparatus. The upper part of this consists of a wide cylinder of refractory clay, in the center of whose cover there is placed an internal tube of refractory clay, which communicates with the channel, G, through a pipe, d. This latter leads the gas into the tube, t, of the smoking furnace, which is perforated with a large number of small holes. The air requisite for combustion enters through the apertures, o, in the cover of the furnace, and brings about in the latter a high temperature. The very hot gases descend into the lower iron portion of this small furnace and pass through a tube, e, into the smoking chamber by the aid of vertical conduits, b', which serve at the same time as gas tuyeres for the extremity of the furnace that is exposed to the fire. [Illustration: GAS FURNACE FOR BAKING REFRACTORY PRODUCTS.] In the lower part of the smoking furnace, which is made of boiler plate and can be put in communication with the tube, e, there are large apertures that may be wholly or partially closed by means of registers so as to carry to the hot gas derived from combustion any quantity whatever of cold and dry air, and thus cause a variation at will of the temperature of the gases which are disengaged from the tube, e. The use of these smoking apparatus heated by gas does away also with the inconveniences of the ordinary system, in which the products are soiled by cinders or dust, and which render the gradual heating of objects to be baked difficult. At the beginning, there is allowed to enter the lower part of the small furnace, S, through the apertures, a very considerable quantity of cold air, so as to lower the temperature of the smoke gas that escapes from the tube, e, to 30 or 50 degrees. Afterward, these secondary air entrances are gradually closed so as to increase the temperature of the gases at will. * * * * * THE EFFICIENCY OF FANS. Air, like every other gas or combination of gases, possesses weight; some persons who have been taught that the air exerts a pressure of 14.7 lb. per square inch, cannot, however, be got to realize the fact that a cubit foot of air at the same pressure and at a temperature of 62 deg. weighs the thirteenth part of a pound, or over one ounce; 13.141 cubic feet of air weigh one pound. In round numbers 30,000 cubic feet of air weigh one ton; this is a useful figure to remember, and it is easily carried in the mind. A hall 61 feet long, 30 feet wide, and 17 feet high will contain one ton of air. [Illustration: FIG. 1]