In the lower part of the smoking furnace, which is made of boiler plate and can be put in communication with the tube, 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.
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, 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, to 30 or 50 degrees. Afterward, these secondary air entrances are gradually closed so as to increase the temperature of the gases at will.

License information: nan
MPAA: G
Go to source: http://www.gutenberg.org/files/8559/8559-h/8559-h.htm#6