THE FORCING GAKDEX. 



533 



inches square, with a circular valve in the centre three inches 

 in diameter. The grating is thirty inches long, and of the 

 same width as the door. The ash-pit is also furnished with 

 a door of the same form and size as that of the furnace, but 

 fifteen inches deep. The circular valve in this door is sup- 

 plied with a handle to turn it by, for the purpose of admitting 

 any quantity of air to the extent of half its diameter. The 

 bottom of this oven is placed twelve inches at least below the 

 level of the bottom of the flues, and is paved with fire-bricks. 

 The walls or sides of the oven or furnace are composed of the 

 same material, as is also the arch or roof, and fire-clay is used 

 instead of lime. Furnaces of this description have given much 

 satisfoction, and are sometimes of larger dimensions, parti- 

 cularly when wood, or wood and peat, are used for fuel, and are 

 sometimes smaller, when coals, or coals and cinders, are used 

 only. 



Small furnaces have an important advantage over larger ones, 

 in requiring much less fuel to heat the flues, which when fully 

 heated or charged with heated air, the heat is shut up in them 

 by means of a damper placed at the extremity of the flue farthest 

 from the entrance of the fire, or where the flues terminate in 

 the shaft for the discharge of the smoke. Flues thus charged 

 with heated air, and prevented from escaping at the chimney- 

 top, gradually lose their heat in the house through the bricks 

 of which the flues are composed, and when once the whole vo- 

 lume of air in the house is heated to the required degree, this 

 gradual supply from the flues will keep up the required tem- 

 perature for a long time. If the doors of furnaces were made 

 double, they would have the advantage of durability, and of 

 preserving the heat from escaping behind, and of not so readily 

 admitting cold air to pass over the fire, which air, of course, 

 will be less heated, and less fit for entering into the flues, than 

 if it entered below and passed through the body of fire in the 

 furnace. Ash-pit doors are of great use, in consequence of 

 their acting as regulators to the current of air for keeping 

 the fire alive, or as a damper or suflbcater, when it is judged 

 necessary to extinguish or diminish the fire. 



In fixing the situation of the furnace, much may be gained 

 in i)oint of economy in fuel, by placing them under the wall of 



