trivance, applicable to nearly every existing grate, and that this result could be attained without impairment of, and often with increase of, the heating power of the fire. This contrivance, which I have named an "Economizer," was simply a shield of iron, standing on the hearth, and rising as high as the level of the grid at the bottom of the grate, converting the hearth-space under the fire into a chamber closed by a movable door.
The effect was twofold: The stream of air, which usually rushes through the bottom of the fire, and causes for a short time rapid combustion at a white heat, was thereby cut off, and the air under the fire was kept stagnant, the heated coal being dependent for its combustion on the air passing over the front and the upper surface. The second point was that this boxing up rendered the chamber hotter, and this increased temperature beneath the fire-grate, i. e., under the fuel, added so materially to the temperature of the whole, even of the cinders coming into contact with the iron grid, that the very moderate supply of oxygen reaching the front and upper surface of the fuel was sufficient to maintain every portion in a state of incandescence. Moreover, I observed that combustion was going on at an orange, not at a white, heat.
Let us contrast a white with the orange heat. A white heat in a fire means rapid combustion, owing to the strong current of air, oxygen, which passes under the grate, through the center of the fire, and up the chimney. As soon as the heart of the fire has been rapidly burned away at a white heat, the fuel cools; the iron grid cools also; and the cinders in contact with the grid are chilled below combustion point. They then cease to burn, and the bottom of the fire becomes dead and choked. The poker must now be brought into play to clear away the dead cinders, and to reopen the slits in the choked grid. New coal is added to the feeble remnant of burning embers, with no reserve of heat in the iron surroundings; and in time, and perhaps very slowly, the fire revives, and rapid combustion sets in afresh under the influence of the renewed current of oxygen passing through the heart of the fire. An orange heat means that the coke, i. e., the red-hot cinder, is burning with a slowly applied stream of oxygen, a degree of combustion which is only possible when the coal is kept warm by the hot chamber beneath, and by a reasonable limitation of loss of heat at the back and sides by fire-brick, either in contact with the fuel, or at least close behind the iron surrounding it. This effect is seen, partially, in the grates with solid fire-brick bottom, but far more perfectly in the grates with the chamber closed by the "Economizer."
This hot chamber has the following effects: The incandescent coal remains red-hot from end to end of the grate, until nearly all is consumed, thus maintaining a larger body of the fuel in a state to radiate effective heat into a room. The cinders on coming into contact with the iron grid also remain red-hot, and so continue to burn
