Furnaces have moreover several apertures: one of these is at the ash-hole, which gives passage to the air, and through which the ashes that fall through the grate are raked out; this aperture is called the ash-hole Door: another is at the fire-place, through which the fire is supplied with fuel, as occasion requires; this is called the mouth or door of the Fire-place, or the Stoke-hole: there is a third in the upper story, through which the neck of the vessel passes; and a fourth in the dome for carrying off the fuliginosities of combustible matters, which is called the Chimney. To conclude, there are several other openings in the several parts of the furnace, the use whereof is to admit the air into those places, and also, as they can be easily shut, to incite or slacken the activity of the fire, and so to regulate it; which has procured them the title of Registers. All the other openings of the furnace should be made to shut very close, the better to assist in governing the fire; by which means they likewise do the office of registers. In order to our forming a just and general idea of the construction of furnaces, and of the disposition of the several apertures in them, with a view to increase or diminish the activity of the fire, it will be proper to lay down, as our ground-work, certain principles of natural philosophy, the truth of which is demonstrated by experience. And first, every body knows that combustible matters will not burn or consume unless they have a free communication with the air; insomuch that if they be deprived thereof, even when burning most rapidly, they will be extinguished at once: that consequently combustion is greatly promoted by the frequent accession of fresh air, and that a stream of air, directed so as to pass with impetuosity through burning fuel, excites the fire to the greatest possible activity. Secondly, it is certain that the air which touches, or comes near ignited bodies is heated, rarefied, and rendered lighter than the air about it, that is, farther distant from the center of heat; and consequently that this air, so heated and become lighter, is necessarily determined thereby to ascend and mount aloft, in order to make room for that which is less heated and not so light, which by its weight and elasticity tends to occupy the place quitted by the other. Another consequence hereof is, that if fire be kindled in a place enclosed every where but above and below, a current of air will be formed in that place, running in a direction from the bottom to the top; so that if any light bodies be applied to the opening below, they will be carried up towards the fire; but, on the contrary, if they be held at the opening above, they will be impelled by a force which will drive them up, and carry them away from the fire.