1224

THEORY AND PRACTICE OF ENGINEERING.

Book IT.

the rays of heat which impinge on one are reflected, consistently with the property of con-cave mirrors, in parallel lines, so as to fall upon the opposed mirror, whence they convergeto its focus, in which is the thermometer, and which will be affected proportionally tothe heat of the original radiating body. As an experiment on the receptive and emissivepowers of surfaces, in regard to radiant heat, it has been proved, that when two vesselsfilled with water are placed before a fire, the one clean and polished, the other coated withlampblack, the former receives the heat more slowly, whilst the latter cools more quickly.

In an open fireplace the fuel should be arranged as near the hearth as possible, in orderthat the air in the lower part of the room may become warmed; and the proportion givento the length of the bars which contain the coal should be equal to the twelfth part of thelength of the room, if it be lofty and of good proportions; in their construction Stour-bridge brick, or Welsh lumps, should be made use of in preference to metal, as they arefar better radiators of heat, and less liable to destruction by too rapid expansion or con-traction. Whenever coke can be used, the heat produced is far more agreeable,.and there isno soot to lodge in the chimney : soft burnt coke, broken into small pieces, when all thegases have been abstracted, is far preferable in point of economy, and when the methodof lighting and maintaining its combustion is acquired, coal is seldom again resorted to.Attention is requisite to prevent too rapid a draught up the chimney, otherwise the greaterportion of heat will be drawn up it, and pass off without warming the room. Thechimney which is said to draw well, consumes well, and is by no means always the best •that which draws sufficiently only, is the most economical in its consumption: all thatneed be desired is to pass off the smoke, or prevent the cold air from descending down theshaft, and disturbing the process of combustion, forcing the dust and smoke into theapartment: different states of the atmosphere require different areas of flue; when theair is very heavy, it often descends in our ordinary chimney more rapidly than it canbecome rarefied ; and the column of warm air, ascending cylindrically in a square flue, thefour angles are so many passages for the descent of cold airs; the less these disturbingforces occur, the less chance is there of a smoky chimney.

Grates or Stoves, which have an open fire, and an apparatus for throwing out a streamof warm air, are much used: this is effected by passing the air received by a pipe fromthe outside of the building through a double casing, which surrounds the fuel in thegrate; in many parts of France , where wood is burnt upon the hearth, the back of thefireplace is hollow, or has a chamber cast in it: by means of a pipe from the outside airis introduced to be warmed, which by another pipe enters at the skirting, or some otherconvenient part of the room.

The great quantity of hot air which passes up the chimney of an open fireplace has ledto the adoption of close stoves in this country, and necessarily to various methods forinsuring perfect ventilation; on the Continent they are universal, generally of earthen-ware, either round or square, and so placed as to warm the air before it is admitted intothe apartment; in most of them the object seems to be, and a most judicious one, toproduce a large volume moderately heated, rather than a small quantity of very hot air,—the latter being apt to char or burn the organic dust which exists in the atmosphere ofour apartments, producing an unpleasant smell: an artificial hypocaustum is an arrange-ment preferred in France ; the small pillars which stand in the midst of the burningembers, and support the oven or chamber above, are hollow ; through these the cold airfrom below ascends, and in its passage through the heated metallic supports becomeswarmed before it enters the chamber, where, on its arrival, it is heated, and then by itsexpansion passes off by pipes, or ornamental apertures, where required.

Metallic Stoves , lined with fire-brick, are now generally substituted; they are found toassist in radiating as well, care being taken so to dispose the lining that the outer caseshall never become red-hot, and only a sufficient quantity of air be admitted to feed com-bustion, without exciting a rapid draught and a great consumption of fuel. All bodieshave the faculty of absorbing and emitting heat, so that radiation takes place, not only atthe surfaces, but also in the interior of solids and liquids, in the same manner as in air:indeed the molecules of all bodies are regarded as so many foci radiating heat, and con-tinuing to do so until the objects near them have arrived at the same degree of tempera-ture : in solids as well as liquids the absorption takes place at very small distances, in airand in gases at very considerable distances.

The low Temperature , or Dr. Arnott's Stove , by means of an ingenious contrivance,economises fuel, and gives out a considerable degree of heat: it is a much more scientificarrangement than that practised in the ordinary close stove : only sufficient air is admittedto keep up a slow combustion, and the heat is communicated to the radiating surface ofthe stove; the air is admitted by means of a self-acting valve, opened or shut by the riseand fall of a thermometer, and the heat is prevented from passing into the chimney in con-sequence of the very gentle motion imparted to it by the small draught maintained.

To determine the quantity of surface of steam-pipes, or vessel necessary to warm a roomin winter when the external temperature is 10° below freezing, and to make the apartment