. The book of the garden. Gardening. HEATING BY FLUES. 145 In estimating the relative merits of the different modes of heating, we should also consider that the difference in the quality of fuel is very variable; and even in that most common of all combustible material, coal, we find a great difference in its heat- ing powers, arising from its differences of chemical composition. Experiments have been made by Watt, Rumford, Black, and many others, as to the amount of heat procurable from Newcastle coal of a fair average quality. According to Watt, 1 lb. of coal will boil 45 lb. of water taken
. The book of the garden. Gardening. HEATING BY FLUES. 145 In estimating the relative merits of the different modes of heating, we should also consider that the difference in the quality of fuel is very variable; and even in that most common of all combustible material, coal, we find a great difference in its heat- ing powers, arising from its differences of chemical composition. Experiments have been made by Watt, Rumford, Black, and many others, as to the amount of heat procurable from Newcastle coal of a fair average quality. According to Watt, 1 lb. of coal will boil 45 lb. of water taken at 55° of temperature. Black's computation is, that the same weight of coal will boil 48 lb. of water taken at a mean temperature; while Rumford makes it capable only of heating 36t2<j lb. to the same degree, the temperature of water being 32°. If we take the medium, therefore, of these experiments, the result will be, that 1 lb. of coal will boil 39 lb. of water taken at a temperature of 32°. From these data we can readily deter- mine the quantity of coal necessary to heat any given length of pipe of hot water; for as 100 feet in length of a 4-inch pipe contains 544 lb. of water, lb. of coal will be required to raise the temperature of this quantity of water 180°—that is, from 32°, or freezing point, to 212°, the boiling point. And, again, if the water loses 1° per minute, the above quantity of coal will supply 100 feet of pipe during three hours, provided the temperature remains constant in the atmosphere to be heated. § 2.—HEATING BY FLUES. Having in a preceding section remarked on flues generally, we shall now proceed to consider them individually. The Chinese, from the earliest ages, used flues in the walls of their houses, as well as under the pavement of the floors of their rooms : the latter of these they call the ti-kang, and the former the tong-Jcang, the heated air and smoke of which ascend into the spaces of a hollow wall,—a mode of wall-buildi
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