Page:Popular Science Monthly Volume 15.djvu/222

210 Coal has now receded in England to the old minimum price of 4s. 6d. at the pit's mouth. Some of our northern railways are paying 6s. a ton for coal. The price of the best Wall's End coal delivered at private residences in London, at the end of January, 1879, was 29s. per ton. Thus, even in the three hundred miles which divide the metropolis from the pit's mouth, it will be seen that the price of coal is so regulated by local conditions, and distance from the collieries, that it is not easy to strike an average. We may therefore assume a price, equal to that of petroleum, of 10s. per ton, for the sake of comparison, and it will then be easy to apply the correction due to the price of coal in any particular spot. The undetermined charges for interest on capital, merchants' profits, and delivery to consumers, may also be roundly taken, for the sake of comparison, as equal for the different materials.

The cost of the manufacture and distribution of gas in London (exclusive of the cost of coal) is about twenty per cent. over the amount realized for the sale of the residual products of distillation, of course excluding the gas. 10,000 cubic feet of gas per ton is a high, though not the highest, production. The price of the residual products, as a rule, is so far regulated by the price of coal at the spot, that it is usually reckoned that the local price of gas in England is nearly independent of the local variation in the price of coal, sales balancing purchases. Thus, if we take 10,000 cubic feet of gas as costing the same as one ton of coal, we shall be within twenty or twenty-five per cent. of exactitude, as a general rule. We have, then, to compare the luminiferous and calorific value of a ton of coal, a ton of petroleum, and 10,000 feet of cubic gas, assuming the approximate price of each of these quantities to be equal.

For lighting purposes, indeed, coal is nowhere. It has been occasionally used for giving light on public works, such as railways, when it was necessary to carry them on by night. But the light of a "devil," or iron basket of live coals, is fitful and costly. As recently as 1815 the dangerous Bell Rock, at the entrance to the Firth of Tay, was lighted by a fire-basket, or "chauffer," of live coals. It is stated in the "Life of Robert Stevenson," the great lighthouse engineer, that the consumption of coal in this "chauffer" was four hundred tons per annum, while the light was never reliable when most required. In violent gales the coal never burned on the windward side of the fire; and the guardian actually laid hold of the bars of the "chauffer," on the windward, to steady himself while putting on more fuel. Thus, in the direction where, and at the time when, the light was most required, it was all but totally invisible. The gas requisite to maintain a light equal to one hundred Carcel burners, or nine hundred and sixty candles, for twelve hours, is producible from half a ton of coal, as distilled in the gas-works. This would yield a splendid light (if the locality were such as to allow of its introduction); while the consumption of twenty-two hundred weight per night of coal only made darkness visible.