After Adam Smith has declared that there are some mines which can only be worked by the owners, as they will afford only sufficient to defray the expense of working, together with the ordinary profits of the capital employed, we should expect that he would admit that it was these particular mines which regulated the price of the produce. If the old mines are insufficient to supply the quantity of coal required, the price of coal will rise, and will continue rising till the owner of a new and inferior mine finds that he can obtain the usual profits of stock by working his mine. If his mine be tolerably fertile, the rise will not be great before it becomes his interest so to employ his capital; but if it be less productive, it is evident that the price must continue to rise till it will afford him the means of paying his expenses, and obtaining the ordinary profits of stock. It appears, then, that it is always the least fertile mine which regulates the price of coal. Adam Smith, however, is of a different opinion: he observes, that "the most fertile coal mine too regulates the price of coals at all the other mines in its neighbourhood. Both the proprietor and the undertaker of the work find, the one that he can get a greater rent, the other, that he can get a greater profit, by somewhat underselling all their neighbours. Their neighbours are soon obliged to sell at the same price, though they cannot so well afford it, and though it always diminishes, and sometimes takes away altogether, both their rent and their profit. Some works are abandoned altogether; others can afford no rent, and can be wrought only by the proprietor." If the demand for coal should be diminished, or if by new processes the quantity should be increased, the price would fall, and some mines would be abandoned; but in every case, the price must be sufficient to pay the expenses and profit of that mine which is worked without being charged with rent. It is therefore the least fertile mine which regulates price. Indeed it is so stated in another place by Adam Smith himself, for he says, "The lowest price at which coals can be sold for any considerable time, is like that of all other commodities, the price which is barely sufficient to replace, together with its ordinary profits, the stock which must be employed in bringing them to market. At a coal mine for which the landlord can get no rent, but which he must either work himself, or let it alone all together, the price of coals must generally be nearly about this price."