is known; and from these data it has been calculated that the coal in these counties will last 360 years. Mr. Bailey, in his Survey of Durham, states, that one-third of the coal being already got, the coal districts will be exhausted in 200 years. It is probable that many beds of inferior coal, which are now neglected, may in future be worked; but the consumption of coal being greatly increased since Mr. Bailey published his Survey of Durham, we may admit his calculation to be an approximation to the truth, and that the coal of Northumberland and Durham will be exhausted in a period not greatly exceeding 200 years. Dr. Thomson, in the Annals of Philosophy, has calculated that the coal of these districts, at the present rate of consumption, will last 1,000 years! but his calculations are founded on data manifestly erroneous, and at variance with his own statements; for he assumes the annual consumption of coal to be only two million eight hundred thousand tons, and the waste to be one-third more,--making three million seven hundred thousand tons, equal to as many square yards; whereas he has just before informed us, that two million chaldrons of coal, of two tons and a quarter each chaldron, are exported, making four million five hundred thousand tons, beside inland consumption, and waste in the working[1]. According to Mr. Winch, three million five hundred thousand tons of coal are consumed annually from these districts; to which if we add the waste of small coal at the pit's mouth, and the waste in the mines, it will make the total yearly destruction of coal nearly double the quantity assigned by Dr. Thomson. Dr. Thomson has also greatly overrated the quantity of the coal in these districts, as he has calculated the extent of the principal beds from that of the lowest, which is erroneous; for many of the principal beds crop out, before they reach the western termination of the coal-fields. With due allowance for these errors, and for the quantity of coal already worked out, (which, according to Mr. Bailey, is about one-third,) the 1,000 years of Dr. Thomson will not greatly exceed the period assigned by Mr. Bailey for the complete exhaustion of coal in these counties, and may be stated at three hundred and fifty years. [1] The waste of coal at the pit's mouth may be stated at one-sixth of the quantity sold, and that left in the mines at one-third. Mr. Holmes, in his Treatise on Coal Mines, states the waste of small coal at the pit's mouth to be one-fourth of the whole.