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.