of the total 750, all the remainder being steam or coking and gas coal.  The greater part even of this 96 square miles has been worked out on the Tyne, and the supply is rapidly decreasing also on the Wear, where the largest bulk of the household coal lies.  The collieries of the Tees possess but six square miles out of the 96, as far as we at present know.  Turning, however, to that part of the coal-field regarded as precarious, and consisting of first, second, and third-rate household coal, we have for future use 300 square miles.  London was formerly supplied from the pits east of Tyne Bridge, where is the famous Wallsend Colliery, which gave the name to the best coal.  That mine is now drowned out, and, like the great Roman Wall, at the termination of which it was sunk, and from which it derived its name, is now an antiquity.  There is now no Wallsend coal, and the principal part of the present so-called coal comes from the Wear, but the seam which supplied that famous pit is continued into Durham, and that seam, or its equivalent, sends a million or two of tons every year into London.  The supply, however, in this district is rapidly decreasing.  Careful calculations have been made as to the probable duration of this coal, of which the following is a summary.  The workable quantity of coal remaining in the ten principal seams of this coal-field is estimated at 1,876,848,756 Newcastle chaldrons (each 35 cwt.).  Deducting losses and underground and surface waste, the total merchantable round or good-sized coal will be 1,251,232,507 Newcastle chaldrons.  Proceeding on this estimate, formed by Mr. Grunwith in 1846, we may arrive at the probable duration of the supplies:  taking the future annual average of coal raised from these seams to be 10,000,000 of tons—­and this is under the present rate—­the whole will be exhausted in 331 years.  A still later estimate was made by Mr. T.G.  Hall in 1854, and he reckoned the quantity of coal left for future use at 5,121,888,956 tons; dividing this by 14,000,000 of tons as the annual consumption, the result would be 365 years; and should the annual demand arrive at 20,000,000 of tons, the future supply of this famous coal-field would continue for 256 years.  The total available coal (1871) in the British coal-fields, at depths not exceeding 4000 feet, and in seams not less than 1 foot thick, is 90,207,285,398 tons, and taking into account seams which may yet become available, lying under the Permian, New Red Sandstone, and other superincumbent strata, this estimate is increased to 146,480,000,000 of tons.  This quantity, at the present annual rate of production throughout the country—­namely, 123,500,000 tons—­would last 1186 years.  Other estimates of various kinds relative to our coal supply have been put forth:  some have asserted that, owing to increasing population and increasing consumption in manufactures, it will be exhausted in 100 years, and between this extreme and that of 1186 years there are many other conjectures and estimates.