In 1773 there were only 13 collieries on the Tyne, and these had increased to upwards of 30 in 1800. The number of collieries in 1828 had increased to 41 on the Tyne, and 18 on the Wear, in all 59, producing 5,887,552 tons of coal. The out-put of coal in Northumberland and Durham in 1854 was no less than 15,420,615 tons, and now in these two counties there are 283 collieries. Mining began on the Tyne and continued on the Wear, where the industry has been largely developed. There are in all about 57 different seams in the Great Northern coal-field, varying in thickness from 1 inch to 5 feet 5 inches and 6 feet, and these seams comprise an aggregate of nearly 76 feet of coal. Taking the area of this field to be 750 square miles--a most probable estimate--we may classify the contents as household coal, steam coal, or those employed in steam-engine boilers, and coking coal, employed for making coke and gas. Of household coal there is only 96 square miles out 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