The inaugural address of Sir William Armstrong, President af " The Institution of Mechanical Engineers," at its annual congress which assembled at Newcastle-on-Tyne, England, on the 3d August, made some encouraging statements in regard to the duration of the English coal fields, a subject that has latterly attracted much attention. He remarked " that coal had a special interest for them in a locality celebrated, since the earliest days of coal mining, for the production of that invaluable mineral. England, with her innumerable steam engines and manufactories, is more dependent upon coal for the maintenance of her prosperity than any other nation, and the question of the duration of her coal fields now very properly occupies the attention of a Royal Commission. The investigations of the Commission are not yet completed, but so far as they have gone the results are reassuring. He concurred in the probable accuracy of the announcement lately made by two of his fellow commissioners, that the total quantity of coal in this island will prove to be practically inexhaustible ; but until the complicated details of quantities collected by that Commission have been pivt together, and expressed in totals, it is difficult to judge with certainty or accuracy on the subject. Although the duration of our coal may, geologically speaking, be practically unlimited, we have still to consider the important question—How long will England be supplied with coal as good and as cheap as at present ? We have unquestionably made greater inroads into our best and most accessible coal beds than other nations have done into theirs ; and if foreign coals should grow better and cheaper, and ours dearer and worse, the balance may turn against us as a manufacturing country long before our coal is exhausted in quantity. It is clear that our stock of good coal is very large, but most of it lies at great depths, and one of the most important questions the Royal Commission has to investigate is the depth at which coal can be worked with commercial advantage. " The chief obstacle" the President continued," to reaching extreme depth is the increase in temperature which is met as we descend. He was justified by ascertained facts in saying that this rate of increase will, as a rule, prove to be not less than 1" Fah. for every 20 yards in depth, and there is reason to expect that it will be even more rapid at greater depths than have yet been attained. The constant temj)srature of the earth in this climate, at a depth of 50 feet, i s 50, and the rate of increase as we descend is to be calculated from this starting point. Adopting these figures, it would be found that the temperature of the earth will be equal to blood heat at a depth of about 980 yards, and, at a further depth of 500 yards mineral substances would be too hot for the naked skin to touch with impunity. It is extremely difficult to form an opinion as to the maximum temperature in which human labor is practicable in the damp atmosphere of a mine, and it is almost equally difficult to determine how much the temperature of the air in the distant parts of an extremely deep mine can be reduced below that of the strata with which it is brought in contact. It is certain, however, that the limit of practicable depth will chiefly depend upon the mechanical means which can be provided for relieving the miners of tho severest part of their labor, for maintaining a supply of sufficiently cool air at the working faces of the coal, and for superseding the use of horses, which suffer even more than men from highly-heated air. " For the relief of labor we must look to coal-cutting machines for improvement of ventilation to exhausting fans, and for the superseding of horses to hauling-engines driven by transmitted power. The employment of coal-cutting machines, worked by compressed air, conveyed into the mine by pipes, is already an accomplished fact; and when the difficulties and the objections which usually adhere, for a considerable time to new mechanical arrangements, are removed from these machines, they will probably attain extensive application. One of the earliest attempts at coal-cutting by machinery was described by the late Mr. Nicholas Wood, at the former Newcastle meeting of this Institution, and all the really practical results as yet obtained date from that period. The cooling influence of the expanding air as it escapes from these machines, will be a collateral advantage of considerable importance in the hot atmosphere of a deep mine. The air discharged from the pneumatic coal-cutting machines now in use in the Hetton Colliery, escapes into the mine at a temperature of 7 below freezing, and the cold air from each machine appears to be sufficient in quantity to lower the temperature of the circulating atmosphere by 1. If, as eeems to be probable, six or seven of these machines can be employed at each working face, we may by this means lessen the heat by a corresponding number of degrees, and thus afford very considerable relief. The employment of compressed air as a motive power, in substitution of horse traction, is also quite feasible, and may be expected to become quite general in very deep workings. As regards ventilation, the fan machines of the several constructions tried have already exhibited great superiority over the old method of ventilating by an upcast furnace shaft; and although the efficiency of tho furnace system of ventilation is increased by depth, there is reason to believe that the fan will maintain its_superiority to greater depths than are likely to be reached in mining." Facts Elicited During the Recent Debato in England Relating to Patents. The recent attempt to subvert the patent system in England and the discussion which followed, have elicited some interesting and instructive facts. For instance, it was shown that for centuries flour was ground under unpleasant conditions. The miller in the time of Chaucer had to work amid a cloud of flour which obscured the air, filled his nostrils, irritated his lungs, and lessened his profits. The millers of this generation had the same difficulty to contend against and the same lament to make, until their chief grievance was removed by an inventor. Countless attempts have been made to remedy the evil. These failed either because tho flour was drawn away too rapidly, and the waste increased, or because the draft was insufficient, and the nuisance became worse than before. At last, the golden mean was achieved by Mr. Bovill. He succeeded in adjusting the several parts of the millstones so as to multiply their efficiency, yet prevent any flour from filling the air. For this he obtained a patent. Instead of being grateful to the inventor and ready to pay a royalty to the patentee, the millers of England combined together to procure the patentee's ruin, by subverting his legal title to the fruit of his brains. What they want is free trade in this invention. Sir Roundell Palmer declares them justified in protesting against the act of him whose ingenuity has conferred a benefit on their trade. The plea i s that in process of time each miller could have made the discovery for himself. The supposition is that if there is a demand for an invention the supply is as certain as is the supply of loaves when corn is abundant. The difference between the two cases is a difference in substance as well as degree. Necessity may be the mother of invention in common speech, but without being so in actual experience. The most pressing demand for a particular improvement has no other effect than that of calling forth a host of suggestions, of which three fourths are foolish and the other fourth is inadequate, AH the millers in England had failed to make the change which Mr. Bovill made in their mills. Those who are constantly engaged in a pursuit have little tima to consider how best to improve upon their system of procedure. Nor are they disposed to admit that improvement is possible, e.ven while convinced that improvement is desirable. They oannot take an outside and impartial view of their position. The required change Js generally made from without. There were engineers before the time of Watt, hut none of them thought of making the improvements which he effected in the steam engine, and some of them did their best to denounce those improvements as visionary.