This material is rich in carbon, and when used in gas-engines will furnish more power than the best Pocahontas coal when steam-engines are used. Thus one-fourth of all our coal is wasted at the mines simply because steam-engines instead of gas-producer engines have been employed. If in the future installation of power this fact is taken into consideration, it will make the cost less to the user, and at the same time utilize a large proportion of our impure coal and save the higher grades for other purposes. (2) In the mining of coal it was formerly the unfailing custom to leave supporting pillars of coal for the over-lying rocks to rest upon, to make suitable working-rooms, etc. These pillars, twelve to eighteen inches square, and higher than a man's head, are scattered throughout the entire mines and are usually of the highest grade coal. In many mines, also, a roof of coal a foot or more in thickness must be left because the material above the coal is not solid enough to prevent cave-ins. When the mine is abandoned and closed these pillars and roofings remain untouched, because removing them constitutes one of the greatest dangers to life, and is one of the frequent causes of mine accidents. It is improbable that the coal thus left in abandoned mines will ever be reclaimed, because not enough is left to make it profitable at present prices to re-open the mines; and frequently the rocks cave in about these pillars and make the task almost impossible. (3) By careless blasting an unnecessarily large amount of coal is blown into powder,--the slack which has not been marketed at all until within the last few years. Much of this slack, which is the best grade of coal in a pulverized form, is left inside the mines. These wastes in abandoned roofing, pillars, and small-sized coal, together make a total which for all the mines in the country will average fully one-fourth more of the coal that is in the ground. It is to be noted, however, that conditions are changing for the better. The most modern mines use fewer supporting pillars of coal, and these are of larger size, so that there is less danger of accidents. Wherever possible they use timbers of wood instead of these smaller pillars of coal. They also mine as near the top of the seam of coal as can be done safely, and so regulate the blasting that much less slack is made than by the heavy discharges. These changes in mining methods save a far larger proportion of coal, and also prevent many accidents, which are the most unfortunate feature of coal mining, and the one which should receive most careful consideration. (See chapter on Health.)