Development.—After a property has been prospected and it is definitely known that it contains mineral deposits of economic import ance the development proper begins by the con struction of shafts, entries or slopes, and the blocking out of the ore or coal which is to be taken out at a later date. This work is usually done preliminary to the building of a mill or other reduction works.
In the development of mines one of the important factors to be taken into consideration is the matter of drainage and ventilation. Of course drill holes and pre liminary shafts indicate to a certain extent the amount of water that will be encountered, but as the mine workings are extended larger areas are opened for the entrance of ground waters, thus increasing the amount of water which in the small mines would be considered negligible. When a mine is opened on a hillside, the drainage is usually taken care of by an adit and all of the mine workings drained from the lowest level, the upper workings in this case being absolutely without water. Where a mine cannot be thus drained, it is necessary to install large 'and expensive pumping plants, as fre quently the amount of water handled is far in excess of the coal or ore produced.
The ventilation of mines is also an important item to be considered, espe cially when mines are opened to a considerable depth. The ventilation of coal mines has been
well worked out in the anthracite and bitumi nous fields by reason of the fact that many of the mines generate noxious or poisonous gases, making it impossible to work without adequate ventilation. The metal mines as a rule do not generate gas, therefore their ventilation has not, in many cases, received proper attention. Natural ventilation in metal mines is usually provided for by surface openings at different elevations which, in most cases, give a very satisfactory distribution of fresh air. Where there is but slight difference in elevation of two or more openings it is necessary to install mechanical ventilation whereby fresh air is supplied by fans. Coal mines, and frequently metal mines,. are often opened by a drift-slope tunnel or adit with no outlet at the inner end, so that it is necessary to use a force fan to send in new air to the working face. The fresh air then circulates through the places in the mines where the men are at work: the air thus circulated becomes more or less vitiated as its ts oxygen content, and is also burdened with gas and dust, before escaping from the mine. Its escape is often assisted by a suction fan. The circulation of large volumes of air is necessary to make working conditions tolerable in gaseous mines.