of the air-current. Formerly all the galleries were united so as to form one long and sinuous airway, of which each extremity emptied into the atmosphere directly, or into the pits. The air descended one of these, traversing its whole length, and finally ascended the upcast shaft. Places were stowed, and doors erected, so as to direct the air-current at various places. The inconveniences of this system arose from the length of the air-course, which became so much more as the mine developed, and which by the friction diminished the volume of air. The current in some part of the mine is pretty sure to meet with gases of some nature or other, so that, if they be mephitic, the air will pass over the workmen in the return, and injure their health; and, if the air has acquired sufficient inflammable gas, the same workmen may explode it with their lamps. In two collieries alone, where this system of ventilation was adopted (Lundhill and Risca), three hundred and twenty-nine lives were lost. The tendency of a current of air is naturally to choose, in passing from point to point, the nearest route, and that which has the largest section. If allowed to pursue its own course, the air will not always be renewed in galleries which require it.

28. To overcome this difficulty, doors and regulators are employed; the former to conduct the air where it

74

MINE VENTILATION.

§ 28.

cas

is needed, the latter to cause a diminution of the volume of air in certain portions of the mine where only a portion of the current is needed. Should permanent stoppings be required for old ways or abandoned passages, they may consist of solid masonry, tightly stowed, so as to leave no passage for a single thread of air: on the contrary, they must be movable if the trams and men pass along the gallery. For this purpose, recourse is had to doors, which are usually placed in pairs, but so far apart that one may be shut before the other is opened. Regulators are employed to prevent more than a required quantity of air from passing into certain districts, and to cause the excess to pass into other parts of the mine. They are simply slidingdoors, whose open area may be increased or diminished as required by circumstances, and which can be locked fast at any area, the key, of course, being retained by the boss.

(a) To illustrate these regulators, let us assume an airway 5' x 5' passing 10,000 cubic feet of air per minute a regulator is put in, which contracts it to 20-foot Find the quantity of air that will pass, power and pressure remaining the same.

area.

From § 23, 5, we find that "the resistance increases as the square of the velocity of the air-current," also we know that sudden contraction of an airway will

cause diminution of the discharge; then, according to § 23, 7, we have

√25 × 25: √20 × 20:: 10000: 7152 cubic feet. Ans.

(b) As we have regulated the discharge, and diminished the quantity, we may increase the discharge by enlarging the airway.

Problem. What volume of air would flow through an airway 10 feet square, when 6,000 cubic feet per minute flow through an airway 5 feet square, the pressure and length being the same?

√20: √40

:: 6000: x, or

√40 × 100 × 6000
√20 × 25

= 33840. Ans.

25: 100

In the above case the area was enlarged four times. The resulting volume would have been increased four times also, viz., 24,000 cubic feet, had it not been that the perimeter only increased to 40 feet instead of four times its former size, or 80 feet. The volume may be found thus:

√80: √40:: 24000: 33840 cubic feet. Ans.

29. THE splitting of the air-current of a mine, when not carried to extremes, is very advantageous, as it secures a greater volume of air at the expense of the same motive-power. The different divisions of a mine are never equally developed; and, if proper care be not taken, districts requiring the most air will receive only a small portion, while other districts requiring but little will receive large quantities. Common sense suggests, that, of two developed districts of a mine, the more developed of the two should receive the larger quantity of air: the air should therefore be distributed in a systematic manner, basing the quantities for each district on to the number of bends, extent of rubbing-surface, and other resistances. This aim is attained, as mentioned above, by the "sliding shutter," which, being placed so as to admit a small volume of air, causes whatever excess there may be to pass into other parts of the mine where the air is more needed.

It may happen that a "blower" of gas is met with, which renders the air explosive in its district. When it is perceived, the regulators should be immediately

opened, and a greater circulation produced, in order to carry away the inflammable gas. During this time work in the other districts need not necessarily be stopped, unless it is found necessary to pass the whole current through the dangerous district.

If the gases be given off in such quantities that an explosion takes place before the miner perceives the explosive state of the air, the workmen in the other districts will be protected from the flame of the explosion, and the "after-damp" will only momentarily check the current; and, if the return airways are of large sectional area and no great length, the current will scarcely be interrupted, save, perhaps, in the district where the explosion occurred, the doors and regulators of which have been destroyed. Where gas is given off at several points in a mine, ventilated by a single current, the aggregate amount may be sufficient to render the current explosive, and cause an accident.

30. Either to increase the ventilating pressure, or to lessen the extent of rubbing-surface exposed to the air circulating in mines, is a very slow and costly manner of increasing the amount of ventilation; but by judiciously dividing the current, and leading it into several airways, the circulation will be much more active than