made in the form of two chambers with a glass front and a connecting aperture, the size of which can be regulated by a tap. This is Dickinson's water-gauge,* which is a brass box 6 inches high, 4 inches wide, and 3 inches deep, with a partition in the middle, making two chambers each 2 inches by 3 inches. A glass front shows the two columns of water, and a scale, graduated into inches and tenths, enables the difference in their heights to be measured.

Efficiency of Ventilating Appliances.-The efficiency of a fan or other ventilator is calculated by comparing the work which it does in drawing air through the mine, with the work done by the steam in moving the piston of the engine that drives it.

The work done in moving air is reckoned from the volume displaced and the pressure; the former is ascertained by the anemometer and the latter by the water-gauge. As a cubic foot of water weighs 62:425 pounds, each inch indicated by the watergauge will represent pressure of one-twelfth of this amount, or 5.2 pounds per square foot. A depression of the water-gauge of 2 inches will mean 2 × 5'2 or 10'4 pounds pressure per square foot. In common parlance the word "depression is understood, and the miner speaks of a water-gauge" " of 2 inches, for instance, meaning thereby a depression of the water-gauge.

66

The work done is looked upon as that of pushing a volume of air through a pipe under the pressure indicated by the watergauge. Let A represent the area of the airway in square feet, V the velocity of the air current in feet per minute, as measured by the anemometer, W the water-gauge in inches, 5'2 pounds being the weight of a column of water one inch high with an area of I square foot, E the useful effect of the ventilator. Then

E =

(A V W × 52) foot-pounds per minute.

To ascertain the horse-power it is only necessary to divide by 33,000, and we may state:

If the quantity of air in circulation, A V, is 100.000 cubic feet per minute, the water-gauge 15 inches, the useful effect of the ventilation will be:

The efficiency of the ventilating plant is the ratio of the horsepower of the ventilation so calculated to the indicated horsepower of the driving engine.

Dickinson, op. cit., p. 12.

Supposing that the indicated horse-power was found to be 45, we should have the ratio of 23'63 to 45 as denoting the efficiency. In other words:

Resistance caused by Friction.-The amount of power required to overcome the friction of the air current in passing through the passages of the mine must be studied, because it is an important factor in the problem of ventilation; and unless its effects are appreciated the best method of arranging the ventilation will not be understood.

The amount of friction depends upon five conditions:

1. The length of the airway, which we may call L.

2. The perimeter of the airway, P.

3. The sectional area of the airway, A.

4. The velocity of the current, V.

5. The nature of the rubbing surface, the effect of which may be expressed by a co-efficient C.

The friction is directly proportional to the length of the airway and its perimeter; in other words, if there is twice as much rubbing surface, there is twice as much friction. It is inversely proportional to the sectional area of the airway-that is to say, a level 7 feet high and 10 feet wide will cause only one-half of the friction produced in a level of the same height, but 5 feet wide. Lastly, the friction increases as the square of the velocity. These relations may be expressed by the general formula :

It is evident from this formula that it is desirable to shorten the path of the air as far as possible; much is done in this direction nowadays by "splitting" the air current-that is to say, dividing it into separate branches instead of causing the whole of the current of the downcast shaft to travel through the entire length of the workings.

With regard to the second factor, the perimeter, it may be well to notice that a circular section is the one with which a given length of perimeter affords the largest area. Take, for instance, the case just cited of a rectangular airway, 7 feet high by 5 feet wide, with a perimeter of 24 feet and an area of 35 square feet. A circle having a circumference of 24 feet would have an area of 45.8 square feet, or 30 per cent. more than the rectangle.

Splitting has also the effect of reducing the velocity required for the passage of a given quantity of air through the mine. Suppose that 90,000 cubic feet are wanted per minute in order to ventilate the mine; if the mine is divided into three equal and similar districts and each is ventilated separately by one

third of the main current, the velocity of the minor currents need be only one-third of what would have been necessary if all the air had had to travel by one road. Reducing the velocity to one-third means, according to the formula, a diminution of the resistance caused by friction to one-ninth.

The co-efficient, C, varies according to the nature of the rubbing surface; in smooth passages, such as those of levels lined by an arching of brick, it will naturally be less than in the irregular airways along the working face, or in an airway with frames of timber, forming a succession of projecting obstacles at short intervals.

If the resistance due to friction, or, in other words, the pressure required to overcome it, is measured in pounds per square foot, then taking L and P in feet, V in thousands of feet per minute, and A in square feet, the co-efficient C varies from 0'002 to 0°014* according to the nature of the airway.

Mining engineers owe a debt of gratitude to M. Murguet for his graphic representation (Fig. 589), which illustrates the influ

* Elwen, "An Account of Experiments on the Resistance to Air Currents in Mines," and Walton Brown in the discussion. Trans. N. E. Inst. M. E., vol. xxxviii., 1888-9, p. 205-218.

↑ "Recherches Expérimentales sur la Perte de Charges dans les Parcours Souterrains," Bull. Soc. Ind. Min., vol. vii., 1893, p. 5; and translation in Trans. Amer. Inst. M. E., vol. xxii., 1893-1894.

ence of the sides of an airway in a most striking fashion. He compares three kinds of airways: one arched, A B C; a second, DEFG, in bare rock; and a third, H I J K, lined with timber; and he shows that, with the dimensions given in the figure, all three airways produce the same amount of friction, or cause the same loss of "head." In other words, the arched passage A B C, in spite of its small dimensions, offers no greater resistance to the air current than the large timbered tunnel HIJ K; whilst you may put the brick lining A B C inside a level D E F G without in any way requiring additional ventilating power. He concludes that it is more important to diminish the friction in the airpassages than to seek for better ventilators, and that the miner can lessen the resistance to air-currents not only by increasing the size of his levels, but also by lining them with brick or stone in place of timber, and by keeping them as straight as possible.

CHAPTER XI.

LIGHTING.

Reflected daylight-Candles, candle-holders-Lamps and lamp oil-Wells light-Magnesium wire-Safety lamps: Davy, Clanny, Mueseler, Marsaut, Hepplewhite-Gray-Locks: lead rivet, magnetic bolt, Cuvelier's lock-Coal gas-Electricity.

MINES are usually lighted by candles, torches, lamps, gas, or electricity.

In a few cases the miner does his work without artificial light. In sinking oil-wells in Burma,* the quantity of explosive gas is so great that no naked light can be used, and even if the workman had a safety lamp, he would be unable to stay below ground long without being affected by the noxious atmosphere. He therefore carries no light at all, and has his eyes bandaged up before he goes down, because otherwise it would take longer for his eyes to become accustomed to the semi-darkness of the bottom of the pit, than the whole time he can stay below ground.

Reflected Daylight. For sinking oil-wells in Japan + reflected daylight is used. A piece of yellowish translucent oilpaper, about 5 feet by 3 feet, is suspended over the well at an angle of 45° and throws light down the pit. The wells are about 3 feet square, and are dug to a depth of 600 to 900 feet.

In driving the Bell tunnel at the New Idria quicksilver mine,‡ in California, there was a disastrous explosion from the ignition of some inflammable gas, and after this occurrence the tunnel was lighted by the reflection of the sun's rays. A mirror was kept at the mouth of the drift at the proper angle to effect this, and with a straight tunnel and in a sunny country like California the device answered perfectly.

Candles.-The candles used by miners are very frequently the so-called "dips"-that is to say, they are made by dipping a wick into molten tallow and allowing it to take up grease; the process is repeated several times, until the thickness of tallow is sufficient. The wick is made of cotton, or of cotton and linen.

*

Noetling, Rec. Geol. Surrey India, vol. xxii., 1889, p. 97.

† Redwood, "Petroleum and its Products," Jour. Soc. Arts, vol. xxxiv., 1886, p. 832.

Becker, "Geology of the Quicksilver Deposits of the Pacific Slope," Mon. U.S. Geol. Survey, vol. xiii., 1888, p. 308.