evolution of gas, or absorption of aqueous vapor in the mine, we have in the ventilator drift, 100000 X 31.25" x (60° +461) =107900 cubic feet. In order to find the amount of work which is expended in producing ventilation, and what amount is lost in overcoming friction, it is necessary to use an indicator, the diagram of which will give us the effective horse-power, which differs from the nominal or theoretical horse-power. Problem. Find the per cent of power used by a fan with the following data: Area of piston 484 inches; stroke 2 feet; speed 60 revolutions, or 240 feet of piston per minute; indicated effective pressure on piston 20.89 pounds per square inch; then, 484 x 240 × 20.89 = 73.54 horse-power engine. The air in fan-drift measures 106,680 cubic feet per minute. Water-gauge in fan-drift measures 2.8". Then 106680 x 2.80 × 5.2 33000 = 47.06 horse-power in the air. Therefore the per cent of power utilized will be 73.54:47.06 :: 100: 64 per cent. Ans. 104 MINE VENTILATION. $39. THE BAROMETER. 39. It is supposed that Torricelli derived from Galileo the definite conception of atmospheric pressure. Pascal, however, was first to state that the mercurial column decreased in length as we ascend. This experiment was for the first time performed at Clermont, on the top of the Puy de Dôme, Sept. 19, 1648. The barometer in its simplest form consists of a tube, about thirty-four inches long, closed at the top. This tube is filled with mercury, then inverted in a vessel containing mercury. The atmospheric pressure on the vessel of mercury will force the mercury up the tube, or let it sink, according as that pressure is greater or less. These risings and fallings are measured by means of a scale. As mercury expands by heating, it follows that a column of warm mercury exerts less pressure than a column of the same height at a lower temperature. It is usual, on this account, to reduce the actual height of the column to the height of a column of mercury at the temperature of freezing water, which would exert the same pressure. The formula for this correction is h= in which h height of mercury at t°, h, height of mercury at freezing-point, mco-efficient of expansion of mercury per degree Fahrenheit = 9990 99900.0001001. When very exact readings are required, corrections must be made for expansion of the scale by which the height of the mercurial column is measured, also for capillarity. (a) The Aneroid Barometer is a thin metallic vessel partially exhausted of air, and sealed: consequently it will expand or diminish in size as the atmosphere is lighter or heavier. This change in size, M. Vidi made use of, and transmitted the movement to an index. The Aneroid Barometer is a very convenient instrument; as it is round, and of small compass. In the second geological survey of Pennsylvania it was used, to a great extent, for determining heights, and making contours, in the anthracite coal-regions. Good Aneroid Barometers are compensated for differences of tempera ture. (b) Atmospheric pressure will, according to the condition of the weather, vary from 28.5 to 31 inches of mercurial column. When the barometer rises, the thermometer usually falls, and vice versa. The discharge of gas becomes greater when the barometer falls, because the atmospheric pressure which before kept the gas pent up is lessened; and hence, wherever the pressure of gas is strong enough to overcome the lessened. 106 MINE VENTILATION. $39. atmospheric pressure, it escapes. The barometer is useful, therefore, as it will give warning when an increased discharge of gas will take place; and hence precautions may be taken to overcome it by increasing the volume of air. Sudden falling of the barometer is much more dangerous than a gradual fall; for in the first case more gas will be given off in less time than in the second. When the barometer is 27 inches, the pressure of the atmosphere per square foot is 1,908.23 pounds; at 28 inches, it is 1,978.90 pounds; at 29 inches, it is 2,049.58 pounds; at 30 inches, it is 2,120.25 pounds; at 31 inches, it is 2,190.93 pounds. The following table will be found useful in order to ascertain the pressure per square foot, or fractional part of a foot, for a given height of the barometer. |