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 in which h height of mercury at t°, h, height of = mercury at freezing-point, m = co-efficient of expansion of mercury per degree Fahrenheit = 0.0001001. 9990 = 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 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. Problem. Require the amount, in cubic feet, of air and gas that may be expected to be given off for 1,000 cubic feet of open space in the goaves, or other waste places, by a falling of the barometer from 30.4 inches to 28.75 inches. The pressure at 30.4 inches = 2,148.52 pounds cubic feet of gas, which, theoretically, may be given off by a reduction of pressure equal to that indicated above. Experience in the use of the barometer in mines has shown that its indications are from one to three hours behind what is actually taking place. On this account it has been asserted that the barometer is not to be relied upon to give warnings. According to the "Colliery Guardian," Jan. 31, 1883, the government issued and sent out in England, during the year 1882, thirtytwo warnings, nineteen of which were justified by subsequent events. Twelve were followed in three days by explosions which caused one hundred and thirty-nine deaths; two were followed on the fifth day by explosions causing forty-three deaths; twenty-three lives were lost on the sixth day after the warning-showing that a total of two hundred and five lives were lost in six days from the issue of the warnings, while five lives were lost on the day of the issue. We have noticed, when there have been explosions. telegraphed from England, that, within a short time, explosions have occurred in our American mines. This warning should never be slighted, whether the explosions are due to falling or rising barometer. These teachings, we are aware, conflict with some of the late writings on this point; but we are so convinced by |