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(a) By the first method it is necessary to select as regular an airway as possible, in which is measured a length of from fifty to two hundred yards. This is traversed in the same direction as the current, a candle being held in the hand, whose flame must be kept vertical. The time of walking the distance, which is the same as the velocity of the air, is measured by a watch. The mean of two or three experiments gives a rough estimate of the velocity.

(b) By light bodies, such as down or powder-smoke, and noting the time it takes for the particles to pass from one point of the gallery to another. Under this head may be classed measurements made by ammonia, sulphuric-ether, etc. To measure air by volatile liquids, small phials containing the liquid are broken at a certain place in the airway, and the time noted that it takes to pass from this point of the gallery to another, the distance between the two having been previously ascertained.

By vapor-measurements Mr. Arnold constantly obtained the same results. This method, so simple in practice, is more exact than measuring by powdersmoke, down, or other light substances. Mr. Arnold considers it to be less subject to error than the best anemometer yet invented.

On the other hand, others claim that—the sense of

smell being more acute in one person than in another, and different for the same persons at different times it is impossible to measure the air as accurately as with an instrument.

Many experiments made with anemometers show a variation in their co-efficient at different velocities of the air-current. M. Guibal, in order to suppress this source of error, sought an approximate velocity; then, by using the co-efficient thus found, he made new experiments, which would give the true co-efficients. Two anemometers are required to do this: if they agree, the measurement is correct; if they do not, one of the two is wrong, and it is then necessary to ascertain which.

(c) Miners have long recognized the importance of knowing the volume of air for all pressures of the atmosphere: hence they regularly measure the air traversing all the main intakes in the returns and at certain points in the various districts. Some use powdersmoke; but that test has been practically superseded by the anemometer: however, whatever method is employed, the measurements are, or should be, made at certain prescribed times. There are many and various instruments for measuring the velocity of the air, among which may be mentioned Devillez's, Dickinson's, Briam's, Robinson's, Casella's, and Casartelli's ane

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MINE VENTILATION.

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mometers. Briam's is the one most generally used in this country, and is a modification of Robinson's as originally made for meteorological use. It consists of a series of vanes, which revolve by the action of wind. Each revolution is transmitted to dials by means of wheels and pinions. These instruments are made of various sizes, from four to twelve inches. The dials are six in number, marked for feet, hundredths, thousandths, etc. Whatever instrument is used, all that is required to ascertain the velocity is to read the figures on the respective dials before and after experiment, then to subtract the first from the second; to the remainder is added a value corresponding to the constant friction, and which will be found with the table that comes with each instrument.

The special formula is of the form

Ꮴ =ar + u

in which requals the number of revolutions per minute, a equals constant proportional to the number of linear feet traversed by the air in a revolution, and u represents the losses due to friction.

For the anemometer generally used in Pennsylvania, the correction is about thirty feet when the instrument is new and clean; but the dirt and grit, to which it is

more or less exposed, have, no doubt, a tendency to increase the friction, or the correction in time. To determine the amount of correction required, the instrument is placed on a whirling table; the anemometer is whirled around by the table revolving; the velocity of the table is then compared with the indications of the anemometer..

38. The method of procedure in conducting experiments to find the useful effect of fan-ventilation varies materially from the ordinary method of ascertaining the velocity of the air, because the revolutions of the fan, the indicated power of the engine, water-gauge, etc., must be considered.

The air is measured by the anemometer in the ventilator-drift if possible. At the place of measurement, strings or wires should be fixed so as to divide the drift into, say, ten divisions of nearly equal area. The anemometer should run at least one minute in each division; one minute interval should be taken for reading the instrument, and moving it to the next division. Simultaneously with the air-measurement, diagrams should be taken from the engine at intervals of three or five minutes. Each diagram should be accompanied with an observation of the water-gauge, and the number of strokes per minute of the engine-piston. The

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neval working-speed should be adopted for the experiment, and it should be maintained as miformly as possible throughout the trial. After completing the drift-measurements, a second air-measurement should be made, either in the intake or return airways, to check, in some degree, the drift-measurement. To make these check-measurements, move the anemometer uniformly over the whole area of the airway, for, say, two minutes, repeating the observation twice to avoid error. During these check-measurements, diagrams of engine, and observations of speed and water-gauge, should be taken, as in the first measurement.

The laws of Mariotte and Gay-Lussac may be applied to correct the volume of air measured in the intake or return airways to the condition of the ventilator-drift at the surface; namely, for pressure and temperature, as follows:

Problem.-Supposing the volume of air measured at the intake to be 100,000 cubic feet per minute, the required volume which it would occupy in the ventilator-drift is found to be 107,900 cubic feet with the following conditions: