Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1911 Excerpt: ...be frequently tested in order to check the correction for friction. The method at one time used by the writer was to have constructed a wooden box 1 foot square and 60 feet in length, one end of which passed through a brick stopping into the main return air-way of the mine, near to the upcast shaft, the other VOL. IV. G end opening into the main intake air-way, so that a strong current of air passed into the return. A little gunpowder was fired at the intake end of the box, and the passage of the smoke observed by means of glass windows in the box. The anemometer was then placed inside and half-way along the box, and the vanes revolved for one minute, and the correction determined, e.g. the time taken by powder smoke to pass over a given distance, say 60 feet, is 23 seconds; therefore 60 cubic feet of air passes per 23 seconds, and 23: 60:: 60: 156 cubic feet per minute. Number of revolutions recorded by the anemometer in one minute = say 105, and 156--105 = 51, which is the addition which must be made for friction. By means of a sliding door or regulator at the "return" end of the box, a very considerable variation in velocities, and a consequent series of corrections, can be secured. When using the anemometer for measuring velocities of air-currents in mines, care should be taken to hold it at arm's length and at right angles to the direction of the current. As the sides of a roadway or shaft offer considerable resistance to the flow of air, the maximum velocity of current will be in the centre of the road. In order, therefore, to correctly determine the volume of air passing along the road, it will be necessary to take an average of the velocities, which can be done either by moving the instrument about in a zigzag direction over the sectional ...
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