170 ANEMOMETER 



dimensions as could bo found, and after measuring off a distance of 100 to 150 yards 

 in length, taking a lighted candle and walking in the direction of the current, 

 holding the flame in such a position as to be fully exposed to the influence of tho 

 current, but taking care to walk at the particular rate required, to cause tho flame to 

 burn in an upright position, without being deflected from the vertical, either by the 

 current or by the progress of tho person carrying it. The time required to traverse 

 the distance measured off, being noted by a seconds-watch, enabled the average rate 

 of walking to be determined ; and the average rate so found, from three or four trials, 

 was assumed to be the velocity of the air-current ; and this, multiplied by the average 

 sectional area of the part of tho air-way selected for the experiment, was taken to 

 represent the quantity of air passing in the unit of time. Formerly, when this mode 

 of measuring the air in mines was in use, it would afford a close approximation to the 

 truth ; but, with the ventilation now existing in many of our large mines, it would 

 not be practicable to walk as quickly as the currents travel in the principal splits ; and 

 running is not a sufliciently steady pace. One of the objections to this, as well as to 

 all other methods that require a considerable distance to be traversed, over which to 

 observe the velocity, is the difficulty of obtaining a gallery of equal area throughout 

 over a sufficient distance ; but in cases where this is attainable, this method admits of 

 great accuracy for velocities up to 400 feet per minute. 



One of the principal of the second modes employed for the measurement of air, 

 consists in observing the velocity of the smoke from an exploded charge of gunpowder 

 in a part of the gallery, of nearly uniform sectional area ; and this, until recently, was 

 the means most generally adopted in the coal mines of this country, for ascertaining 

 the velocity of air-currents ; and although it has of late been largely superseded by 

 the use of the anemometer, the practice is still in considerable use, and, so far as 

 regards shaft-velocities, remains tho only method. It is, therefore, desirable to 

 ascertain how far the results obtained by this, and similar methods of measuring air- 

 currents, can be relied upon for accuracy ; and to investigate the various sources of 

 error connected with them, with a view of either avoiding or making proper allowances 

 for their effects, po far as may be practicable. 



The sudden explosion of gunpowder in the confined passages of mines produces 

 several effects, which tend to cause inaccuracies in the results obtained by noting the 

 passage of the smoke, as an index of the velocity of the current. 



Experiments prove (as, indeed, might have been anticipated, considering the small 

 quantities of gunpowder used), that in general neither the increase of bulk due to the 

 introduction into the current of the products of combustion, nor that due to the elevation 

 of temperature, have any appreciable effect on its velocity. But other experiments 

 show that the force of the explosion, when a considerable quantity of gunpowder is 

 used in a feeble current, gives an impulse to the current, and creates a velocity in 

 excess of the normal one. A revolving anemometer was placed in an air-passage 

 traversed by a feeble current, so regulated as to be just strong enough to produce 

 thirty revolutions of the instrument per minute. The explosion of a cubic inch of 

 gunpowder, at a distance of seventy feet, did not in any way affect the instrument ; 

 but when the charge of gunpowder was increased to twenty cubic inches, the explo- 

 sion caused a sudden and violent increase of its rate of revolving, acting as a tem- 

 porary impulse, the revolutions very quickly decreasing to the original number again. 

 The amount of error arising from this source, and which tends to increase the 

 apparent velocity, depends on the quantity of gunpowder used, the sectional area of 

 the air-way, and the velocity of the current, increasing with the quantity of gunpowder 

 employed. 



These errors may be overcome by using anemometers, or apparatus of various 

 forms ; and these may be divided into three classes : (a) Anemometers having vanes 

 or wands, made to revolve by the current of air impinging upon them, the rate at 

 which they revolve being indicated by pointers on dials forming a part of the instru- 

 ment the pointers being made to revolve by means of wheels connecting them with 

 the axis of the vanes or wands. The anemometers of Combes, Biram, Whewell, Osier, 

 and Robinson, are instances of this class of instruments now in use in this country, all 

 of which require a correction for friction. (6) Instruments which are affected by the 

 force or impulse of the wind, without being subjected to any continuous revolving 

 motion, such as Dr. Lind's, Henaut's, Bongui's, and Dickinson's anemometers, (c) 

 Anemometers of a more complex character, such as Leslie's. 



One of the most common forms of anemometer is that devised by Mr. Combes. 

 This consists of a delicately mounted axle turning with the utmost freedom, upon 

 which are mounted four rectangular piano wings, equally inclined as to a plane 

 perpendicular to the axis. In tho middle of the axle is an endless screw which drives 

 a wheel with a hundred teeth : this is adjusted so that it advances one tooth for each 

 revolution of the axle. Tho first wheel carries a cam, which acts upon the teeth of a 



