ANEMOMETER 



66 



Every one -who has occasion to use this anemometer should bo aware' that it does not 

 register the actual velocity of the air, especially in feeble air-currents ; nor yet the 



number of revolutions of the -wands, but 

 only a number proportional to the latter ; 

 and although it is of great value, as indi- 

 cating an increase or decrease in the velocity 

 from time to time, such as the periodical 

 variations in any particular current, it is of 

 comparatively little value, as generally 

 used, for ascertaining real velocities, such, 

 for instance, as occur in changing or split- 

 ting air-currents, when it is of great impor- 

 tance to know the actual quantities. To 

 obtain, -with this instrument, accurate 

 results, available for all purposes, it is 

 necessary, as -with Combes' anemometer, 

 to apply a formula to its recorded revolu- 

 tions, or, rather, to the number indicated 

 by the index, in order to ascertain the actual 

 velocity of any current; each particular 

 instrument requiring special experiments to 

 bo made with it, in order to determine the 

 value of the constants required to be em- 

 ployed in the formula. These constants, 

 however, remain the same for the same 

 instrument, so long as it remains in the 

 same condition, and are independent of the 

 velocities of the currents of air in which it 

 is employed. 



To obtain the constants of this formula, as applicable to any particular instrument, 

 it is absolutely necessary, in making tho experiments, to know correctly the true 

 velocity, as a standard of comparison. As before explained, none of tho ordinary 

 modes employed for ascertaining tho real velocities are reliable ; tho investigators, 



therefore, had a Whirling Machine constructed, tho wand of which, in revolving, 

 described a circle of 25 feet in circumference ; tho number of its revolutions being 

 indicated by a pointer on a dial. 



In tho first instance, this Whirling Machine was turned by tho hand, but as this 

 did not give a sufficiently uniform velocity, a small drum, and a rope with a descending 

 weight attached to it, was employed, to give motion to tho machine ; and worked thus, 

 it gave extremely accurate results, so far, at least, as regards tho uniformity of its own 

 velocity. By fixing tho anemometer on tho end of tho wand, tho velocity with which 

 it passes through tho air can bo ascertained and compared with tho revolutions of the 

 anemometer, as indicated on its dial. Fig. 67 represents this machine. 



