100 



NEGBETTI AND ZAMBRA, HOLBORN VIADUCT, E C., 



FIG. 110. FIG. 111. 



The above engraving of the Dial is the exact size of the Dial of the Instrument. 



The Meter may be fi^ed in the current on a rod, fitted into the socket, which screws into 

 the bottom of the instrument. 



To take a measurement fix the position of the hands (by moving the catch) write down 

 the reading, and place the Meter in the current of air to be measured. Now put the 

 hands in action by again moving the catch at the same moment, note the time by the 

 second hand of a watch, allow the fans to run in the current for one minute, at the end of 

 which time again put the hands out of action, and again read their position, subtract the 

 first reading from the second, and the result gives the velocity of the air in feet per minute 

 (unconnected). 



The Meter may be allowed to run ia the current of air for any convenient length of 

 time ; but, if for longer than one minute, the difference of the first and second readings 

 must be divided by the number of minutes of the running. This gives the (uncorrected) 

 velocity of air for one minute. 



A table is supplied with each instrument, showing the necessary correction for friction, 

 &c., at various velocities per minute. In the -second column of this table will be found the 

 correction (opposite the velocity shown by the Meter in the first column). This correction' 

 if applied to a measurement of more than one minute, must be multiplied by the number of 

 minutes of the measurement, and added to or subtracted from (according to the sign) the 

 difference of the two readings, 



EXAMPLE. 



Suppose the first reading to be . 



And the second after a running of ten minutes is 



5260 



11060 

 5260 



The running per minute would be . . , . 



Say the correction for 580 shown by the meter per minute is 



The real or corrected velocity per minute would be 



580 

 82 



662 



