2 Cramp, Measurement of Air Velocities. 



(3) Acceleration and retardation must exist in con- 



nection with the wheel, so that the operator 

 never knows in timing it when it attained its 

 final in the duct. 



(4) Friction must make an enormous difference to its 



readings, and it is impossible to eliminate this 

 or to make it follow any definite law. 



Thus in nine cases out of ten occurring in ordinary 

 practice the anemometer is unreliable. The sole instance 

 in which it may be of use is that in which the Admiralty 

 adopt it, viz., to measure the volume of air at the 

 open end of a tube carrying air, by a series of velocity 

 readings taken uniformly over the open end. Generally 

 speaking, the author has found it entirely unsatisfactory, 

 and he regards it more as an indicator than a scientific 

 instrument. 



The facing gauge consists of a tube with one end 

 square with its axis placed or shaped so that this end 

 faces the current to be measured, the other end being 

 connected to a manometer. If the other end of the 

 manometer be open to atmosphere, the pressure registered 

 will be the sum of the static head above or below atmos- 

 phere at the gauge tip and the kinetic head due to the 

 velocity at the same place. Thus the manometer reading 



where h is the static head of air, v its velocity, and p and p^ 

 the densities of air and the manometer fluid respectively. 

 Since p and p^ are usually known with sufficient accuracy, 

 V can be deduced if h is known, and the instrument being 

 small and accurate it can be used where anemometers are 

 impossible. Now careful tests show that/ can be obtained 

 accurately if the facing tube be {a) small in diameter, {b) 



