lO Qkkui?, Measurement of Air Velocities. 



set, and velocity changes were then obtained by means of 

 the shunt motor driving the fan, the speed being varied 

 by means of a shunt resistance. 



The velocity readings in these tables were obtained 

 by a carefully-constructed facing gauge forming part of 

 the Brabbee tube. The disturbance caused by this tube 

 in the current of air is probably less than that due to any 

 other form of tip, its tapered and thin shape allowing the 

 air to flow smoothly past it with little eddying. A further 

 advantage is that the actual velocity and pressure are 

 measured at nearly the same point in the air stream. 



Criticism of Results. 



In the first series of figures the difference in reading 

 between the Brabbee and the Disc tip is nearly imm. as 

 a maximum, and this would mean an error in the velocity 

 of nearly 30%, so that the question of the accuracy of the 

 pressure gauge is of enormous importance. The readings 

 generally showed so much more difference than I antici- 

 pated that I sought for a reason. I found that the air 

 many feet before the eye of the fan showed a whirling 

 motion due to change of direction on entering the eye. 

 This is a curious fact which was hardly to be expected, 

 and shews that air behaves much more like an elastic 

 solid than might have been anticipated. The distance 

 along the pipe from the eye at which this whirl had con- 

 siderable effect was as much as 20 feet. I therefore in- 

 serted just before the eye of the fan a wooden cross about 

 4 inches wide, forming four guide blades to prevent 

 rotation of the air. The results are shown in the second 

 series, and clearly indicate hat this whirl affected the 

 Brabbee very little compared with the disc. 



The total differences are shewn at the bottom of the 

 columns. When these are worked out per metre change 



