Manchester Me)iioirs, Vol. Iviii. ( 1 9 1 4), No. 7. 5 



obtained with the tip shewn in Fig. i, and so this was 

 adopted throughout their subsequent work. 



The gauge, however, which Messrs. Heenan and 

 Gilbert used would only read to 1/44 of an inch of water, 

 and further, by their very method of adopting baffles 

 they reduced the movement of the air. This, while 

 tending to increase the inductive action on the tip, would 

 also tend to produce a condition of the air quite different 

 from that existing in air having the same relative velocity 

 with respect to a stationary gauge. 



Some three years ago the author had occasion to test 

 accurately a number of fans, and he very soon discovered 

 that the tip recommended by Heenan and Gilbert gave 

 results inconsistent with its supposed accuracy. He 

 therefore determined to test it, if possible, by a method in 

 which the conditions should be those existing in the ex- 

 periments which he was about to carry out. To obtain 

 such a method was, however, very difficult, as it involved 

 separating the true pressure from the " induction effect " 

 without being able to determine the actual value of the 

 latter. Since the equation was : — 



Measured pressure = true static head + induction effect. 

 The only possible method seemed to be that of measuring 

 the pressure at a point where the true static head was 

 known. Now the only value of the latter that could be 

 properly known was zero : and an apparatus was therefore 

 constructed in which a zero point could be found, and 

 which was of such a size as to be a test under practical 

 conditions. 



Apparatus. This consisted of an ordinary six-bladed 

 centrifugal fan having a wheel 6^/^ in. wide at the outer peri- 

 phery, and 1 8 in. in diameter over the blades. The case was 

 properly constructed with diffuser and volute, the latter 

 having a mean outside diameter of 30 in. The eye was 10 in. 



