1 6 Cramp, Measurement of Air Velocities. 



From which it is evidently easy to determine the velocity 

 from the manometer reading. 



The great advantage of the instrument is that static 

 head is entirely eliminated, and the velocity is measured 

 practically independently of any other condition. Thus, 

 if the manometer be read in millimetres and the velocity 

 in metres per sec. for o° C. and 760 m/m Bar. the con- 

 stant is : — 



^' = 3-33 VA 

 while for 15° C. it becomes : — 



t;=3.4i Jp. 



I have not been able to carry out careful tests with 

 this instrument, but so far as I have been able to use it 

 it agrees well with the Brabbee tube measurements, and 

 therefore it must be fairly accurate. 



Where clean gases are being dealt with, the form of 

 Pneumometer invented by Krell is likely to be more 

 accurate than that invented by Prandtl ; for the former 

 has no obstruction near the disc which can vary the 

 pressure or cause eddy readings. Where, however, as is 

 usually the case, there is dust in the stream, the small 

 holes in the middle of the disc are liable to become 

 choked, and its superior theoretical construction is then 

 less useful than the more practical form of the other 

 inventor. In the latter case it will be noted that even 

 large pieces of material in the air would not influence the 

 reading so long as the instrument is not damaged. 



My acknowledgments are due to Messrs. Henry 

 Simon, Ltd., who placed at my disposal the apparatus here 

 mentioned, and to Mr. C. P. Kinnimonth, B.Sc.Tech., my 

 assistant, who took many of the readings and helped me 

 in every possible way. 



