14 Cramp, Measurement of Air Velocities. 



unit area expressed above as c* It will be seen that 

 the value of c varies slightly with change of velocity, 

 but that the least value measured was little less than 

 0'9. With no slip at the boundaries, and assuming 

 the value of yu for air to be constant (as it is practically 

 if the temperature be constant) the curves shown in 

 Fig. 4 should be parabolas, in which case the value of c 

 would be 0'5. It is very clear that over an area of 

 diameter = about half the pipe diameter, the velocity in all 

 the tests is extremely uniform, and that the difference in 

 the value of c is due more to the slope of the velocity 

 curve from the edge of this diameter to the boundary than 

 to any uniform change over the pipe area as a whole. 



It is also worth while remarking that in the case of 

 pipes carrying water the ratio c is of the order 0"85. 



Note on the Fneumometer, 



Recently there has been introduced into Germany 

 an instrument for velocity measurements known as the 

 Fneumometer. It was originally invented by Dr. Krell, 

 but a more convenient and more recent form is that 

 invented by Dr. Prandtl {Fig. 5). The chief advantage 

 of the latter lies in the fact that it may be used in a 

 stream of air carrying considerable quantities of dust 

 without becoming choked. The construction of the 

 instrument is clear from the figure, and its action is as 

 follows : — 



When a circular lamina or disc e is immersed in a 

 moving stream of fluid there is a small region on either 

 side of the lamina where the fluid is at rest. If the plane 

 of the lamina be at right angles to the stream, this region 



* Note. — If the velocity = (?>);- where r is the radius, the total flux 

 — J 2'nf {(p)rdr ; in the table the integration has, of course, been carried out 



graphically as <p is very complex. 



