366 On the Value ofthePitot Constant. 



was 01 mm. from the side. This was evidently due to a 

 change in the disturbance of the flow ; in the former case the 

 flow was restricted to one side of the pitot only, while in the 

 latter it was more uniform. This effect was still more marked 

 in the case of a pitot formed by making a hole in the side of 

 a tube stretching across a diameter of the pipe : the flow past 

 such a pitot close to the wall of the pipe is still further 

 modified. 



Owing to the constriction introduced into one side of the 

 gauge connexions by these narrow pitots, it was difficult to 

 obtain accurate results because the two sides of the gauge 

 never filled up at equal rates ; a small disturbance, such as a 

 slight change in velocity or a sudden draught, thus often 

 produced a temporary displacement of the gauge-surfaces 

 which, at low velocities, was comparable with the pitot 

 pressure itself. In fact all the low velocity work, even with 

 the large pitot, could only be undertaken on days of atmo- 

 spheric calm. The disturbing effects were decreased, but 

 never eliminated, (1) by placing a capillary tube in the 

 circuit on the other side of the gauge, and (2) by making the 

 volumes of the connecting tubes and of the air spaces in the 

 gauge as small as possible. Many more readings will, 

 however, be necessary before a quantitative discussion of the 

 readings with wedge pitots can be presented, but the results 

 as yet obtained seem to show a similar apparent drop in K, 

 with decreasing velocity. 



Summary. 



1. The value of the pitot constant obtained by a centrifugal 

 method is 1'002 and is constant between velocities 60 and 

 1400 cms. per second. 



2. The method of determining the pitot constant by 

 measuring the distribution of pitot pressure across a pipe is 

 unreliable at small velocities. 



3. If a small correction proportional to the mean rate of 

 flow through the pipe is added to all the pitot pressures across 

 a section of the pipe, this method also leads to a value 1*00 

 between the limits 6-2000 cms. per second. 



4. A very small pitot possessing very thin walls gives in 

 general a pitot constant several per cent, above unity. This 

 is readily explained in terms of effects known to be present 

 at the edges of plane surfaces placed normal to a current 

 of air. 



