220 HYDRAULICS AND ITS APPLICATIONS 



heights of the two columns) as thus obtained were 1*006, '993, and 

 1-034. 



In modern types of the Darcy tube the static pressure is taken from a 

 series of small openings in the walls of a tube held parallel to the 

 current, and by this method, using extremely small openings, the effect 

 of eddy formation is greatly reduced. Such a tube, as used for pipe flow 

 work by Professor G. S. Williams, 1 is shown in Fig. 101. Here the 

 statical pressure is transmitted through four openings, each % inch 

 diameter, while the impact orifice itself is -% inch diameter. The tube 

 was manipulated through a stuffing-box in the wall of the pipe, this 

 permitting the orifice to be adjusted to any required radius. 



The tube shown in Fig. 101 was rated in two different ways. By 

 moving it at a known speed through still water in a circumferential 

 trough, of approximately 12 feet diameter and of 72 square inches 

 cross sectional area, the mean value of G was '926, while using it to 

 determine the velocity at the centre of a 2-inch pipe whose discharge 

 was caught and weighed, on the assumption that the central velocity 

 was 1*33 times the mean velocity the mean value of C was found to be 

 895. The values obtained by this second method may reasonably be 

 expected to be lower than those obtained by the first method, since the 

 velocity at the section of the pipe containing the statical pressure 

 orifices is of necessity greater than in the plane of the orifice, and the 

 pressure as recorded by the static column will consequently be less than 

 in the latter place. This effect will be more marked the greater the ratio 

 of the cross section of the tube to that of the pipe, and unless calibrated 

 in a pipe of approximately the same dimensions as that in which it is to 

 be used, the results of such ratings can only be approximate. 



On the whole it would appear preferable for pipe work to use a simple 

 Pitot tube to measure the impact pressure, and to obtain the static 

 pressure from an orifice in the pipe walls at the same level as, and in the 

 plane of, the impact orifice. Using the tube in this manner C is found 

 to be very sensibly equal to unity. 



When used for measuring the velocity of flow in an open jet the 

 necessity for the static pressure tube vanishes, and under such circum- 

 stances the point of a stylographic pen has been found to give good 

 results as the impact branch of the tube. 2 



(4) The velocity may be obtained by Venturi meter (Art. 196), when for 



1 See u Proc. Arn. Soc. C.E.." vol. 27. 



2 For the description of a Pitot tube for high velocity jet work see a paper by W. R. 

 Ekman, " Proc. Inst. Mech. Eng.," 190910. 



