PITOT TUBE 



219 



The instrument as improved and used by Darcy and Bazin for stream 

 measurements 1 is shown in Fig. 100. It consists essentially of two 

 tubes, one drawn to a point and facing up-stream, and the other straight 

 and with an opening at its lower end whose plane is parallel to that of 

 motion of the stream. The air in both tubes is partially exhausted so as 

 to bring the water columns to a convenient height on the scale. A 



Opening '/32 -diam. 



FIG. 101. 



cock E' is provided by which the columns may be simultaneously isolated 

 from their orifices. The instrument may then be removed to a con- 

 venient position and the readings taken at leisure, Darcy and Bazin 

 rated this meter in flowing water by surface floats, and also by observing 

 the velocity at many points in a cross section and comparing the mean 

 so obtained with the known mean velocity ; and in still water by taking 

 readings from a boat towed with a known mean velocity. The respective 

 mean values of (7, in v = C V 2 g h' (h f being the difference in the 



1 Darcy and Bazin, " Recherches hydrauliques," 1865. 



