MEASUREMENT OF VELOCITIES 



36') 



The Hydrometric Pendulum.. Where flow takes place through an uniform 

 channel of small dimensions, and where the velocity can be initially 

 determined by some accurate method, as by a weir, the hydrometric 

 pendulum may be calibrated so as to record this 

 velocity at any future time. The instrument 

 consists of a pendulum having a submerged 

 Spherical bob which is heavier than the water 

 which it displaces, and which hangs vertically 

 when the water is at rest. When in motion the 

 pressure of the water causes the pendulum to 

 take up an inclined position, the angle of inclina- 

 tion being a measure of the velocity, and from 

 a previous calibration, this may be read off 

 directly. 



Measurement of Flow in a Parallel Conduit. A 

 method recently devised 1 for measuring the dis- 

 charge from a parallel conduit consists in the 

 provision of a light but rigid apron of canvas 

 over a framework of angle iron, suspended verti- 

 cally from- a light carriage which runs on rails 

 fixed on either side of the conduit. When 

 lowered, the apron fills the conduit with very 

 little clearance and is carried along with the 

 same velocity as the stream. Its velocity is then 

 measured by a chronometer and electrical con- 

 tacts. This method necessitates a conduit of 80 

 to 100 feet in length, and has been applied to 

 conduits up to 20 feet wide and 12 feet deep, for 

 the measurement of the water supplied to tur- 

 bines under test. 



ART. 102. STREAM RATING TABLES. 



The usual object of velocity measurements in a 

 river is the construction of a rating table, which 

 shall show the relation at a given point between 

 the height of water, referred to some permanent bench mark, and the 

 discharge of the river. In order to prepare such a table it is necessary to 

 )btain the discharge at various stages of a stream, covering the usual 



FIG. 162. Pitot Tube for 

 Steam Rating. 



1 By Prof. E. Andersson of Stockholm. 

 April 20, 1907. 



See Zeltschnft des Verelns Deutseher Ittgenieur, 



