734 



HYDRAULICS AND ITS APPLICATIONS 



-Metallic Cord to Recording 

 Mechanism 



Discharge is then practically continuous and the machine has no dead 

 centre, while the smallest flow is registered. The positive type has the big 

 advantage over the inferential meter that whereas the correct registration of 

 the former is unaffected by an increase in the friction of the moving parts, 

 such an increase in friction may seriously affect the speed of the latter 

 for a given flow. Its chief drawbacks consist in its liability to water 

 hammer unless worked at a very low speed, and its consequent large 

 dimensions and high first cost. 



(4) One of the best types of meter for the determination of the varying 

 flow in a pipe, and hence of leakage, is that of Mr. Deacon (Fig. 354). 

 Here the water flows through a conical tube containing an axial rod 

 which carries a circular disc D. Any axial movement of this disc is 



resisted by a spring, and is recorded 

 on the registering apparatus. The 

 varying pressure on the disc, pro- 

 duced by any variation in the rate 

 of flow, may then be registered, and 

 by suitably calibrating the recording 

 mechanism the position of the rod 

 may be arranged to indicate the 

 flow in gallons per minute. 



(5) The Venturi water meter, in- 

 vented by Herschel in 1881, and 

 called by him after Venturi because 

 of the experimental work of the latter 

 on the physical properties of diverg- 

 ing tubes, depends in its principles on the truth of Bernoulli's theorem. 

 It is at once the simplest, and for large quantities of water the most 

 satisfactory meter yet designed, and simply consists (Fig. 355) of a pipe 

 passing the whole quantity of water to be measured, and fitted with a 

 portion BC, uniformly converging to a short parallel throat CD. At D 

 the pipe again diverges to its full diameter at E. The usual proportions 

 of the meter are indicated in the figure in terms of the pipe diameter, 

 experiments showing that an angle of divergence of 5 6' gives the best 

 results in the reconversion of kinetic to pressure energy from D to E. 

 If the pipe be horizontal, and if A and a be the areas of main pipe and of 

 throat, then, as shown onp 80, with a perfect fluid 



9 (PA - P a ) 



Outlet 



Inlet 



FIG. 354." Deacon " Meter. 



