GENERAL PRINCIPLES OF HYDRAULICS 441 
deli of which is to be measured. The axis of the water main in the 
neighbourhood of the meter is horizontal. The water enters the meter 
at A, and leaves at D. A hollow belt is cast round the pipe AB at EF, 
and the interior of this belt communicates with the interior of the pipe 
by four small holes, the positions of which are shown in the cross section, 
. 718. These holes are bushed with vulcanite to prevent incrustation. 
_ A copper tube leads from the annular space at EF to the top of a vertical 
cast-iron cylinder containing mercury. ‘The throat is lined with a gun- 
D 
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4 
J 
; Fig. 716. 
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1 
| 
2 Wa SS ST 
Y 
A Tf 
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LSS KW 
| Qe= BD YYWUu«La‘ 
& 
B 
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iy ae Bh! Y -dilldlg 
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Fig. 719. 
metal casting, having an annular space round its centre which communi- 
cates with the interior of the throat. by four small holes arranged as at 
EF, and shown in Fig. 719, which is a cross section at the throat. A 
second copper tube leads from the annular space round the throat to the 
top of a second cast-iron cylinder containing mercury. The two vertical 
cylinders containing mercury communicate with one another at their 
bottom ends, so as to form the equivalent of a U tube mercury gauge, 
