PIEZOMETERS 



365 



Tho fluid rises in tlu'se coUateral tubes or piczonwicr.s to a height 

 proportional to the pressure in the main conduit. In otlier words, 

 the level of the liquid in those pressure-gauges is accurately- 

 adjusted to the peripheral resistance encountered by the liquid as 

 it passes their points of insertion. Such a system is represented 

 in Fig. 87. The power furnished by the liquid, in the constant-level 

 reservoir (R), is the downward pressure of gravity. The pressure at 

 various points is manifested by the height of the fluid in the 

 branch tubes (A) 1. 2, 3, etc. If the levels of the column of 

 liquid in each of these piezometers be joined by a straight line 

 which is produced to the reservoir wall at (y), the mass of liquid 

 will be divided into two portions. The lower portion (r) represents 

 the portion of the energy of the total spent in overcoming the 



FALL OF PRESSURE. 



fNCA^TUBE OF UNlFORJn DIAMF.TtR )N(B) TUBE OF VARYING DIAMETER. 



y 6 5 •* 3 2 1 R£5E<?vr>lR. I 2345676 9 10 



R. 



FIG. 87. 



resistance, and is consequently known as resistance-pressure. Of 

 the remainder, a certain amount (o) is spent in forcing the fluid 

 through the orifice into the tube. The actual driving force or 

 velocity pressure comes from the mass (v). 



If the main tube is not of uniform bore — suppose {B) it increases 

 in sectional area, at first gradually (a to b) and then somewhat 

 suddenly (at b) — corresponding alterations in pressure may be seen 

 in the manometers. Increase in width means smaller resistance, 

 and therefore a smaller resistance-pressure is required to drive 

 the fluid along the tube. As the total mass in the reservoir is 

 kept constant, the amount not required in r goes to increase v. 

 There being relatively a greater head of pressure, the levels shown 

 by the manometers will tend to decrease progressively at a slower 

 rate than before. If, on the contrary, the bore of the tube is 

 diminished as at c, the fall of pressure will become more rapid. 

 Further, if at b a constriction is produced, resistance to flow is 



