HAEMODYNAMICS 



279 



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 

 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). 



FALL OF PRESSURE 



1N(A)TUBE OF UNIFORM DIAMETER IN(B) TUBE OF VARYING DIAMETER. 



765432 I RESERVOIR. I 23456769 IO 



R, 



FIG. 60. 



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 

 augmented, and therefore there is a heaping up of the fluid in the 

 earlier tubes 1, 2 and 3, a rapid fall to tube 4, and thereafter a 

 fall of pressure at the same rate as in the earlier part of the system. 

 All the above is stated in terms of pressure. Putting the same 

 matter in terms of velocity of flow, one may say that if a tube be 

 used, the second segment of which is wider than the first and 

 third, the speed of flow will be decreased in the central one. 



In the preceding experiment, the head pressure has always 

 been kept constant by making provision for a steady influx of 



