350 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



is immediately followed by a rise of the liquid all the way to the reservoir, because 

 under this condition the collaterals are converted into mere recesses of the main 

 vessel. 



It must be clear, therefore, that the power furnished by the liquid in the 

 reservoir {H) is the downward pressure of its constituents. A large, portion of it 

 (h) is utilized in overcoming the resistance and is therefore known as the resistance- 

 -pressure. The remainder (h^) constitutes the actual driving force and is com- 

 m.only spoken of as velocity-pressure. The amount of each may be determined 

 very readily by joining the levels of the liquid in the piezometers by a straight 

 line and by extending this line until it meets the reservoir (y-n). It should be 

 noted, however, that their sum total is not absolutely equal to the head-pressure 

 {H). This discrepancy indicates that a fraction of the latter {x) is used up in 

 overcoming the friction encountered by the liquid in its passage through the 

 orifice of the reservoir. The initial energy (H) may also be produced in other 

 ways than by means of the position or ''head" of the liquid in a reservoir, for 

 example, by the movement of a piston within a cylinder. But the results remain 

 the same irrespective of the source of the pressure. 



FiQ. 182. — A Pressure Vessel, P, With A Horizontal Outflow Tube, 0-n, into 

 Which Vertical Tubes or Manometers are Inserted (a, 6, c, d, e, and /) . 



If the tube attached to the reservoir, does not retain the same diameter through- 

 out, but changes from large to small, or from small to large, the dynamical con- 

 ditions resulting therefrom may readily be deduced from the foregoing data. 

 Thus, if the median portion is the larger, the speed of flow is diminished in this 

 particular segment, because the velocity is inversely proportional to the cross- 

 section. Moreover, in as much as the resistance is less here, the initial energy or 

 head-pressure is used up more slowly in this section. Consequently, the lateral 

 pressure declines less rapidly here than nearer the reservoir. On entering the 

 third segment which possesses the same diameter as the first, the original velocity 

 is again established, while the increased resistance in turn insures a more rapid 

 fall in pressure. 



If a tube is now used, the second segment of which is narrower than the first 

 and third, the speed of flow is increased in the central one. This implies that the 

 resistance is also increased, while the head pressure is considerably diminished. 

 This change is clearly indicated by the fall in the lateral pressure. On reaching the 

 third section of the tube, the velocity and resistance are decreased as is betrayed 

 by a less rapid fall in the pressure. In the preceding experiments the head- 

 pressure has always been kept constant by making provision for a steady influx 

 of water into the reservoir to compensate for its outflow. But if the initial energy 



