878 



PHYSIOLOGY 



in the tubes show a continuous descent, so that the line joining the tops of the fluid 

 in the various tubes is a straight one. The movement of the fluid from B to c can be 

 regarded as due to the difference of the pressure between B and c, i.e. P 2 -P 3 . It will 

 be noticed in the diagram that the straight line joining the tops of the fluid does not 

 strike the surface of the fluid in B, but falls a little below it. Of the total pressure in B, 

 H, the large portion h f is employed in overcoming the resistance of the tube AG, while a 

 small portion h represents the force necessary to give to the fluid as it leaves the reservoir 



at A a certain velocity. If the flow of fluid be diminished by partially clamping the 

 end at G the rate of fall of the pressures will be diminished. The same effect will be 

 produced either by raising the level of G oj by lowering the level of the reservoir and so 

 the pressure at A. 



The difference of pressure between any two points, i.e. between D and E, may be 

 regarded as that pressure which is necessary to maintain a certain velocity of the fluid 



against the resistance offered by the friction of the fluid in contact with the walls of 

 the tube. This friction, and therefore the resistance to the flow, can be altered by 

 diminishing the diameter of the tube, when a larger difference of pressure will be 

 necessary in order to maintain the same velocity of flow. This can be shown by in- 

 troducing a resistance between D and E by partially clamping the tube at this point 

 (Fig. 383). The continuity of the fall of pressures in the vertical tube is at once abolished. 

 Between A and D there is a continuous fall, which is succeeded by a steep fall between 

 D and E, and this again by a gradual fall between E and G. In any system of tubes 

 therefore through which fluid is flowing the fall of pressure between any two points 



