CIBCULATION OF THE BLOOD. 



519 



walls of the tube ; for the movement of the portion of liquid in contact 

 with the walls of the tube is delayed by adhesion, and the movement of 

 the central column is delayed by friction with the external layers. It is 

 evident that the resistance due to friction along the sides of the tube 

 will depend upon the length of the tube. This may be illustrated by 

 the accompanying diagram. 



If a horizontal tube be connected with a reservoir of liquid and a 

 number of vertical side arms be connected with the horizontal tube, the 

 liquid will rise in the branch tubes to different heights inversely as the 

 distance of the vertical tube from the reservoir, for the propelling force 

 in the horizontal tube will diminish from point to point on account of 

 the uniformly acting resistance (Fig. 214). The vertical tubes will, there- 

 fore, enable us to measure the pressure exerted by the fluid upon the 

 walls of the tube through which it is flowing, and shows us that the pres- 

 sure at an 3^ point of such a tube will be less the greater the distance from 



FIG. 214. ESTIMATION OF PRESSURE OF LIQUID IN A HORIZONTAL, OUTFLOW-TUBE 

 CONNECTED WITH A CYLINDRICAL, VESSEL FILLED WITH WATER. (Landois.) 



a b, outflow-tube, along which are placed at intervals vertical tubes I. II, and III to estimate the pressure. The 

 line D, Dl, D2, D3, D4 indicates the rapidly decreasing pressure. 



the propelling force. Further, the resistance increases with the velocity 

 of the current ; for it is evident that, the resistance being mainly depend- 

 ent upon friction, if the column of fluid is at rest there is, consequent^, 

 no friction and no resistance, while the greater the rapidity of motion 

 the greater will be the friction, and, consequent!}^, the greater the resist- 

 ance. It follows from this that the smaller the tube the greater will be 

 the resistance, for the smaller the tube the greater will be the velocity 

 of motion. It may, therefore, be said that in a moving column of liquid 

 " the resistance is directly proportional to the length of the tube and is 

 inversely proportional to its cross-section, and increases with the speed 

 of the stream." 



It has been mentioned that the friction of the central moving column 

 on the outer layer is a source of resistance, consequently increase in the 

 cohesive nature of the fluid will increase the retardation of the central 



