THE CIRCULATION OF THE BLOOD 



333 



the tube is of uniform diameter the velocity through each cross-section will be the 



same. 



As the water flows through the horizontal tube it meets with resistance, namely, 

 the cohesion and friction of its molecules, and the adhesion between the walls of 

 the tube and the water which must be overcome if the flow is to continue. Because 

 of the fact that water will moisten most surfaces with which it comes in contact 

 there will be an adhesion between the walls of the tube and the outer layer of the 

 column of water, in consequence of which it will become more or less retarded in 

 its flow. Between the outer retarded layer and the axis of the stream, there is an 

 infinite number of layers of molecules, the cohesion of which one for the other is 

 more and more overcome by the pressure in the vessel, P. The force of adhesion 

 between wall and fluid, together with the force of cohesion between the molecules 

 of the fluid give rise to the resistance of the fluid to the flow. 



As a result of the resistance the forward movement of the water under the 

 pressure in P, is somewhat retarded, and as a consequence it will exert a lateral or 

 radial pressure against the walls of the tube. That such a pressure exists is shown 



FIG. 151. A PRESSURE VESSEL, P, WITH A HORIZONTAL OUTFLOW TUBE, O-n, INTO WHICH 

 VERTICAL TUBES OR MANOMETERS ARE INSERTED. 



by the rise of the fluid in each of the vertical tubes, and the height to which it rises 

 in each tube is a measure of the pressure at its base. In the tube /, the fluid rises 

 to but a slight extent for the reason that the resistance yet to be overcome .is slight 

 in amount It is, however, a measure of the resistance or friction between the base 

 of the tube and the orifice of outflow. In the tube e the fluid rises twice as high 

 as in / because of the additional friction between the bases of the tubes e and /. 

 What is true of these two points is equally true of the points at the base of the tubes 

 d t c, b, a. Lines drawn to the pressure vessel from the top of the fluid in each tube 

 and parallel to the horizontal tube will show how much of the pressure force is 

 utilized in overcoming the friction in each section of the horizontal tube, 

 amount of the lateral pressure at any given point is theref ore indicated and measured 

 by the height to which the water rises in the tubes. For this reason these tube 

 termed pressure tubes or piezometers. 



Since the resistance in a tube of uniform diameter is proportional to its length 

 the lateral pressure will gradually but progressively decrease from *e reservoir 

 the outlet Therefore the pressure at any given point is proportional to the r 

 ance yet to be overcome and conversely the resistance to be overcome is indicate 

 by the amount of the pressure. (In the conduct of an experiment the propelling 



