278 CIRCULATION 



tend to reduce the kinetic energy of fluid flowing through a tube, 

 viscosity or internal friction and external friction on the walls of 

 the tube. On account of the latter, the outermost layers of the 

 fluid adhere to the walls of the tube and become more or less 

 stationary. The molecules of the layers of fluid next to the outer- 

 most tend to cohere to the stationary layer on one hand and are 

 pulled along by their cohesion to the next inner layer. As a 

 result, their velocity is decreased. The net result is that a whole 

 series of cylindrical layers is produced each with a different rate 

 of flow ranging from the almost stationary outer layer to the 

 central axial column, which is retarded least of all and, therefore, 

 possesses the greatest kinetic energy. In a straight tube of uni- 

 form bore, such as is under consideration, this retarding influence 

 reduces the average rate of flow to half that of the axial stream. 

 It is obvious, therefore, that a considerable amount of the potential 

 energy of the liquid in the reservoir is absorbed in overcoming the 

 peripheral resistance caused by cohesion and adhesion. Resist- 

 ance to flow also depends on the area of cross-section of the tube 

 the wider the tube the larger the number of cylindrical layers 

 over which the adhesive resistance spends itself and, therefore, 

 the less the resistance met by the axial stream. A tube so narrow 

 that only an outer layer and a central column could pass along 

 would move with infinite slowness. Except in instances in which 

 the conducting tube has a very large or a very small diameter, 

 the rate of flow is proportional to the area of cross-section. 

 Furthermore, since the resistance in a tube of uniform diameter 

 is proportional to its length, the energy of the fluid must decrease 

 gradually from the reservoir to the outlet of the tube. The energy 

 of the fluid is shown by the pressure it exerts. Pressure may be 

 measured by some form of manometer. It is sufficient to insert a 

 number of vertical glass tubes, of uniform bore and open to the air, 

 at various points of the conducting tube to see the fall of pressure 

 with distance from the source of power. The fluid rises in these 

 collateral tubes or piezometers to a height proportional to the 

 pressure in the main conduit. In other words, the level of 

 the liquid in those pressure-gauges is accurately adjusted to the 

 peripheral resistance encountered by the liquid as it passes their 

 points of insertion. Such a system is represented in Fig. 60. 

 The power furnished by the liquid, in the constant-level reservoir 

 (R), is the downward pressure of gravity. The pressure at 

 various points is manifested by the height of the fluid in the 

 branch tubes (A) 1, 2, 3, etc. If the levels of the column of 



