2i2 PHYSIOLOGICAL PHYSICS. [Chap. xx. 



of the current encounter resistance by reason of the 

 adhesion of the outer molecules. Naturally, the re- 



*/ ' 



sistance due to the friction along the sides of the tube 

 will depend on the length of the tube. It will be 

 greater the longer the tube, and vice versa. Thus 

 at the point a (Fig. 102) the resistance will be the 

 amount due to the friction encountered along the 

 whole tube 6, at i it will only be the friction 

 to be encountered between I and 6, at n only that 

 between n and. the outlet, and it is therefore a con- 

 stantly diminishing amount to the outflow point 

 where the water issues freely, and where the resistance 

 is consequently 0. Now the friction exerted on the 

 sides of the tube means pressure, and the deter- 

 mination of this pressure will give the amount 

 of resistance. In Fig. 102 vertical tubes are seen 

 communicating at intervals with the horizontal tube. 

 These being in free communication with a6, the 

 water will rise in them to a height which, accord- 

 ing to what has been previously seen, will be an 

 expression of the pressure exerted by the fluid upon 

 the walls of the tube through which it is flowing. 

 These vertical tubes are thus measures of pressure, of 

 pressure only at the point where they communicate 

 w r ith the horizontal tube. They are called PIEZOMETERS. 

 On filling up the apparatus shown in Fig. 102 it is 

 found that the height of the column of liquid regularly 

 diminishes in each tube, and is reduced to zero at b, 

 if the outlet there is free. So that a line joining the 

 surfaces of the fluid in each tube takes up a position 

 shown by the dotted line P P : P 2 , etc., experimentally 

 proving what has been said as to the diminution of 

 pressure onwards to the outlet. Now had the opening 

 at o been a free outlet, the water would have issued 

 from it with a velocity determined by the height of 

 the column of liquid above it, that is, by the pressure 

 HO, which is called the hydrostatic pressure. The 



