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TEXT-BOOK OF PHYSIOLOGY. 



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 y c, 6, 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. The 

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

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

 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 the reservoir to 

 the outlet. Therefore the pressure at any given point is proportional to the resist- 



FIG. 150. A PRESSURE VESSEL, P, WITH A HORIZONTAL OUTFLOW TUBE, O-n t INTO WHICH 

 VERTICAL TUBES OR MANOMETERS ARE INSERTED. 



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

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

 power should be kept constant by permitting fluid to flow into the reservoir as 

 rapidly as it flows out of the horizontal tube.) 



The power or force which overcomes the resistance in the horizontal tube and 

 imparts velocity to the fluid is the downward pressure of the water in the reservoir, 

 represented by H. The amount of this power utilized in overcoming the resistance 

 is approximately indicated by the height of the fluid, y t at which point the line 

 uniting the upper limits of the water in the vertical tubes intersects it. The height 

 of the fluid at this point is a measure, therefore, not only of the resistance but also 

 an indication of the relative amount of the pressure used in overcoming it and is 

 therefore known as the pressure height. 



The amount of the pressure consumed in imparting the observed velocity is 

 determined by ascertaining the height from which a particle must fall in empty 

 space to acquire this velocity. This is obtained by dividing the square of the veloc- 



v 2 

 ity by twice the accelerating force of gravity as expressed in the formula, - ; the 



quotient is the height and is known as the velocity height. Conversely if the mov- 

 ing fluid were discharged into empty space through an opening in the tube at n, 



