CH. XXI.] 



FLUID PRESSURE 



265 



flow out is given by putting the kinetic energy = work done. In 

 other terms : 



= gh ; hence v = 



or k = 



A liquid, however, has not necessarily a free surface, but may be 

 completely enclosed, as is the water in a system of hydraulic pressure 

 mains, or the blood in the circulatory system. The pressure in such 

 a system at any point may be measured by inserting at that point 

 a vertical tube at right angles to the blood-vessel ; the blood would 

 rise in it to a point, and would form a free surface a certain distance 

 up this tube ; the head h in the above calculation must be reckoned 



FIG. 267. Schema to illustrate blood-pressure. 



from this free surface downwards. If, instead of using a tube of fine 

 bore for this purpose, we employ a wider tube, say of ten times 

 greater area, the height or head to which the fluid rises will be the 

 same as in the narrow tube, though naturally the actual weight of 

 fluid supported will be ten times greater ; but the weight per unit of 

 area is the same in both cases. When, therefore, we measure the 

 pressure of fluid in terms of the height of a column of fluid, like 

 mercury, which it will balance, we really mean that the force of the 

 blood is equal to the weight of the mercury it supports per unit of 

 area, and this will naturally be proportional to the height of the 

 column. 



Let us next consider the simple case of a fluid flowing from a 

 reservoir, E (fig. 267), along a tube, which we will imagine is open at 

 the other end. 



