16 HYDRAULICS AND ITS APPLICATIONS 



in cases where, as when under the influence of capillary action, the effect 

 of cohesion is great. When dealing, however, with water in bulk, impreg- 

 nated as it usually is with air in solution, the effect of cohesion may be 

 nr^K'ctrd. and when at rest water may be considered as satisfying the 

 tial condition of a perfect fluid, viz., that it exerts a normal pressure 

 on all surfaces with which it may be in contact. All the laws governing 

 the statical equilibrium of a perfect fluid, depending as they do solely on 

 this property, can then be applied to that of water, and are included 

 among the principles of Hydrostatics. 



ART. 6. PRESSURE AT A POINT. 



The average pressure intensity over any area A equals the total 



p 

 pressure P on the area divided by the area, or equals -j. If the pressure 



varies from point to point of the area, and if this be divided into a large 



number n of small areas 5 A, the pressure 

 on any one of these containing a given 

 point being b P, then the limiting value to 



8 P 

 ==r-|^ which the ratio sj tends, as n is made 



infinitely large, is taken as the pressure 

 FlG 4 intensity or more shortly the pressure 



at the point under consideration. 



The units in which pressure intensity is measured depend on those of 

 force and space. 



In English practice the unit is usually the pound per square inch, or 

 per square foot. In the metric system of units the usual unit is the 

 kilogramme per square centimetre. 



These units are connected by the relationship that 1 kilogramme per 

 square cmm. = 14 '228 Ibs. per square inch. 



ART. 7. IN A LIQUID AT BEST, THE PRESSURE INTENSITY is EVERYWHERE 

 THE SAME AT THE SAME DEPTH, AND IS THE SAME IN ALL 

 DIRECTIONS. 



The truth of the first of these propositions may be seen by considering 

 the equilibrium of a small vertical column of the liquid of cross sectional 

 area a and having its base at the depth h below the surface (Fig. 4). 

 \\IK-IVYM iliis column is taken in the liquid its weight will be the same 

 and must be balanced by the vertical upward pressure on the base, which 

 will therefore be the same. Since the sectional area of the column is 



