CENTRE OF PRESSURE 

 The resultant of these will pass through P, where 



31 



i.e., the Centre of Pressure divides the middle third of the median line in 

 the inverse ratio of the pressures at the two ends. 



A 



The magnitude of the resultant R = - 

 Taking moments about F we have 



6R.FP 



2.R 



QE.FP 



2E 



FIG. 15. 



If F P = , then Pl = 0. 



, then p 2 = 0. 



If F P = 



I.e., when the resultant acts through either extremity of the middle 

 third of E F, the pressure at one end ie zero. 



For both pi and p% to be positive, the C. P. must lie within the middle 

 third. Wherever the pressure intensity varies uniformly across a surface 

 as in this case, this deduction holds true. One particular case, of great 

 importance to the hydraulic engineer, occurs in a masonry dam or 

 retaining wall exposed to water pressure on one side. Here the pressure 

 across any horizontal joint varies uniformly, and in order that the 

 jointing material should not be exposed to a tensile stress the C. P. of each 

 joint should lie within its middle third. 



The following table gives the position of the centre of pressure in some 

 cases of frequent occurrence in practice (Fig. 15 A, B, C, D). The 



