CENTRE OF PRESSURE. 21 



20 ft. and the lower 26 ft. below the surface of the water. 

 Find the pressure it sustains. Ans. 32 tons.* 



6. A cylinder, closed at both ends, is immersed in a 

 liquid so that its axis is inclined at an angle 0, to the hori- 

 zon, and the highest point of the cylinder just touches the 

 surface of the liquid. Find the whole pressure on the cyl- 

 inder, including its plane ends. 



[Let r = the radius of the base and h = the length of 

 the cylinder.] Ans. gpnr (h -f r) (h sin 6 + 2r cos 6). 



7. A hemispherical cup is filled with water, and placed 

 with its base vertical. Find the pressures on the curved 

 and plane surfaces. 



. ( Pressure on the curved surface = 2gpna s . 

 \ Pressure on the plane surface = gprra?. 



This example shows the distinction between the total 

 pressure of a fluid on a curved surface, and on that portion 

 of it which is perpendicular to any given plane. The press- 

 ure on the vertical plane side of the hemispherical cup 

 might be obtained by finding the sum of the horizontal 

 components of the actual pressures on all the elements of 

 the curved surface. This latter pressure, called the result- 

 ant horizontal pressure of the liquid on the surface, is 

 equal to the pressure of the liquid on the plane base, other- 

 wise the cup would have a tendency to move in a horizontal 

 direction. 



16. Centre of Pressure. The centre of pressure 

 of a plane area immersed in a fluid is the point 

 of action of the resultant fluid pressure upon the 

 plane area. It is therefore that point in an immersed 

 plane surface or side of a vessel containing a fluid, to which, 

 if a force equal and opposite to the resultant of all the press- 



* One ton = 2240 Ibs. 



