Fluid Pressure 83 



mutually attract one another causes them to move 

 towards each other, the motion of the unsupported 

 body being readily followed by the eye. A balloon, 

 however, although unsupported by any visible means, 

 does not fall to the Earth's surface, but even moves 

 during part of its flight in the opposite direction. 

 Similarly a cork dropped into water does not fall to 

 the bottom of the liquid. In these cases a force is 

 brought into play which is strong enough to overcome 

 the force of gravity, and the consideration of the nature 

 and magnitude of this force exerted in common by 

 all fluids such as the air and the water in the examples 

 above quoted will form the subject of this chapter. 

 49. The way in which a fluid substance makes its 

 presence felt in the case of a body in contact with it 

 is by exerting what is termed a pressure upon all parts 

 of the surface of the body which touch the fluid. When 

 a 56 Ib. weight is placed upon the ground, that portion 

 of the ground upon which it rests is said to be under 

 a pressure of 56 Ib. If the area of the base of the 

 weight be 28 sq. in., each sq. in. of the ground beneath 



f\f\ 

 it supports ^ = 2 Ib. weight. In this case we call 



Zo 



the pressure one of "2 Ib. per sq. in." and similarly we 

 generally measure pressure by the magnitude of the 

 force exerted upon unit area. We will next investigate 

 the nature of the pressure exerted by fluids, by means 

 of some simple experiments. 



EXPERIMENT 27. Remove the brass plate from a 

 "deflagrating spoon". Measure its diameter in cm. 

 and calculate its area. Pass a length of fine string 

 through the hole in the centre of the plate and push 

 in a tight-fitting cork, so that the string is held firmly 

 and the arrangement is watertight. Weigh the disc, 



62 



