USES OF WATER 



111 



Experiment 30. A very simple experiment may be arranged 

 to determine whether the pressure changes as the depth of the 

 water is increased. Stretch a rubber diaphragm over the mouth 

 of a thistle tube and attach the tube to a pressure gauge (Figure 

 100). The working of the apparatus may be tested by pressing on 

 the diaphragm with the finger. The drop of ink moves out as the 

 pressure is increased and returns to its former position when the 

 finger is removed. Test the pressure at different depths and also 

 at the same depth with the tube in a number of different positions. 

 What is the effect of doubling the depth? Trebling the depth? 

 What is the effect of changing the direction of the pressure with 

 the depth remaining the same? 



The upward force on any horizontal surface in a liquid 

 is equal to the weight of a column of the liquid whose 

 base is the given 

 surface beneath 

 the free surface 

 of the liquid and 

 whose height is 

 equal to the depth 

 of the liquid. 



Since the above FIG. 101. Irregular Shapad Vessels having Equal 

 , . Bases. 



statement is true 



and since of course the downward force on such a surface 

 is exactly equal to the upward force, it will be seen that 

 these forces are not dependent on the shape of the vessel 

 but simply upon the area of the surface considered and 

 the depth and density of the liquid. For example, if the 

 four vessels of Figure 101 have bases of the same area and 

 are filled with the same liquid to the same depths, the 

 downward pressure on the bases will be exactly the same 

 in each case. This conclusion seems unreasonable at first, 

 since it means that the pressure in some cases is much 

 more than the total weight of the water in the vessel; 



