Chap, xix.] EQUILIBRIUM OF LIQUIDS. 



193 



well down into the water, and, holding it vertically, 

 proceed to pour water into it. The disc will continue 

 to close the end of the cylinder against the downward 

 pressure of the water being poured into it, until the 

 water inside the cylinder reaches the same level as 

 the water outside, and then the disc sinks away. This 

 shows that the disc w T as retained in position by an 

 upward pressure yielded by a column of water whose 

 base Avas the area of the disc, and whose height was 

 the distance between the disc and the level of the 

 fluid outside of the cylinder. 



i/ 



Equilibrium of liquids in communica- 

 ting' vessels. A further deduction from Pascal's 

 principle declares that where a series of vessels, com- 

 municating with one another, are filled with the same 

 liquid, equilibrium can only exist if the liquid stands 

 at the same height in each vessel, that is, if the free 

 surface of the liquid in each vessel is in the same hori- 

 zontal plane. This is readily understood from what 

 has been already said. Let c and 

 B (Fig. 95) be two vessels com- 

 municating with one another by the 

 tube A. ISTow, in order that the mole- 

 cules of the fluid in the tube A may 

 be at rest, they must be submitted 

 to the same force from the directions 

 B and c. If the force from the end 

 C, for example, predominates, the 

 fluid in A would move towards B. 

 In order that the pressure may be 

 the same at each end, the height of the 

 column of liquid at each end must be the same. In 

 the same way any number of vessels might be com- 

 municated with B and c, of any diameter and shape, 

 equilibrium is only established when the level of the 

 fluid is the same in each. If the equilibrium be 

 overthrown, for instance, by the addition of more 

 N 7 



Fig. 95. Equili 

 brium of Liquids 

 in communicating 

 Vessels. 



