LIQUIDS IN COMMUNICATING VESSELS. 



179 



the solution of salt is 1*2 times as heavy, the pressure 

 is 480 x 1-2 - 576 81 . 



In general, then, the pressure, expressed in grammes, 

 upon the horizontal base of a vessel containing a liquid is 

 found by multiplying the number of square centimetres in 

 the area of the base into the perpendicular height of the 

 liquid in the vessel, and into the specific gravity of the 

 liquid. 



20. Communicating Vessels. Buoyancy. Principle 

 of Archimedes. It has been already stated that the 

 pressure at any point in a liquid at rest is the same in 

 every direction. Hence any surface of which the lower 





Y7 



FIG. 128. 



side is in contact with a liquid is pressed upwards with 

 te same force with which an equal surface is pressed 

 lownwards, of which the upper side is in contact with 

 te liquid, provided that both surfaces are at the same 

 lepth below the top of the liquid. If a vessel have the 

 >rm fig. 128, A, e f and a b being equal and in the 

 same horizontal plane, the upward pressure upon the 

 surface e f will be equal to the downward pressure upon 

 the surface a 6, for in both cases it is equal to the weight 

 of the liquid column a b c d, or to that of an equal 



N 2 



