70 ELEMENTS OF SCIENCE 



about half a pound for the square inch; and as we 

 increase the height a foot, the pressure increases half 

 a pound. In a cubical vessel the pressure of a liquid 

 filling it is, as before said, equal on all sides, and its 

 pressure on each side is equal to half the weight of the 

 liquid. Therefore a liquid in a cube exercises, on base 

 and sides, three times as much pressure as that pro- 

 duced by its weight alone. Let us suppose its weight 

 exercises a pressure of one pound, then the pressure 

 exercised on each side of the cube will be half a pound 

 that is, a pressure of two pounds, besides the pressure, 

 due to gravity, of one pound on the base of the cube. 



Any solid body immersed in a liquid, necessarily dis- 

 places a quantity of that liquid exactly equal to its own 

 bulk. If it also exactly equals this displaced quantity 

 in weight, it will remain indifferently at any depth in 

 the liquid without any tendency to rise or sink. If its 

 weight is greater it will of course sink, and if less, it 

 will rise. Not that, of course, it has any spontaneous 

 tendency in itself to rise ; it simply rises because the 

 greater pressure pushes it upwards. But a body which 

 sinks, apparently loses just as much of its own weight 

 as the water it displaces weighs, as may easily be ascer- 

 tained experimentally. 



Since liquids press equally in all directions, any 

 object immersed in them must be at least as much 

 pressed upwards by pressure from below, as it is 

 depressed by pressure from above. Thus fishes can 

 swim with ease at depths where they must be sub- 

 jected to enormous pressure from above, since they are 

 sustained by a somewhat greater pressure from below. 



If a solid body be first weighed in air and then in 

 water ; if its weight in the latter be subtracted from its 

 weight in the former and its weight in air be divided by 



