182 



HYDRODYNAMICS. 



larger vessel rises as much more slowly than it de- 

 scends in the smaller, as the large one exceeds the 

 smaller ; thus acting on the principle of virtual veloci- 

 ties in precisely the same manner that a heavy weight 

 on the short end of a lever is upheld by a small weight 

 on the long end. The great mass of water is support- 

 ed directly by the bottom of A, in the same way that 

 nearly all the weight on the lever is supported by the 

 fulcrum. A man who was seeking a solution to the 

 absurd mechanical problem of perpetual motion, and 

 Fig 153. who supposed that the large mass in A 

 would overbalance the small column in 

 B, and drive it upward, constructed a 

 vessel in the form shown in Fig. 153, so 

 that the small column, when forced up- 

 ward, wovild flow back into the larger 

 vessel perpetually. He was, however, 

 greatly surprised to see the fluid in both 



Attempted Perpetual o .y x 



Mutiun. divisions settle at the same level. 



This principle may be further explained by the fol- 

 lowing experiment : A B {Fig'. 154) repre- 

 sents the inside of a metalhc vessel, with 

 a bottom, C, which slides up and down, 

 water-tight. If water be poured in to fill 

 the lower or larger part only, it will be 

 found to press on the sliding bottom with 

 a force exactly equal to its own weight ; 

 that is, if there is a pound of water, it will 

 ^' press on the botton with a force equal to 

 one pound. Now, if the bottom be pushed upward, so 

 as to drive the water into the narrow part of the ves- 

 sel, the pressure upon the bottom becomes instantly 



