HYDR OSTA TICS. 



125 



held in place against the force of gravity until the level of 

 the water within the tube is very nearly the same as that 

 in the outer vessel. The disc will not fall until the weight 

 of the water in the tube plus the weight of the disc equals 

 the upward pressure. 0I& 



Note. A lamp-chimney answers the purpose of this experiment. 

 On the glass disc pour a little fine emery powder, and on this rub 

 the end of the lamp-chimney until they fit accurately. The string 

 may be fastened to the disc with wax. 



22S. Rule for Upward Pressure. To find 

 the upward pressure on any horizontal surface, 

 find the weight of an imaginary column of the 

 given liquid whose base is the same as the given 

 surface, and whose altitude is the same as the 

 depth of the given surface below the surface of 

 the liquid. 



229. The Hydrostatic Paradox. It may seem 

 strange at first thought that vessels whose bottoms are 

 subjected to equal pressure, like those represented in Fig. 

 75, do not exert equal pressures upon the stand supporting 

 tli em ; in other words, that they do not weigh the same. 

 The difficulty will be removed by remembering that the 

 pressure on the bottom of the vessel is only one of 

 the elements which combine to produce the pres-* 

 sure upon the 



stand. By refer- C L 1 D c|| K s I D K c 



ence to the figure, 

 which represents 

 three vessels of un- 

 equal capacity but 

 having equal pres- 

 sures upon the bot- 



