ATMOSPHERIC PRESSURE. 159 



275. What Supports the Mercury Column ? 



- To answer this very important question, consider the 

 horizontal layer of mercury molecules in the tube at the 

 level of the liquid in the bath. Under ordinary circum- 

 stances, they would hold their position by virtue of the 

 tendency of liquids to seek their level. But in this case, 

 they hold it against the downward pressure caused by the 

 weight of the mercury column above, which is equivalent 

 to fifteen pounds to the square inch. Being in a condi- 

 tion of equilibrium, they must be acted upon by an upward 

 pressure of fifteen pounds to the square inch. It is evident 

 that the pressure of the mercury in the bath is not able to 

 do this work, its powers being fully tasked in supporting 

 the mercury in the tube up to the level of the particular 

 molecules now under consideration. This upward pres- 

 sure then must be due to some force acting upon the sur- 

 face of the mercury, and transmitted undiminished by that 

 liquid. The only force, thus acting, is atmospheric 

 pressure, which is thus measured. The original column 

 of thirty-six inches fell because its weight was greater 

 than the opposing force. As it fell, its weight diminished, 

 continuing to do so until an equality of opposing forces 

 produced equilibrium. (See Appendix H.) 



276* Pascal's Experiments. Pascal confirmed 

 Torricelli's conclusions by varying the conditions. He 

 had tho experiment repeated on the top of a mountain and 

 found that the mercury column was three inches shorter, 

 showing that as the weight of the atmospheric column 

 diminishes, the supported column of mercury also dimin- 

 ishes. He then took a tube forty feet long, closed at one 

 end. Having filled it with water 3 he inverted it over a 



