TORKICELLIS EXPERIMENT. 



Fig. 



height of 32 feet, since in this case her abhorrence was limited 

 to 30 inches? In fine, Torricelli soon perceived the true cause 

 of this phenomenon. 



The weight of the atmosphere acting upon the surface of the 

 mercury in the cistern supports the liquid in the tube. But the 

 surface E being excluded from contact with the atmosphere, is free 

 from the pressure of its weight ; the 

 column, therefore, of mercury F, being 

 pressed upwards by the weight of the 

 atmosphere, and not being pressed down- 

 wards by any other force, would stand 

 in equilibrium. 



This explanation was further con- 

 firmed by the fact, that on admitting 

 the air to the upper end of the tube B, 

 by breaking off the glass at that point, or 

 opening a stop-cock placed there, the 

 column of mercury in the tube instantly 

 dropped into the cistern. This was 

 precisely the effect which ought to ensue, 

 inasmuch as the admission of the pres- 

 sure of air upon the column E balanced 

 the pressure on the surface in the 

 cistern, and there was no longer any 

 force to sustain a column of mercury in 

 the tube, and consequently it fell into 

 the cistern. 



11. This experiment and its explana- 

 tion excited, at the epoch we refer to, the greatest sensation 

 throughout the scientific world, and, like all new discoveries 

 which have a tendency to explode long-established doctrines, 

 was rejected by the majority of scientific men. The celebrated 

 Pascal, wl^o flourished at that epoch, however, had the sagacity 

 to perceive the force of Torricelli' s reasoning, and proposed to 

 submit his experiment to a test which must put an end to 

 all further question about it. " If," said Pascal, "it be really 

 the weight of the atmosphere under which we live that sup- 

 ports the column of mercury in Torricelli' s tube, we shall 

 find, by transporting this tube upwards in the atmosphere, 

 that in proportion as it leaves below it more and more of the 

 air, and has consequently less and less above it, there will be 

 a less column sustained in the tube, inasmuch as the weight 

 of the air above the tube, which is declared by Torricelli to be 

 the force which sustains it, will be diminished by the increased 

 elevation of the tube." 



103 



