the upper extremities of the columns of water and mer- 

 cury had no communication with the atmosphere, but 

 that the lower extremities had a communication, and he 

 attributed the elevation of the columns in the tubes to 

 the weight of the atmosphere. 



The curious may amuse themselves with the action of 

 the weight of the atmosphere in the following manner: 

 Take a glass tube of uniform bore, open at both ends ; 

 fit a cork to it, and cement a wire into the cork, which 

 will form a piston to the tube ; place the piston even 

 with the lower end of the tube ; and in that situation 

 place the same end of the tube in mercury ; hold the 

 tube steadily and pull up the piston ; the mercury will 

 follow the piston, and will fill that part of the tube 

 which is below the piston. By this means the weight 

 of the atmosphere is removed from off the mercury, 

 which is forced into the tube as far as the piston, by the 

 weight of the atmosphere on the rest of the surface of 

 the mercury in the bason; when the mercury in the 

 tube balances the weight of the atmosphere, it remains 

 stationary ; and on pulling the piston higher, the space 

 between it and the mercury is called a vacuum, or space 

 void of air. 



In 1646 Pascal at Rouen repeated Torricelli's experi- 

 ments with similar results. He also varied them by 

 employing liquids of different specific gravities, and he 

 perceived that the lighter the liquid the higher it as- 

 cended in the tube ; but the agency of an invisible fluid 

 was still doubted, and he therefore determined to make 

 an experiment on the top of the mountain Puy de Dome, 



