CH. xv. TORRICELLT THE BAROMETER. 



that his friend and follower Torricelli (born 1608), who was 

 a mathematical professor at Florence, hit upon the reason. 



Torricelli asked himself, 

 'Why does the water rise in 

 the tube at' all? something 

 must force it up.' Then it 

 occurred to him that air must 

 weigh something, and that it 

 might be this weight on the 

 open surface of the water which 

 forced the water up the pump 

 where there was no air press- 

 ing it down. To understand 

 this you must picture to your- 

 self all the air round our 

 globe to be pressing down 

 upon the surface of the 

 earth. Now, so long as the 

 tube also is full of air the 

 surface of the water will' all 

 be equally pressed down, and 

 so will remain at one level 

 at w B w. But when the pis- 

 ton A is drawn up, it pushes 

 the air above it out of the tube, and so lifts the weight off 

 B, the water at which will immediately be forced up the 

 tube by the pressure of the air on the water outside from w 

 to w. This will go on till the water has riser* about 34 feet 

 to c, and then the column of water c B in the tube will 

 press as heavily on the water at B as the air does on the 

 water outside from w to w, so all the water w B w will 

 again be equally pressed upon, and no further rise will take 

 place in the tube. 



FIG. 14. 



Section of a Suction-tube. 



A, Tight-fitting piston, c, Greatest 



height to which the water will rise. 



\v 13 w, Natural level of the water. 



