CH. XV. 



rORRICELLI—THE BAROMETER. 



117 



Fig. 



Z4^ftH 



34 feet it would not mount any higher, even though the tube 

 between the surface of the water c, and the piston a, had no 

 air in it. He could not, how- 

 ever, find out why the water 

 should stop rising just at this 

 point, and it was not till after 

 his death that his friend and 

 follower Torricelli (born 1608), 

 who was a mathematical pro- 

 fessor 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 pressing it down. To un- 

 derstand this you must picture 

 to yourself 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 piston a is drawn up, it pushes the air above 

 it out of the tube, and so lifts the weight off the water at 

 B, 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 risen about 34 feet to c and 



Section of a Sucnon-tube. 



A, Tight-fitting piston, c, Greatest height 

 to which the water will rise, w B w, 

 Natural level of the water. 



