118 THE ATMOSPHERE AND ITS SERVICE TO MAN 



utilizes air pressure for " lifting " water. When the piston 

 of the pump is raised from the bottom of the cylinder a 

 partial vacuum is created in the cylinder. The air pressure 

 on the water in the cistern forces the water up the pipe and 

 through the valve B into the cylinder. When the piston 

 descends the valve B in the bottom of the cylinder is closed 

 by the weight of the water and the valve A in the piston 

 opens allowing the water to flow through 

 to the upper side of the piston. As the 

 piston is once more raised the valve A 

 closes and the water above the piston is 

 lifted and flows out the spout. Air pres- 

 sure again forces more water up the pipe 

 and through the valve B into the 

 cylinder. The water continues to rise 

 into the cylinder and to be lifted out as 

 long as the pump is worked. 



Lift pumps were in use for 2000 years 

 before any one successfully explained 

 their operation. Galileo observed that 

 in the best lift pumps the water could 

 not be made to rise higher than 32 feet, 

 but he died without being able to ex- 

 plain why. When Torricelli, his pupil and friend, performed 

 the experiment with the mercury tube, and found that atmos- 

 pheric pressure would support a column of mercury about 

 30 inches high in a vacuum, he explained what had puzzled 

 Galileo. Since mercury is about thirteen and one-half 

 times as heavy as water, the pressure of the atmosphere 

 would support a column of water thirteen and one-half 

 times as high as the column of mercury. In a perfect 

 vacuum, therefore, the pressure of the atmosphere would 



64. DIA- 

 OF A LIFT 



