Gravitation 19 



water, forces it up into this empty space from which 

 the air has been pushed. But air pressure cannot force 

 water up even into a perfect vacuum farther than about 

 33 feet. If your glass pump were, say, 40 feet long, 

 the water would follow the plunger up for a little over 

 30 feet, but nothing could suck it higher ; for by the 

 time it reaches that height it is pushing down with its 

 own weight as hard as the air is pressing on the water 

 below. No suction pump, or siphon, however perfect, 

 will ever lift water more than about 33 feet, and it will 

 do well if it draws water up 28 or 30 feet. This is 



FIG. 9. A glass model suction pump. 



