LESSONS IN PHYSICS. 41 



tube with water, then closing the end of the longer arm with the 

 finger, place the end of the short arm in a glass of water and 

 open the end of the long arm, when the water will flow from the 

 glass through the tube. If the water be allowed to run into 

 another glass it will flow until the water is at the same level in 

 the two glasses. If we take a glass in each hand, by changing 

 the level slightly the water may be made to run either way. 

 Almost any substance may be used for a tube. The first siphon 

 the writer ever saw was made of dandelion stems. In studying 

 the barometer we learned that the pressure of the air would sus- 

 tain a column of water that was over thirty feet high, in case the 

 air pressure was removed from the column. In the case of the 

 barometer the air pressure was removed by closing the end of 

 the tube. In the siphon the air pressure is removed by bending 

 the tube, so that both ends are under water. When both arms 

 are of the same length, that is when the water in each glass 

 is at the same level, the air pressure sustains equal columns of 

 water, and there is no motion ; but if we change the level of the 

 two glasses of water the pressure on the water in the lower glass 

 has a longer column of water to sustain than is sustained in the 

 upper glass, and water is pushed over into the lower glass to 

 establish an equilibrium. The long arm and short arm means 

 the perpendicular distance from the arch of the siphon to the 

 surface of the water on either side. 



The common pump depends upon atmospheric pressure for its 

 usefulness. It consists of two barrels separated by a valve open- 

 ing upward. In the upper barrel there is a piston with a valve 

 opening upward. When the piston is raised the pressure of the 

 air is removed from the water in the lower barrel, and the press- 

 ure of the air on the water outside forces it up the barrel 

 through the lower valve. When the piston is pushed down the 

 lower valve closes and the valve in the piston opens so that the 

 water rises above the upper valve; as the piston is raised again 

 it lifts the water above the upper valve and allows water to be 

 forced through the lower, and so on. In the ordinary force pump 

 the piston is solid, and a valve, opening outward in the side of the 



