146 



THE WATERS OF THE EARTH 



as borax or washing soda are often used to soften water for 

 laundry work. These substances combine chemically with 

 the mineral salts in solution and leave the water free to 

 form an emulsion with soap. 



Pressure in Water. Experiment 67. Tie a piece of thin sheet 

 rubber (dentist's rubber) tightly over the mouth of a small, short 

 thistle tube. Attach tightly to the neck of the thistle tube a 

 flexible rubber tube about two feet long. Bend a glass tube into 

 the shape of a U, making one arm slightly longer than the other. 

 Put colored water into the U-tube until it stands about two inches 

 high in each arm of the tube. Fasten a meter 

 stick in a perpendicular position and tie the 

 U-tube to it so that the long arm lies along the 

 scale. Attach the open end of the rubber tube 

 to the short arm of the U-tube. When you 

 press on the rubber sheet at the mouth of the 

 thistle tube, the water rises in the long arm of 

 the U-tube. You have made a simple pressure 

 gauge. (Figure 75.) 



Nearly fill a battery jar with water. Slowly 

 push the thistle tube down into the water and 

 notice the action of the column of water in the 

 U-tube. How does increasing depth affect pres- 

 sure? Being careful to keep the center of the 

 rubber diaphragm at the same depth, face it up, down, and side- 

 ways. Does the pressure in different directions vary at the same 

 depth? Hold the thistle tube at equal depth in the battery jar 

 and in a pail or tub of water. Does the greater volume of water in 

 the pail make any difference in the pressure at the same depth? 



Pressure in water varies directly as the depth, and at the 

 same depth pressure is equal in all directions. At a given 

 depth the volume of the water makes no difference with the 

 pressure. The pressure would be no greater in a lake six 

 inches below the surface than at the same depth in the 

 battery jar. For that reason, the pressure on a water main 



FIGURE 75 



