AIR AND WATER AS SERVANTS OF MAN 113 



ance it exactly by weights in the other pan. Now blow up the 

 football, forcing just as much air into it as you can. Put it on 

 the scale pan again and you will notice that it is slightly heavier, 

 due, of course, to the added air that has been forced into it. 



Next we need to demonstrate that the pressure of a fluid 

 is exerted equally in all directions. A tin can with glass tubes 

 set into its top, sides, and bottom and filled with water will show 

 that the water stands at the same level in all the tubes (Fig. 47). 

 See the Field and Laboratory Guide in Physical Nature-Study, 

 page 51. Pressure, therefore, of the water must be exerted in 

 all directions in order to maintain the columns of water in all 

 of these tubes. The pressure in the tube let 

 into the bottom of the can must be downward 

 at the end of the tube that projects into the 

 can. It must be likewise sideways at the 

 ends of the tubes inserted into the sides of 

 the can. Another simple device for showing 

 that the pressure in the water is equal in all 



directions is made as follows: Tie a piece | M\ 



of sheet rubber tightly over the mouth of 



a thistle tube. Cut off the stem of the FlG - 47.-Diagnun 



,,. ,, , , f ,. ,. , ... , of can with tubes in it 



thistle tube (see directions for cutting glass to show water pressure, 

 in the Field and Laboratory Guide, p. 50) 

 about i inch from the bulb of the tube. Slip a 3 -foot piece 

 of rubber tubing on to the short stem of the thistle tube. Draw 

 out the cut-off stem of the thistle tube at about its middle 

 point so as to make a fine glass tube. Break this at the 

 middle point and put one of the pieces into the other end of the 

 rubber tubing, the fine end out. Press lightly on the stretched 

 rubber over the mouth of the thistle tube, insert the fine end of 

 the glass tube into red ink, and slowly release the pressure. The 

 red ink is drawn up now into the fine end of the tube. By rubber 

 bands fasten this glass tube on to a ruler or meter stick. Now 

 stick the thistle tube down into a larger beaker of water. As the 

 thistle tube goes down notice that the red ink moves in the tube. 



