70 



Our Surroundings 



the chimney equals that in the jar the piece of cardboard will no 



longer keep its place. Can you tell why? 



To show that water exerts pressure upon surfaces and that 



this pressure can be measured and increased, perform the follow- 

 ing experiment : 



Cover the large end of a 

 thistle tube with a piece of 

 very thin rubber sheeting. 

 Fasten this tube to the lower 

 end of a standard so that 

 the rubber sheeting may face 

 upward, downward or side- 

 wise. To the other end of 

 the thistle tube fasten, by 

 means of rubber tubing, one 

 end of a U-tube containing 

 colored water or mercury. 

 With the rubber sheeting 

 faced upward, allow the this- 

 tle tube to sink in water and 

 notice how the level of the 

 liquid in the U-tube changes. 

 Can you explain this ? Notice 

 how the amount of change 

 differs as the thistle tube sinks 



An EXPERIMENT TO SHOW WATER 



lower. 



Try the same experiment with the rubber sheeting on the 

 thistle tube facing downward, and then sidewise. Compare the 

 pressures as shown by the liquid in the U-tube for each of the 

 positions of the thistle tube. The pressures indicated on 

 the rubber sheeting are equal to the pressures on anything 

 located at the same depth as the sheeting. These experiments 

 may be performed equally well with three different thistle 

 tubes. 



The relation between depth and pressure may be shown by the 

 use of a can having three or more holes in its side equally spaced 

 from top to bottom. Plug the holes and fill the can with water. 



