58 ELEMENTARY SCIENCE 



her that atmospheric pressure is not an unlimited thing. 

 It is limited, as you have noted, to fifteen pounds to the 

 square inch. Of course it will not force a column of water 

 up an indefinite distance. About thirty-two feet is the limit. 

 Atmospheric pressure is well illustrated by the siphon 

 (see Fig. 24). The siphon has been used since ancient 

 times for drawing liquid from a higher level to a lower level 

 over the side of the higher vessel. The tube must be 

 filled with the liquid in order to start the process. As 

 long as the distance from a to d is less than the height 

 at which atmospheric pressure will hold 

 a column of water, and as long as the 

 tube remains filled with water, it will 

 continue to flow from the bottom of 

 the tube. For as fast as it flows down 

 and away, it is replaced by water 

 rising to the top of the arch on account 



FlG ' of 4 'the T s l iphon ndple * the atmos P neric pressure. 



What effect should you expect atmos- 

 pheric pressure to have upon the boiling-point of water ? Will 

 it take less heat or more heat to make water boil at the top 

 of a mountain? Amateur campers in the mountains have 

 found that the water may all boil away without cooking the 

 beans, or whatever it is they are trying to boil. Why is this ? 



Measuring Atmospheric Pressure. An aneroid ba- 

 rometer is an instrument much used in mountain travel. It 

 enables you to estimate how high up you are, and it does 

 this by recording the atmospheric pressure. There is a 

 little metal box with a vacuum inside, that is, a space from- 

 which the air has been withdrawn. The pressure of the 

 air outside causes imperceptible movements of the metal 



