THE ATMOSPHERE 



55 



tie air in the upper end of it. Remove the mouth from the 

 short tube, and observe the water column. What is now 

 true of the air pressures within and without the bottle? 

 Exhaust some of the air from the bottle through the short 

 tube, and observe the height of the water column. By 

 what is the weight of the water in the long tube sustained? 



4. 1 Fill with mercury a glass tube that is at least thirty 

 inches long and closed at one end. Holding the forefinger 

 over the open end, invert the tube 

 and put this open end down under 

 mercury in a dish (a small mortar) . 

 Cautiously remove the finger, and ob- 

 serve the upper end of the mercury 

 column. Explain why the mercury 

 column stands in the tube above the 

 mercury level in the dish. Tip the 

 tube back and forth sidewise for short 

 distances, being careful to keep the 

 lower end at all times below the sur- 

 face of the mercury in the dish. What 

 seems to be true of the space above 

 the mercury in the tube? 



Measure the distance vertically up- 

 ward, both in inches and in centimeters, from the surface 

 level of the mercury in the dish to the level in the tube. 

 How long will this height remain unchanged? What is 

 true of this difference in levels as the tube is again tipped 

 sidewise? As mercury is 13.6 times heavier than water, 



1 The part of this experiment making use of mercury should be done by 

 the teacher. 



FIG. 21. A simple 

 barometer. 



