116 THE ATMOSPHERE AND ITS SERVICE TO MAN 



In the preceding experiments atmospheric pressure ac- 

 counted for the various things that happened. When the 

 steam in the can cooled, it condensed and occupied less 

 space. The pressure of the atmosphere from the outside, 

 therefore, pushed the sides inward. With the atmospheric 

 pressure lessened inside the Magdeburg hemispheres, the 

 full atmospheric pressure on the outside held them together. 

 The inverted glass kept the atmosphere from pressing down 

 on the surface of the water immediately under it. The up- 

 ward pressure of the atmosphere on the paper was greater 

 than the downward pressure of the water. When you 

 withdrew air from the glass tube, the pressure of the at- 

 mosphere on the surface of the water forced the water up 

 into the tube to take the place of the air that had escaped. 



Variation in pressure due to heating and cooling of air 

 explains circulation and drafts. A column of cold air is 

 denser and therefore heavier than a corresponding column 

 of warm air. The cold air, therefore, presses the warm 

 air up, and takes its place below. ^ 



Measuring Atmospheric Pressure. Experiment 48. 

 (Teacher's Experiment.) Take a thick-walled glass tube of about 

 \ cm. bore and 80 cm. length. Close it at one end. Fill the tube 

 with mercury. (Be sure to place the closed end of the tube in a 

 large vessel so as not to waste the mercury if you spill it.) Place 

 the thumb tightly over the open end of the tube and invert it in 

 a vessel of mercury. If you are at or near sea level, the mercury 

 column will drop to a height of about 75 cm. (about 30 inches) 

 and will stand there. This is known as Torricelli's Experiment, 

 because Torricelli first performed it and explained it. 



The space above the mercury is without air, and there- 

 fore no atmospheric pressure is exerted at the top of the 

 column of mercury. The column of mercury is pressing 



