112 OUR PHYSICAL WORLD 



fills with steam driving out the air. When this happens, remove 

 the can from the stove or flame and force the cover in tightly. 

 As the can cools the water which was in the form of steam con- 

 denses and becomes water again. There was, of course, very- 

 little water in the can to start with, so that as condensation occurs 

 the water occupies only a very small part of the space. Since the 

 air was driven out by the steam there is little or no air in the can. 

 The air pressure on the outside will crumple in the can. 



Take a glass tube some 3 feet long and close one end of it 

 by heating it in the flame. Directions for handling the glass 

 tube in the flame are given in the Field and Laboratory Guide in 

 Physical Nature-Study. When the tube has cooled, fill it with 

 water, put the finger over the end and set the tube held in a 

 vertical position, mouth down, in a bowl of water. When the 

 mouth of the tube is under water remove the finger. Since 

 there is no air in the tube above the water to exert its pressure on 

 the column of water, the water in the tube is held up by the 

 pressure of the air on the surface of the water in the bowl. If 

 possible, repeat this experiment, using mercury in place of water. 

 Mercury is much heavier than water so the air pressure will only 

 support a column of mercury in the neighborhood of 30 inches 

 high. The column of water supported in a similar way by air 

 pressure is 33 feet high, for mercury is nearly fourteen times as 

 heavy. This apparatus is the barometer and as the height of the 

 mercury varies it shows the variations in the pressure of the air. 

 At sea-level normal pressure is about 30 inches; but this may vary 

 considerably, dropping as the pressure decreases or rising as the 

 pressure increases. A barometer is a very useful instrument to 

 indicate sudden changes of atmospheric pressure that herald the 

 approach of storms. 



Such experiments will help the child to appreciate the fact 

 that air has pressure. When we say that air has pressure we 

 simply mean that air has weight. We may demonstrate the 

 fact that air has weight in another way. On a pair of scales lay 

 a football that is distended but not blown entirely full and bal- 



