20 Common Science 



because a perfect vacuum cannot be made. There is 

 always some water vapor formed by the water evaporat- 

 ing a little, and there is always a small amount of air 

 that has been dissolved in water, both of which partly 

 fill the space above the water and press down a little on 

 the water within the pump. 



If you had a straw over 33 feet long, and if some one 

 held a glass of lemonade for you down near the side- 

 walk while you leaned over from the roof of a three-story 

 building with your long straw, you could not possibly 

 drink the lemonade. The air pressure would not be 

 great enough to lift it so high, no matter how hard you 

 sucked, that is, no matter how perfect a vacuum you 

 made in the upper part of the straw. The lemonade 

 would rise part way, and then your straw would be 

 flattened by the pressure outside. 



Some days the air can force water up farther in a 

 tube than it can on other days. If it can force the 

 water up 33 feet today, it will perhaps be able to 

 force it up only 30 feet immediately before a storm. 

 And if it forces water up 33 feet at sea level, it may 

 force it up only 15 or 20 feet on a high mountain, for 

 on a mountain there is much less air above to make 

 pressure. The pressure of the air is different in differ- 

 ent places ; where the air is heavy and pressing hard, 

 we say the pressure is high ; where the air is light and 

 not pressing so hard, we call the pressure low. A place 

 where the air is heavy is called an area of high pressure ; 

 where it is light, an area of low pressure. (See Section 44.) 



What makes winds? It is because the air does not 

 press equally all the time and everywhere that we have 



