5 6 ELEMENTARY SCIENCE 



scious of it at all, any more than fish are conscious of the 

 weight of water in which they swim. Though we are 

 not conscious of this weight of air through our sensations, 

 yet it is a very real thing, and it does a great deal of work 

 which is for our advantage. 



You take advantage of air-pressure every time you 

 suck lemonade through a straw. As the straw stands free 

 in the glass, the air-pressure within it and the air-pressure 

 outside of it are equal, and the lemonade stands at the 

 same level within and without the straw. But the instant 

 you put your lips to the straw and draw into your mouth 

 the air which is in it, what happens? The lemonade 

 immediately follows the air up to your lips. Why does 

 it do this? It is because you have removed the air which 

 was holding it down. The air outside is pressing down on 

 the surface of the lemonade, and it is this pressure which 

 makes the lemonade go up the straw. Suppose there was 

 an air-proof cover over the glass except where the straw 

 protruded. Could you still suck up the lemonade? Try 

 it and see. Have you ever tried to pour liquid out of 

 a can with only one hole in it? Why is it necessary to 

 have two holes in order that the liquid may flow readily 

 out of one of them ? 



Now it is this very principle which operates when a 

 pump works. The surprising thing is that it was so long 

 before any one thought of taking advantage of it in this 

 way. But we should remember that things which seem 

 simple and easy to us now were not at all simple and easy 

 for our ancestors. If we had been in their place we should 

 have done no better than they, for they were quite as 

 bright as we. But we have the great advantage that we 

 may learn quickly and easily what mankind learned very 



