22 ELEMENTARY SCIENCE 



Gravity is just as important as any of the other great 

 things that we have started out to study. It has done just 

 as much as any of them in making the earth so that we 

 can live on it. 



Gravity is a property or quality of the earth. It is not 

 a thing that we can see or feel; it is a "law of nature." 

 It is the "pull" of the earth upon all things on its surface 

 or near it. It is the power that makes an apple fall to the 

 ground, and makes water run down-hill. It is the power 

 that makes it easier for us to go down than to go up. It 

 is the power that keeps water and air from flying off into 

 space. It is the power of Mother Earth to hold things to 

 her surface. 



There is a law of nature that all solid bodies have power 

 to attract other bodies toward them. Thus it is the at- 

 traction of the moon that causes tides in the sea. This 

 law is called the law of gravitation. Gravity is simply the 

 law of gravitation as applied to the earth. No one has 

 explained gravitation. Gravitation explains other things, 

 but it does not explain itself. It is one of those things that 

 you will have to accept without any explanation. 



This is just a beginning of the study of water. You have 

 learned an explanation of its movement. You have found 

 that heat and gravity are the chief causes of the movements 

 of both air and water. Now we must see what the water 

 does as it moves about on the surface of the earth. 



QUESTIONS 



1. How is water distributed? 



2. What is "humid" air? 



3- When is evaporation most rapid? 



