INTRODUCTION 3 



all the machinery in the world is run by power that came 

 in the form of sunlight long, long ago, and was stored up 

 in the form of coal. Nature, you see, is the very founda- 

 tion of man's work, either in city or in country. So, if we 

 are to have a real understanding of our lives we must un- 

 derstand about nature, and our place in nature, as well as 

 about our fellow men and our relations to them. 



Let us begin by thinking of the world as a whole, and 

 try to form a picture of it in our minds somewhat as you 

 learned to do in geography. Then we will consider it 

 more carefully, one thing at a time. 



There are three parts to the earth. We live on top of 

 two of these parts, and nearly at the bottom of the third. 

 These three parts are the solid, the liquid, and the gaseous 

 parts. Most of the earth is solid rock, the liquid is water, 

 and the gaseous part we call the atmosphere or air. Per- 

 haps you have heard the statement that we live at the 

 bottom of a great sea of air. It lies over the whole surface 

 of the earth, and extends more than a hundred miles above 

 it. It is a huge, soft cover. It flows more easily than 

 water. When air moves so that we can feel it we call it 

 wind. Though it is invisible it has weight. It is much 

 more dense and heavy at the bottom than it is at the top. 

 Most of the atmosphere, by weight, is within four miles of 

 the surface of the earth. Above that it is very light and 

 thin. 



Our earth is very old. It has not always been the pleas- 

 ant place that it is to-day. There was a time when we 

 could not have lived here. There was a time when there 

 were no plants. Before that there was a time when there 

 was no soil. Nearly the whole earth was covered with 

 water. The only land was bare, brown rock. Not one 



