18 GENERAL SCIENCE 



and caused marked differences in their customs and lan- 

 guages. The same barriers confined animals and plants 

 to certain localities, thus fostering and developing their 

 peculiar characteristics. 



The advancement of science, the invention of various 

 means of travel and communication, and the progress 

 of civilization have done much toward overcoming these 

 natural barriers, so that now the earth may be considered 

 a relatively small body, with almost every portion of its 

 surface accessible to the active traveler. 



Structure of the Earth. For convenience of study, 

 the earth is divided into several parts or spheres : (1) the 

 outer, gaseous envelope, or atmosphere; (2) the liquid 

 envelope, the water or hydrosphere; (3) the solid rocky 

 part, the lithosphere; (4) the center, the nucleus or 

 centrosphere. 



Motions of the Earth. The earth has three motions, 

 two of which greatly influence all things living upon it. 

 The first is a daily motion, or rotation on its axis; the 

 second is a yearly motion, or revolution about the sun; 

 the third, which is usually disregarded because of its 

 lack of noticeable effects, consists of an onward motion 

 through space which the earth has, together with the 

 other parts of the solar system. 



Rotation. - - The earth turns once on its axis every 

 twenty-four hours (Figure 16). As this rotation takes 

 place, the sun is shining upon one half of the earth's 

 surface, leaving the other half in darkness. The effect 

 to us is that of the sun rising at one point in the horizon, 

 moving over the sky, and setting at some point in the 

 horizon nearly opposite. In reality we are being turned 

 into and out of the sunlight. The same effect is noticed 

 when riding on a swiftly moving train. The train seems 



