4 AGRICULTURE. 



force of gravity, without which nothing could 

 remain upon the surface of the earth, owing to 

 the centrifugal force caused by the rotation of 

 the earth. 



Other forms of energy are the molecular^ 

 forces of cohesion^ and adhesion, ; and the 

 atomic force of chemical affinity, all of which 

 exist within the substances themselves, and act 

 at insensible distances. 



II. The Sun's Energy. 



The great source from which we derive, either 

 directly or indirectly, most of our energy is the 

 sun. " The circulation of winds and waters, 

 the changes of temperature, and the activities 

 of living beings all depend upon the sun's 

 energy," without which there could be upon 

 the surface of the earth no motion and no life. 



The sun's energy comes to us, it is believed., 

 by means of waves in the ether of space. Some 

 of these waves produce the various colors, or are 

 what we might call light waves ; others are not 

 perceptible to the human eye, but are heat 

 waves ; still others are especially productive of 

 chemical changes, as is manifested in photog- 

 raphy. 



When the sunshine falls upon the soil a por- 

 tion of it is absorbed, and the molecular motion 



t Terms thus marked (double dagger) throughout the book are 

 found in the Glossary. 

 * Scott's Geology^ p. 29. 



