20 Introduction. 



of energy may be transformed into another, and one kind 

 of substance may be decomposed and others made from the 

 components, but in these transformations there is neither 

 annihilation nor creation. The small amount of ashes 

 left from the winter's supply of coal or wood seems to 

 point to a destruction of matter, but their weight added 

 to that of the products which pass up the chimney is even 

 greater than that of the original fuel by the amount of oxy- 

 gen which was required to burn the fuel. So, too, the 

 energy of 10 horses expended in threshing grain seems 

 to be annihilated but it is only transformed. Heat of fric- 

 tion and concussion, sound and material raised into new 

 positions, from which it may fall, when added together will 

 make a sum equal to that developed by the horses. Again 

 we appear to realize in the increase of our domestic ani- 

 mals or in milk produced much less weight than has been 

 used by them in feed and drink but this is because such 

 large quantities of the materials eaten, breathed and drank 

 escape in an invisible form through the skin and lungs. 



23. The Source of the Earth's Energy. The real source 

 of the earth's energy is the sun. All the rivers of the 

 world flowing to the sea and the rush of the winds swaying 

 the tree-tops and lashing the ocean into billows represent 

 so much water and air lifted from a lower to a higher level 

 by the sun's heat and now being pulled by gravity back 

 to their original level to be raised again and to again re- 

 turn, just as a pendulum rises and falls while swinging 

 through its arc. 



The wood burned in the stove, the coal burned in the en- 

 gine and the food consumed by the horse are all the prod- 

 uct of sunshine which lifted the constituents of soil, 

 moisture and air into such combinations as readily per- 

 mits of their return to other forms, setting free the energy 

 which was consumed in producing them. 



24. Solar Energy. When the sun rises the temperature 

 increases, usually becoming higher and higher until past 



