Solar Energy. 21 



noon, then when the sun sets the temperature falls again, 

 continuing to do so until once more the sun is above the 

 horizon. So, too, as our days grow longer and longer with 

 the approach of summer in the middle and higher lati- 

 tudes, making more hours of sunshine in every twenty- 

 four, the mean daily temperature increases hut falls away 

 again when the nights became longer than the days. Such 

 and many other facts prove the sun to be a source of 

 energy which in some manner is being transferred to our 

 earth. More than this, since the earth travels entirely 

 around the sun once each year and yet each day receives 

 heat and light from it, it follows that solar energy is con- 

 tinually leaving the sun in all directions, so that the 

 amount arrested by the earth forms a very small portion 

 of the whole. 



25, How Solar Energy Reaches the Earth. To under- 

 stand how the energy of the sun reaches us, coming across 

 93,000,000 of miles we must learn that the energy travels 

 in the form of waves through some medium filling space, 

 which has been named ether, but whose real nature is not 

 yet understood. 



Something similar to the process in question would be 

 represented by a man at the center of a pond throwing 

 its water into waves. These waves would spread in all 

 directions and when reaching the beach a portion of the 

 energy of the waves would be absorbed or transferred to 

 whatever body they chanced to strike. The energy, 

 therefore, generated in the muscles, is changed first into 

 wave energy in the water and conveyed away from the 

 man in all directions, but afterward when arrested at the 

 beach, the waves may move the pebbles, making them 

 grind upon one another, wearing themselves into sand, 

 or their sliding may change a portion of the wave energy 

 into heat and thus the person in a small degree may warm 

 the pebbles lying on the distant margin of the lake, not di- 

 rectly by the heat of his body, but by the waves set up in 



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