CHAPTER IV 



TRANSMISSION OF HEAT 



RADIATION 



The Sun as a Source of Heat. The most important source of 

 heat is the sun, which is continually sending to us light and heat 

 waves. The sun has a temperature of about 10,800 F. It has 

 been estimated that a solid column of ice 2J miles in diameter 

 reaching from the earth to the sun, a distance of about ninety-two 

 million miles, would melt in a single second if the entire heat of the 

 sun were concentrated on the ice. If the sun were composed of 

 solid coal, and we derived our heat from the burning of the coal, 

 it would burn out in less than five thousand years. Since the earth 

 is millions of years old, the sun cannot be burning. It is really 

 heated to glowing point like a piece of white-hot iron. A scientist 

 named Helmholtz first satisfactorily explained why the sun continues 

 to give off heat. He advanced the idea that the sun was contract- 

 ing about 250 feet each year. In other words, the enormous weight 

 of the sun causes tremendous internal contraction which produces 

 great quantities of heat. Newcomb, another scientist, has estimated 

 that in about seven million years the sun will be one-half its present 

 size.- (Compare the size of the sun with that of the earth in Chapter 

 XIII.) 



How the Heat of the Sun Reaches the Earth. We know how 

 much colder the air gets as we ascend. About 10 miles above the 

 earth the temperature of the atmosphere is about 90 below zero 

 Fahrenheit, while the space beyond our atmosphere is estimated to 

 have little, if any-heat, the temperature being about 459 F. below 

 zero. This would seem to tell that the sun does not send us heat 

 directly. Across this enormous cold space light and heat waves 

 travel without warming any part of space, because, as is believed, 



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