The Sun 



Our knowledge of the sun's mean density is 

 practically the only experimental fact known to 

 us regarding its internal nature ; this is not sur- 

 prising since we know absolutely nothing con- 

 cerning the interior of the earth which we inhabit. 

 All that remains to be told about the sun con- 

 cerns its surface, the only part which sends radia- 

 tions to us; these radiations constitute actually 

 the sole intermediary between us and planetary 

 space. Fortunately we have many methods for 

 examining them. 



A first method consists in determining what 

 is called the solar constant, that is to say, the 

 total quantity of energy radiated by the sun 

 to the surface of the earth. This is obtained 

 with ease by exposing to the sun's action actino- 

 meters, that is to say, thermometers with their 

 bulbs coated with lamp black, which absorb 

 all the rays falling on their surface. The 

 increase of temperature recorded by these 

 instruments enables us to measure the quantity 

 of heat radiated by the sun. By proceeding 

 in this manner Pouillet, Crova and Violle 

 ascertained that the quantity of solar energy 

 radiated annually to our globe would be capable 



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