732 THE UNIVERSE. 



buried under the thick incandescent layers of the star 

 which gives us hght.^ 



In his Theogony, Hesiod, wishing to give an idea of the 

 height of the firmament, tells us that an anvil of brass, 

 falling from the summit of heaven, would sink nine days 

 and nine nights through space before reaching the earth. 



How vastly the imagination of the poet of Boeotia is 

 below the truth ; a truth which quite confuses one ! In- 

 deed, on one hand, physics prove that a solid body, fall- 

 ing by gravitation during this space of time, would o\^j 

 ti^averse 143,000 leagues; whilst, on the other hand, the 

 astronomy of the nineteenth centiu-y teaches us that a 

 ray of light issuing from Alcyone, the most brilliant of 

 the Pleiades, takes five years to traverse the intervening- 

 space before reaching our eyes. And yet light is so rapid 

 that in the tenth part of a second one of its vibi-ations Avill 



^ The volume of the sun is more than 600 times as large as that of all the 

 planets J3ut together. It turns round its axis in twenty-five days and a half. 

 We may form an idea of the immense bulk of this star relative to that of the 

 earth, by means of a comj^arison mentioned by Arago in his Astronomie Popii- 

 laire. "A professor of Angers," he says, "hit upon the idea of counting the 

 number of grains of average size contained in the measure of capacity called a 

 litre; he found there were 10,000. Consequently a decalitre ought to contain 

 100,000, a hectolitre 1,000,000, and 14 decalitres 1,400,000. Having collected 

 the 14 decalitres of wheat, he showed his audience a single grain, and then said 

 to them, 'This is the size of the earth, while the heap represents the sun.' This 

 comparison occasioned infinitely more surprise among the students than the 

 statement about the relative size of 1 to 1,400,000 in abstract numbers had done. 



If we wish to compare tlie weight of the sun with that of the earth, astronomy 

 weighs them with as much precision as though each were placed in one of the 

 scales of a balance. The weight of the sun is 2,096,000,000,000,000,000,000,000,000 

 tons. That of the earth is only 5,875,000,000,000,000,000,000. 



The physical constitution of the sun has only been made out loy the astrono- 

 mers of our epoch. The body of this star is almost entirely dark, but it is 

 surrounded by three envelopes, one formed of vapours which touch it; another, 

 which is luminous, placed at a great distance, and which is called the photosphere; 

 and, lastly, a third, which covers the latter, and in which float the clouds. The 

 spots on the sun are occasioned by perforations in the photosphere which allow 

 us to see the earthy nucleus of the star. — See Guillemin, Le CieL Paris, 1865. 



