52 



which have the sun for the common centre of their 

 periodical revolutions : whilst the others, which are 

 called secondary, move round one principal planet, 

 which they accompany like satellites, m its annual re- 

 volution. 



Venus and the earth have each of them their satel- 

 lite: one will undoubtedly be some time or other dis- 

 covered in Mars. Jupiter has four, Saturn five, and a 

 ring or luminous atmosphere which seems to perform 

 the office of a number of small moons ; being situate so 

 far from the sun, he would have received too faint a 

 light from it, if his satellite and ring did not augment it 

 by reflection. 



We have discovered twenty-seven planets, which at 

 present compose our solar system ; but we are not cer- 

 tain that there are not more. Their number has re- 

 ceived a great increase by the invention of telescopes : 

 more perfect instruments, and more accurate observers, 

 may probably make farther additions to them. The 

 satellite of Venus, discovered in the last century, gives 

 room to hope for still greater success. 



4. The comets also are now found to be planetary 

 bodies, whose long routes our astronomers calculate, 

 foretel their distant returns, and determine their place, 

 appearances, and tract. Upwards of thirty of these 

 bodies at present acknowledge the empire of our sun, 

 and the orbits which some trace round are so extensive, 

 that they do not complete their course till the end of 

 a long series of years, and even many ages. 



The stars are innumerable ; and the constellations, 

 which antiquity reckoned to be but few in number, 

 amount to thousands. 



If the diameter of the great orbit, which our planet 

 describes round the sun, is more than sixty millions of 

 leagues, yet this vast circumference vanishes into no- 



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