292 NINETEENTH CENTURY. FT. in. 



time it was expected, and was seen each night aftenvards as 

 usual till January 12. On that night, however, when 

 Lieutenant Mauiy looked at it from the observatory at 

 Washington, in the United States, he saw, not one comet, but 

 two distinct and separate comets moving along together. This 

 seemed so strange that it would scarcely have been believed 

 if several astronomers had not watched the comet for 

 more than a month, and satisfied themselves that it had 

 really split up into two parts, each part being a perfect 

 comet, with a bright head and a glowing tail ! These two 

 comets returned in 1852, still keeping each other company 

 at the same distance apart as in 1846, but since then they 

 have never been seen again. Many other comets have been 

 discovered besides these, and we know that many thousands 

 or even millions must be wandering through space, but of 

 these we cannot speak here. 



Adams and Leverrier determine the Position of an 

 Unknown Planet by its Influence on the Orbit of Uranus, 

 18431846. The next discovery which w r e must consider is 

 one of the most remarkable in the history of astronomy, 

 because it was not made with the telescope but was worked 

 out independently by two men entirely by means of Newton's 

 theory of gravitation. You will remember that in 1781 Sir 

 William Herschel discovered the planet Uranus moving out- 

 side all the other planets (see p. 272). Now many astrono- 

 mers had noticed this body in earlier ages, and supposing it 

 to be a star, had marked its position from time to time in the 

 heavens, and from these observations it was now possible 

 to calculate its path round the sun. When this was done 

 it was found, however, that the planet did not move as 

 it ought to do according to the theory of gravitation. The 

 pull of the sun and the known planets did not account for 



