88 The Realm of Nature CHAP. 



were made, and the path of Uranus along the star-dome 

 calculated, it was found that the planet did not keep to its 

 time-table. The English astronomer Adams and the French 

 Leverrier made calculations on the assumption that this 

 irregularity was produced by an unknown planet beyond 

 Uranus. In 1846 their work was finished almost simultane- 

 ously, and each predicted the position of the hypothetical 

 planet in the sky. The very day that the information from 

 Leverrier reached the observatory of Berlin, the German 

 astronomer Galle turned his telescope to the part of the sky 

 indicated, and there discovered the new planet which was 

 named Neptune. Like Uranus it had previously been 

 recorded as a star, and it was only by mistrusting his 

 observations that an earlier astronomer failed to detect its 

 true nature. One satellite has been observed which re- 

 volves, like those of Uranus, from east to west. 



132. Comets. Occasionally a luminous body appears 

 in the sky, brighter in some cases than the planets, and usu- 

 ally enswathed in a long flowing tail of gauzy texture, from 

 which peculiarity it is called a comet. Many comets have 

 been found to travel in elliptical orbits, much more elongated 

 than those of the planets, but like them with the Sun in one 

 focus. As a comet pursues its path, it approaches the Sun 

 with increasing velocity, sweeps round and sometimes almost 

 touches the solar surface, and then flies on with ever 

 diminishing speed to its aphelion. Halley's comet was the 

 first the regular return of which was noticed ; its period 

 is 76 years, and it should next return to perihelion in 

 1910. It will then pass within the Earth's orbit, but its 

 aphelion lies outside the orbit of Neptune. Several 

 comets have their farthest points from the Sun near the 

 orbit of Neptune ; others show a similar relation to Uranus 

 and to Saturn, while quite a number of comets of short 

 period are associated with the orbit of Jupiter. Many of 

 the grandest comets that have been seen pursued a path 

 shaped like a parabola or hyperbola, and after passing the 

 Sun swept out of the solar system for ever. It is supposed 

 that the orbits of comets are naturally parabolas, but when 

 the comet happens to pass near enough to a planet the 



