THE OUTER PLANETS. 121 



discovery was confirmed in July 1847 by the 

 discoverer himself, and shortly afterwards by 

 Bond and Otto Struve. Regarding the globe 

 of Neptune, we know practically nothing. No 

 markings of any kind have been observed on its 

 surface. However, in 1883 and 1884, Maxwell 

 Hall, an astronomer in Jamaica, noticed certain 

 variations of brilliance which suggested a rota- 

 tion-period of eight hours, but this was not 

 confirmed by any other astronomer. The spect- 

 rum of Neptune has been investigated by various 

 observers, who have found it to be similar to 

 that of Uranus. 



The existence of a trans -Neptunian planet 

 has been suspected by many astronomers. In 

 November 1879 the first idea of its existence 

 was thrown out by Flammarion in his ' Popular 

 Astronomy.' Flammarion noticed that all the 

 periodical comets in the Solar System have 

 their aphelion near the orbit of a planet. Thus 

 Jupiter owns about eighteen comets ; Saturn 

 owns one, and probably two ; Uranus two or 

 three ; and Neptune six. The third comet of 

 1862, however, along with the August meteors, 

 goes farther out than the orbit of Neptune. 

 Accordingly, Flammarion suggested the existence 

 of a great planet, assigning it a period of 330 

 years and a distance of 4000 millions of miles. 



