Biographical Memoir of Sir William Herschel. f3 



which the planets carry along with them in their course ; but it 

 was not discovered, till the close of the last century, that there 

 existed an immense planet beyond the orbit of Saturn. This 

 discovery was destine^- to be the fruit of Hcrschers labours. He 

 pursued with constancy the enterprise which he had formed of 

 examining successively the various regions of the heavens, and 

 of noting down all the remarkable phenomena which occur- 

 red. At Bath, on the 13th March 1781, while examining, with 

 one of his best telescopes, the constellation of Gemini, he ob- 

 served a star, the light of which appeared to him very differ- 

 ent from that of the neighbouring stars, and somewhat to resemble 

 that of Saturn, but much feebler. The perfection of the instru- 

 ment permitted him to see a well defined disk. Having con^ 

 tinued his observations, he discovered that this star had shifted 

 its place, although its motion with relation to the other stars 

 was very slow, for it had been stationary during twelve days 

 preceding. This observation was transmitted to Maskelyne 

 and Lalande, and was confirmed at Paris, Milan, Pisa, Berlin, 

 and Stockholm. The star was generally considered as an ex- 

 traordinary comet free of all nebulosity ; and astronomers were 

 occupied in determining the parabolic elements of its course. 

 The President Bochard de Saron, of the Academy of Sciences 

 of Paris, and Lexel, an astronomer of St Petersburg, who was 

 in Lond.on at the time, were the first who discovered its circular 

 form, and calculated the dimensions of the orbit. It was now 

 no longer doubted, that HerschePs star was a new planet ; and 

 all subsequent observations verified this unexpected result. We 

 have here a striking proof of the perfection of modern theories ; 

 for the laws regulating the motion of this new planet, were de- 

 termined before it had accomplished the tenth part of its course, 

 and that motion was not less accurately known than that of 

 other planets which had been observed during so many centu- 

 ries. Its distance from the sun is double that of Saturn, that 

 is to say, upwards of 660,000,000 of miles ; its volume is more 

 than seventy times as large as that of the earth ; it may be seen, 

 in favourable weather, without the assistance of a glass. The 

 period of its revolution is about eighty-four years ; and its tem- 

 perature, situated at the extremities of the known planetary sys- 

 tem, is more than forty degi'ees below that of ice. Some idea 



