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make it clear, that some of these planetary perturbations are ofno small moment, and help to indicate how difficult it was todetermine the probable position of Neptune from the disturb-ance of the movements of Uranus , when it is seen howNeptune itself is perturbed.*
Far away from the influence of the Sun ’s genial warmth, orperhaps we should rather say, almost beyond any possibility ofbenefit from it, although not beyond the control of its mightygravitating attraction, Neptune pursues its tedious journey.The observations made since its discovery prove its meandistance from the Sun to measure very nearly 2,800,000,000miles. It is therefore about thirty times as far away as theEarth from the centre of the Solar System .
It may be well to notice that such a distance is verydifferent from that which Bode’s so-called law would involve.To correspond with this series, it should be 800,000,000 milesgreater than it is ; which would also involve a periodic timefor the planet’s rotation round the Sun about sixty years longerthan its real period, which is nearly 165 years.
Neptune ’s velocity as it travels round the Sun is about 34miles per second. Its speed is therefore only about £th of thatof the Earth , and |4h of that of Mercury, but nevertheless200 times as great as that of an express train. Its probablediameter—as to which there is, however, some doubt, sinceits immense distance makes it difficult accurately to measurethe apparent width of its disc-—is about 35,000 miles, i.e., itis about Jj-th greater than that of Uranus .
* It afterwards proved that the hypothetical planet employed in thecalculations of Le Verrier had been supposed to be located in an orbit,the mean distance of which from the Sun was about 570,000,000 milesgreater than that of Neptune , while its mass was about twice as great.In like manner, the mean distance, assumed by Mr. Adams for his planet,exceeded that of Neptune by about 675,000,000 miles, and its mass wasthree times the real value. Nevertheless, the effect that each would haveproduced upon the movement of Uranus , in the portion of its orbit whichit described in'about twenty years on either side of the date of the year1822, was so nearly the same as that of Neptune itself, that in each casethe result of the calculations made almost exactly indicated the truedirection in which the unknown planet was to be found in 1846.