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They were Mr. Adams (now Lowndean Professor of Astronomyin the University of Cambridge ), and M. Le Verrier* (lateDirector of the Observatory of Paris ), whose comparatively recentdeath is mourned as that of one who, by his untiring assiduityand his splendid ability, apart from the discovery of the planet Neptune , lias conferred benefits almost beyond all estimationupon the science he so much loved. The honour of the dis-covery is so great that it can well afford to be divided betweenthese two distinguished astronomers.
The planet was practically found, and its place was pointedout, within a degree of longitude, by Le Verrier, and within lessthan 2^ degrees by Mr. Adams, simply and solely as the resultof theoretical calculations of the most elaborate character. Butthe difficulty of the problem thus solved was immense. Someidea of it may perhaps be gathered from the following state-ment.
When the observations made of the planet Uranus duringsome fifty years after its discovery in 1781 were compared withthe previous observations of it, to which we have referred inthe earlier part of this lecture, i.e., with the records of its place,which were found to have been made at various times withoutits real character being known, it was noticed, that there wasapparently a considerable amount of irregularity in its pastmovements ; and that this irregularity showed a tendency toincrease again. For a considerable number of years before theyear 1822, and especially after the year 1800, the planet seemedto have gained speed in addition to that which would properlycorrespond with its distance from the Sun; but, as the year1822 drew nigh, the rate of increase in its speed diminished,and after that date it seemed that some retarding influencebegan to act upon it, the effect of which, in a few years’ time,became decidedly vigorous.
Fig. XCII. illustrates the hypothesis by which it was sug-gested that this peculiarity in the movement of Uranus mightbe explained. It was thought, that an additional planet mightpossibly be situated in an orbit exterior to that of Uranus , theattraction of which might have acted (although the real effect* It may be noticed that we have put the two names in alphabetical order.