The superb success which had attended Le Verrier’s efforts to explain the cause of the perturbations of Uranus, naturally led this wonderful computer to look for a similar explanation of certain other irregularities in planetary movements.  To a large extent he succeeded in showing how the movements of each of the great planets could be satisfactorily accounted for by the influence of the attractions of the other bodies of the same class.  One circumstance in connection with these investigations is sufficiently noteworthy to require a few words here.  Just as at the opening of his career, Le Verrier had discovered that Uranus, the outermost planet of the then known system, exhibited the influence of an unknown external body, so now it appeared to him that Mercury, the innermost body of our system, was also subjected to some disturbances, which could not be satisfactorily accounted for as consequences of any known agents of attraction.  The ellipse in which Mercury revolved was animated by a slow movement, which caused it to revolve in its plane.  It appeared to Le Verrier that this displacement was incapable of explanation by the action of any of the known bodies of our system.  He was, therefore, induced to try whether he could not determine from the disturbances of Mercury the existence of some other planet, at present unknown, which revolved inside the orbit of the known planet.  Theory seemed to indicate that the observed alteration in the track of the planet could be thus accounted for.  He naturally desired to obtain telescopic confirmation which might verify the existence of such a body in the same way as Dr. Galle verified the existence of Neptune.  If there were, indeed, an intramercurial planet, then it must occasionally cross between the earth and the sun, and might now and then be expected to be witnessed in the actual act of transit.  So confident did Le Verrier feel in the existence of such a body that an observation of a dark object in transit, by Lescarbault on 26th March, 1859, was believed by the mathematician to be the object which his theory indicated.  Le Verrier also thought it likely that another transit of the same object would be seen in March, 1877.  Nothing of the kind was, however, witnessed, notwithstanding that an assiduous watch was kept, and the explanation of the change in Mercury’s orbit must, therefore, be regarded as still to be sought for.

Le Verrier naturally received every honour that could be bestowed upon a man of science.  The latter part of his life was passed during the most troubled period of modern French history.  He was a supporter of the Imperial Dynasty, and during the Commune he experienced much anxiety; indeed, at one time grave fears were entertained for his personal safety.

Early in 1877 his health, which had been gradually failing for some years, began to give way.  He appeared to rally somewhat in the summer, but in September he sank rapidly, and died on Sunday, the 23rd of that month.