those made at most of the other national
observatories; but a volume relating to one subject is issued whenever
the work is done. When I was there, the volumes containing the
earlier meridian observations were in press. Struve and his chief
assistant, Dr. Wagner, used to pore nightly over the proof sheets,
bestowing on every word and detail a minute attention which less
patient astronomers would have found extremely irksome.

Dr. Wagner was a son-in-law of Hansen, the astronomer of the little
ducal observatory at Gotha, as was also our Bayard Taylor. My first
meeting with Hansen, which occurred after my return to Berlin, was
accompanied with some trepidation. Modest as was the public position
that he held, he may now fairly be considered the greatest master of
celestial mechanics since Laplace. In what order Leverrier, Delaunay,
Adams, and Hill should follow him, it is not necessary to decide.
To many readers it will seem singular to place any name ahead of
that of the master who pointed out the position of Neptune before
a human eye had ever recognized it. But this achievement, great
as it was, was more remarkable for its boldness and brilliancy than
for its inherent difficulty. If the work had to be done over again
to-day, there are a number of young men who would be as successful
as Leverrier; but there are none who would attempt to reinvent
the methods of Hansen, or even to improve radically upon them.
Their main feature is the devising of new and refined methods of
computing the variations in the motions of a planet produced by the
attraction of all the other planets. As Laplace left this subject,
the general character of these variations could be determined without
difficulty, but the computations could not be made with mathematical
exactness. Hansen's methods led to results so precise that, if they
were fully carried out, it is doubtful whether any deviation between
the predicted and the observed motions of a planet could be detected
by the most refined observation.

At the time of my visit Mrs. Wagner was suffering from a severe
illness, of which the crisis passed while I was at Pulkova, and
left her, as was supposed, on the road to recovery. I was, of
course, very desirous of meeting so famous a man as Hansen. He was
expected to preside at a session of the German commission on the
transit of Venus, which was to be held in Berlin about the time of
my return thither from Pulkova. The opportunity wa