s therefore open
of bringing a message of good news from his daughter. Apart from
this, the prospect of the meeting might have been embarrassing.
The fact is that I was at odds with him on a scientific question,
and he was a man who did not take a charitable view of those who
differed from him in opinion.

He was the author of a theory, current thirty or forty years ago,
that the farther side of the moon is composed of denser materials
than the side turned toward us. As a result of this, the centre
of gravity of the moon was supposed to be farther from us than the
actual centre of her globe. It followed that, although neither
atmosphere nor water existed on our side of the moon, the other
side might have both. Here was a very tempting field into which
astronomical speculators stepped, to clothe the invisible hemisphere
of the moon with a beautiful terrestrial landscape, and people it as
densely as they pleased with beings like ourselves. If these beings
should ever attempt to explore the other half of their own globe,
they would find themselves ascending to a height completely above
the limits of their atmosphere. Hansen himself never countenanced
such speculations as these, but confined his claims to the simple
facts he supposed proven.

In 1868 I had published a little paper showing what I thought a
fatal defect, a vicious circle in fact, in Hansen's reasoning on
this subject. Not long before my visit, Delaunay had made this paper
the basis of a communication to the French Academy of Sciences, in
which he not only indorsed my views, but sought to show the extreme
improbability of Hansen's theory on other grounds.

When I first reached Germany, on my way from Italy, I noticed
copies of a blue pamphlet lying on the tables of the astronomers.
Apparently, the paper had been plentifully distributed; but it was
not until I reached Berlin that I found it was Hansen's defense
against my strictures,--a defense in which mathematics were not
unmixed with seething sarcasm at the expense of both Delaunay
and myself. The case brought to mind a warm discussion between
Hansen and Encke, in the pages of a scientific journal, some fifteen
years before. At the time it had seemed intensely comical to see two
enraged combatants--for so I amused myself by fancying them--hurling
algebraic formulae, of frightful complexity, at each other's heads.
I did not then dream that I should live to be an object of the same
sort of attack