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Chopin is the color genius of the piano, his eye was attuned to hues the most fragile and attenuated; he can weave harmonies that are as ghostly as a lunar rainbow. And lunar-like in their libration are some of his melodies glimpses, mysterious and vast, as of a strange world.
The same may be said in regard to the phenomena of libration; the inclination of the moon's axis to the plane of her orbit is really small, but is purposely exaggerated in this apparatus in order to make the results apparent; in the position represented, it is quite obvious that an observer upon the earth can see a little past one pole, and cannot quite see the other, as well as that this condition will be reversed after half a revolution.
It is hardly necessary for me to state that the result obtained was negative. The admirable memoir of Lagrange upon the libration of the moon seemed to have exhausted the subject. This, however, was not the case. The motion of revolution of our satellite around the earth is subject to perturbations, technically termed secular, which were either unknown to Lagrange or which he neglected.
So, then the moon displays invariably the same face to the earth; nevertheless, to be quite exact, it is necessary to add that, in consequence of certain fluctuations of north and south, and of west and east, termed her libration, she permits rather more than half, that is to say, five-sevenths, to be seen.
The tide was going out, but in order to be able to pass, we had to wait until the breakers receded. We watched them approach us. They dashed against the rocks, swirled in the crevices, rose like scarfs on the wind, fell back in drops and sprays, and with one long, sweeping libration, gathered their green waters together and retreated.
This was called libration, and the explanation was clear when it was understood that in showing always the same face to us she makes one revolution a month on her axis uniformly, and that her revolution round the earth is not uniform. Galileo said that the mountains on the moon showed greater differences of level than those on the earth. Shroter supported this opinion. W. Herschel opposed it.
In this fit of torpor or quiescence of a part or of the whole of the system, an accumulation of the sensorial power in the affected fibres is formed, and occasions a second paroxysm of exertion by the application only of the natural stimulus, and thus a libration of the sensorial exertion between one excess and the other continues for two or three days, where the stimulus was violent in degree; and for weeks in some fevers, from the stimulus of contagious matter.
At this phase, if libration is favourable, the manifold details of its very uneven and apparently convex floor are best seen. On the S.W. side is a group of large craters associated with a number of low hills, of which Schmidt shows five; but I have seen many more, together with several ridges between them and the E. wall.
The inner slopes of this vast rampart are very complex, especially on the E., where many terraces and depressions may be seen under suitable illumination. There are three large ring-plains in the interior, all of them S. of the centre; and at least five smaller ones near the inner foot of the E. wall, which can only be well observed when libration is favourable.
I had contributed, in a regular and very assiduous manner, during nearly two years, to the observations which were made day and night with the transit telescope and with the mural quadrant at the Paris Observatory. I had undertaken, in conjunction with M. Bouvard, the observations relating to the verification of the laws of the moon's libration.
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