[7] "Researches on Solar Physics," by De la Rue, Stewart and Loewy;
_R. S. Phil. Trans_., 1869, 1870.

[8] "The Sun as Photographed on the K line"; _Knowledge_, London,
1903, p. 229.

[9] _R. S. Proc._, xv., 1867, p. 256.

[10] _Acad. des Sc._, Paris; _C. R._, lxvii., 1868, p. 121.



13. THE MOON AND PLANETS.


_The Moon_.--Telescopic discoveries about the moon commence with
Galileo's discovery that her surface has mountains and valleys, like
the earth. He also found that, while she always turns the same face to
us, there is periodically a slight twist to let us see a little round
the eastern or western edge. 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. But Beer and Madler measured the
heights of lunar mountains by their shadows, and found four of them