OP ASPECT, ROTATION , LIBKATION, ETC. 173

simply carried round the end A as a centre, a similarresult would follow. But it is interesting to show thatso far as the rotation of the water within the basin isconcerned, the condition of the basin B is exactly thesame as that of a basin at C turning simply on apivot immediately beneath it.

Another experiment may be tried with the sameapparatus. The water in C and B may, without muchtrouble, be set rotating at the same rate. If this bedone, and then the rod be carried round at the samerate, so that the floating rod in C retains an un-changed position with respect to the bar A B or tothe arrow at A, it will be found that the water in Bbehaves precisely as the moon’s globe behaves (so farat least as the general relation we are dealing with isconcerned), turning always the same portion towardsthe centre C. Thus we learn that it is only by anadditional rotational movement that such a relationcan be preserved.

The moon then turns once upon her axis as shecompletes the circuit of her orbit. Yet it is notstrictly the case that the moon turns always the sameface towards the earth. We see somewhat more thanone half of the moon’s surface. Let us inquire howthis is brought about.

In the first place, the moon’s axis is not at rightangles to the plane of the path in which she travelsround the earth. (Let it be noticed, in passing, that>t is the inclination of the moon’s axis to this plane,and not to the plane of the ecliptic, which affects her