In recent times, as most of my readers know, the paradox that the moon does not rotate has been revived more than once. In 1855 it was sustained by Mr. Jellinger Symons, one of whose staunchest supporters, Mr. H. Perigal, had commenced the attack a few years earlier. Of course, the gist of the argument against the moon's rotation lies in the fact that the moon always keeps the same face turned towards the earth, or very nearly so. If she did so exactly, and if her distance from the earth were constantly the same, then her motion would be exactly the same as though she were rigidly connected with the earth, and turned round an axis at the earth. The case may be thus illustrated: Through the middle of a large orange thrust one short rod vertically, and another long rod horizontally; thrust the further end of the latter through a small apple, and now turn the whole affair round the short vertical rod as an axis. Then the apple will move with respect to the orange as the moon would move with respect to the earth on the suppositions just made. No one in this case would say that the apple was turning round on its axis, since its motion would be one of rotation round the upright axis through the orange. Therefore, say the opponents of the moon's rotation, no one should say that the moon turns round on her axis. Of course, the answer would be obvious even if the moon's motions were as supposed. The moon is not connected with the earth as the apple is with the orange in the illustrative case. If the apple, without rigid connection with the orange, were carried round the orange so as to move precisely as if it were so connected, it would unquestionably have to rotate on its axis, as any one will find who may try the experiment. Thus for the straight rod thrust through the apple substitute a straight horizontal bar carrying a small basin of water in which the apple floats. Sway the bar steadily and slowly round, and it will be found (if a mark is placed on the apple) that the apple no longer keeps the same face towards the centre of motion; but that, to cause it to do so, a slow motion of rotation must be communicated to the apple in the same direction and at the same rate (neglecting the effects of the friction of the water against the sides of the basin) as the bar is rotating. In my 'Treatise on the Moon' I have described and pictured a simple apparatus by which this experiment may easily be made.