Belfast, 26th December, 1869. Srit—I see the controversy about the
moon's motion has been reopened in your columns, and Mr. Jelinger Symons repeats his plausible and fallacious appeal to common sense comparing the motion of the moon round the earth to that of governor-balls round the vertical axis that carries them : as if the line connecting the moon with the earth (technically called her radius vec- tor) were a rod or a string. But if the conditions of the governor-balls' motion were assimilated as nearly as possible to those of the moon's, by letting each of them rotate on an axis passing through its own centre and resting in the end of the rod that carries the ball, and were the friction of these axes entirely got rid of, then, on the governor being set in motion, the balls would not move like the moon ; they would present the same faces to the same points of the compass, and constantly changing faces to the axis of the governor.
If the controversy between Mr. Symons and the astronomers is regarded as a merely geometrical question, it is only a dispute about words, and may be answered in this way : the moon does rotate with respect to the stars, but not with respect to the earth. But as a mechanical question, it is more than a dispute about words : a point of fact is involved,which shows the astronomers to be decidedly in the right. A certain quantity of mechanical force (technically, vis viva) is due to the moon's orbital motion, whatever may be her rotation ; and over and above this a certain quantity of force is clue to her rotation on her axis. Now this latter quantity is exactly what she would have if she had no motion at all except that of rotation on her axis once in eitry lunar month ; and it would not exist if she were always to present the same faces to the points of the compass, and conse- quently a constantly changing face to the earth.
Respeotfully yours, J. J M. If this is difficult to understand, the difficulty is in the subject : quantity of force is not a familiar idea, and the elementary principles of mechanics are not self-evident, like those of geometry.
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