THE ANNUAL OF THE MOON.

MR. TELINGER STMONS returns to the charge in defence of the position which he has taken up with regard to the rotation of the moon. As our readers know, astronomers usually describe the moon as performing a ro- tation on its own axis exactly equal in time to its sidereal revolution round the earth ; the consequence of the remarkable coincidence of the two rota- tions being that we always see the same face of the moon. In a recent and fifth edition of his Outlines of Astronomy, Sir John Herschel has appended to a passage on the subject the following note.

" Strange to say, there are persons who find it difficult to regard as a ro- tation on its own axis that peculiarity of the moon's motion which consists in keeping its face towards the earth. Should any of our readers be in this predicament, we recommend him to plant a staff upright in the ground, and i

grasping it with both his hands, walk round it, keeping as close to t as possible, with his face always turned towards it ; when the unmistakeable sensation of giddiness will effectually satisfy him of the fact of his rotation on his own axis. Or he may walk round a tree, always facing it, and carry- ing a compass in his hand, and while watching the needle during a few cir- cuits, endeavour to persuade himself that he does not turn upon his own centre."

Mr. Jelinger Symons objects to the confusion introduced into the e*- position of mechanics as well of astronomy, and as an educationist he protests against misleading the general learner. He accordingly analyzes Sir John H' erschel's illustration.

"Suppose, to follow out your first experiment, adapting it still more closely to the facts, we expand the arc of the circle, and take a segment ex- actly similar to the are actually described by the moon in her orbit, to run along it sideways with one's face to its centre, would certainly not produce giddiness ; ergo, according to your own test, the moon does not rotate on i her axis. This s a fair inference from your own argument. Your second experiment simply shows that the box of the compass is, like the moon turning round, while the card is held in one direction by the magnet ; this

nowise proves that the box is* rotating round an axis within itself.

"Let a carriage travel round the globe or any circular surface. Its wheels rotate on their axes ; and no matter how slowly they do so, a new face or part of the tire is continually being presented towards the centre of the globe or circle round which the carriage is revolving. The motion of these wheels is, I apprehend, precisely that of the earth round the sun, and is composed—lat, Of the simple revolution of the carriage round the centre of its orbit ; and 2d, Of the wheel round its own axis. This is not the 'po- tion of the moon, for she preserves the same face to the centre of her orbit. Now drag one of the wheels; it will at once partake of the simple motion of translation which the carriage has round its circular path, and will, like it, preserve the same face or part of the tire to the centre of its orbit, being confined to that one motion of simple revolution ; it will, therefore, now move exactly as the moon moves, because its axial rotation is stopped, and thus, the singular coincidence' of keeping the same face to the centre of its orbit, is proved to be the result not of its having, but of its not having, a rotation round its own axis.

"I asked an excellent mechanist who was showing me, the other day, a beautiful steam-engine at work which he had just constructed, What should you say to the man of science who told you that each of those governor balls, revolving round their central shaft, were also at the same time rotating round their own axes ; that it was only by a wonderful coin- cidence that the same face was always turned towards it, and that their mo- tion was the same as that of yonder wheel spinning round its axis ; and that the motion of each being the same, ought to be described by the same term ? ' His reply showed how perilous it might beoome to the validity of patents, were the confusion of terms you have unhappily sanctioned to ob- tain adoption. There is, therefore, a practical object in pursuing this sub- ject,—I trust, to a speedy and final settlement. Bear with me, therefore, If I add one more to the homely illustrations of which you have set the ex- ample."

Mr. Symons is well aware that expressions employed by astronomers with regard to the celestial bodies are conventional. As we said when the subject was first mooted, we are disposed to infer from evidence on

the subject, that although since their attention has been drawn to the point; astronomers clearly understand the precise nature of the motion

performed by the moon, yet when they first adopted their• mode of ex- pression, it was the result of an inaccurate conception. They had a pre- conceived idea that the heavenly bodies rotate on their own axes, and then explained to themselves that the constant presentation of the same face of the moon to a spectator on the earth,—we are making no account of the fibration, which is irrelevant to the present issue,—was caused by

the slowness of the rotation, which exactly coincided with the rotation of the moon round the earth. Now on this assumption four remarks may be made well worth the consideration of so candid and popular a writer as Sir John Herschel.

Is it established that satellites rotate on their own axes, or is it not possible, from the experience of our own planet, that they rotate on the

axis of the planet to which they severally belong ?—a question opening large considerations, not, we believe, alien to speculative inferences al- ready partially adopted by astronomers. The relative position of the moon to the earth would be produced if the satellite rotated (nearly) round the axis of the earth itself: it is against ordinary logic to assume a second cause for a single effect of which one cause is sufficient.

In the case of one of the planets, the satellites move in their orbits in- versely. In his Outlines, Sir John observes the evidences that some

power is perceptible in the planetary movements distinct and different from gravitation. Astronomers, therefore, have not finally arrived at a comprehension of the law which controls the rotation of the planets ; a law, perhaps, subject to more various exception than they have yet recognized.

The conventional expression can only be justified on the assumption, that the axis of the moon's rotation is excentrie to the moon itself; but if the expression begets an erroneous idea amongst amateurs,—and perhaps also amongst the most acientific,—that the body in question actually " rotates ' or turns upon itself as a wheel does, it then tends to perpetuate those false conceptions which form as it were a sort of crystallized igno- rance, and are the most stubborn obstacles to arriving at a better and simpler understanding.