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rotation of a body on its own axis and its revolution round an independent body; that, if it be once endued with the former motion, no force is required to keep it in the state of rotation; whereas, revolution in an orbit is due to the continued attraction of the central body, or, to speak strictly, to their mutual attraction. The force which produces revolution has no effect upon rotation. Attraction acts upon the mass of the body, preventing it from straying beyond certain limits, but not altering the direction in space of a single line in the body. Whenever, therefore, as in the case of the moon, one diameter preserves a fixed direction in space, and all others are continually changing theirs, that (by the laws of physics) can be due to nothing else than rotation. If the moon were to be released from all attraction, she would move in a straight line; but her rotation, if it exist, would go on as before; not a single line would have its direction influenced by the cessation of attraction. But even Mr. Perigal admits that if the moon were moving in a straight line instead of a curve, the fact of her shifting (as she does) the direction of any particular point of her surface would be proof of her rotation. It is equally so when she revolves in a curve by the previous reasoning, or else we should come to this curious conclusion, that the cessation of attraction would cause rotation.
My answer to "Nauticus" is, that I had no intention of answering particular arguments of the other side, but to show that their explanation failed to account for the actual facts.
I am, yours faithfully,
THE MOON CONTROVERSY.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER.
Sir,—On reviewing the whole controversy respecting the moon's rotation, or non-rotation, on her own axis, it will be observed that mathematicians are ranged on one side, and practical mechanics and engineers on the other, and that theory is at variance with practice. Mathematicians have adopted a quasi definition of rotation, which leads to innumerable absurdities, e.g. that every atom of a spinningtop rotates on its own axis; that the ball on St. Paul's cathedral rotates on its own axis once in 23b. 56m. 4'o9o8s., &c. &c. They use the terms "rotation" and "rotation on its own axis" as though they were synonymous in meaning, quite overlooking the distinction between axial and radial rotation. It would surely be quite possible for them to be more concise in the use of language without calling in question any single mathematical deduction or truth of any sort or kind. It would be better to understand the term "rotation" in the sense used by mechanics, engineers, and men of plain common sense.
It is really astonishing how one writer after another, like Mr. Bird in the last No. of the Register, can nersuade themselves that " turning round" necessarily involves " rotation," either axial or radial.
Before closing the correspondence on this most interesting subject it may be as well to notice Sir John Herschel's illustration of axial rotation. He " plants a staff in the ground, and, grasping it in both hands, walks round it, keeping as close to it as possible, with his face always turned towards it," when the "unmistakeable sensation of giddiness effectually satisfies him of the fact that he has rotated on his own axis I" Mr. Bertram Mitford, of Cheltenham, in a letter addressed to the Practical Mechanics' Journal, Dec. i, 1859, thus effectually disposed of this most transparent fallacy by the following arguments:— t. Because giddiness may be produced in various other ways, by looking over a precipice, &c.
2. Because (mechanically speaking) no man can walk round his own axis in any way, much less turn upon it when holding on to any fixed object, which must be external to his body. He may walk his axis round any object he pleases, but he cannot in any sense walk round his own axis.
3. Because a man holding by a stick and revolving round it turns on the axis of the stick and not on his own axis, the axis of the stick in this case becoming the common axis of both man and stick.
4. The alteration of the relative position of the objects round him is no proof that the man has turned on his own axis; he has merely dragged his axis after him, and has not turned upon it.
5. "Keeping as close to it as possible" is certainly the nearest approach to turning on his own axis, but until he is impaled on the stick and turned on it he does not revolve (mechanically speaking) on his own axis.
Your correspondent "Nauticus " makes a strange mistake in his illustration. He takes the moon through the earth's orbit instead of round and round it, and by that means he contrives to make the moon's path always concave to the sun. There is in reality exactly the same amount of convexity in her path as there is" of concavity.
Fearing to trespass at too great a length on your space,
I have the ionour to be, very faithfully yours,
Buckland: Nov. 12, 1864. ACADEMICUS.
NOTES AND GLEANINGS.
New Catalogue Of Nebulae.—In the Ast. Nach., No. 1500, is printed a new catalogue of Nebulse, compiled by M. D'Arrest, and observed by him between 1861 and 1864 at the Copenhagen Observatory. The objects enumerated amount to 215. An English translation of the notes would form a valuable supplement to Sir John Herschel's new catalogue recently published.
Solar Physics.—An important memoir on the Physical Constitution and Functions of the Sun, by Mr. E. W. Brayley, appears in the Companion to the Almanac, for 1865, just published. The author enters into a detailed account of the present state of our knowledge of the sun, and summarises all the recent speculations thereon.
The Comet, visible last August to the naked eye in England, was followed in Australia by Mr. Tebbutt up to Sept. 17. We are wholly unable to understand Professor Coxwell's letter, Asst. Reg. vol. ii. p. 283. We have not heard of anything like a tail of 600 having been seen in Europe.
A New Minor Planet, No. 82, was discovered by M. Luther at Bilk on November 27.
The Minor Planets Eurynome And Terpsichore.—The following elements have been calculated for these planets:—
The Great Object-glass.—A correspondent informs us that, in November last, the "25J in. object-glass for the Leviathan Telescope (by Messrs. T. Cooke and Sons) was tested in the huge wooden tube erected for that purpose at the Buckingham Works, York, and that they had every reason to be satisfied with its performance. It was the impression that it would be three or four months into next year (1865) before the instrument would be completed."
Eoyal Society.—Upon the award of the Royal Medal to Mr. De la Rue, at the last anniversary meeting of the Royal Society, the President, General Sabine, observed:—" No one who has not seen Mr. De la Rue's pictures of the moon can form an idea of their exquisite sharpness and beauty of definition—a result due to his employment of a reflecting telescope of exquisite defining power, the large mirror of which was figured by his own hands, and by peculiar machinery of his own contrivance." To him is mainly due the construction of the photo-heliograph in use at Kew, which is regarded as a model instrument in taking instantaneous sun-pictures: and as he has fully described in print his processes and instruments, he "has thus deepened the feelings of obligation towards him by giving others the benefit of his long experience in the art."—Athenmum.
THE SUITS DIAMETER.
As observed at Greenwich and the Cape of Good Hope about the time of opposite Solstices.
The following observations were taken by means of the Transit Circle of each Observatory—those for the horizontal diameter by the Chronograph at Greenwich, and by eye and ear at the Cape.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER.
Sir, I enclose herewith the comparison of the sun's diameters observed at Greenwich and the Cape, about the time of opposite solstices, December 1859 and June 1860.
Yours truly, Cape of Good Hope Royal Observatory: TÉOS. MACLEAR.
October 24, 1864. [The above timely communication from Sir Thomas Maclear supplies the actual data asked for by those who maintain that the varying diameter arises from atmospheric causes, owing to different altitudes, and not from any increase or decrease of distance.--Ed.]