your impression for the same month against the correctness of measuring only the vertical diameter of the sun. Mr. Howlett, namely, found the horizontal, the vertical, and all the diameters of the sun, measuring sensibly the same at 16° above his horizon. It is more difficult to detect a disproportion in the different solar diameters, arising from the effects of refraction, at moderate altitudes, than to observe the alternate enlargement and contraction of the sun's disc in periphery and area, depending upon its variable distance from the earth. This takes place alternately in winter and summer to the extent of th part of the sun's apparent circumference, and th part of its apparent superficies. And the alteration is not diminished or increased more than both part by the effect of parallax from geographical position. I am, Sir, your most obedient servant,

ALEXANDER S. HERSCHEL.

6 35

2°

"From the table, it will be seen that at 8 A.M., and at an elevation to-day of 16°, all the solar diameters showed equal."

F. HOWLETT, F.R.A.S. St. Augustine's Parsonage, Hurst Green:

Oct. 7, 1864.

ON THE VAST ANTIQUITY OF THE LUNAR
SURFACE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,―The views I entertain on the subject in question are these: namely, that as a direct consequence of the small mass of the moon, and its comparatively large surface, it must have parted with its original cosmical heat with much greater rapidity than in the case of the earth, and consequently the moon must have assumed a final condition of surface-structure ages before the earth had ceased from its original molten condition. And as the moon had, in all reasonable probability, never possessed an atmosphere or water-envelope (it certainly has none such now), while the earth has both, the action of the earth's atmosphere, and especially that of its ocean, when it existed in the first instance as a vast vapour-envelope ere the earth had cooled down so as to permit the ocean taking, up its final position as an ocean, this mighty vapour-envelope must have retarded the escape into space of the cosmical heat of the earth millions of ages after the moon had assumed its final condition as to temperature. Therefore, it is from

such considerations I am led to the conclusion that the surfacefeatures and details of the moon present to us a sight of objects the antiquity of which is so vast as to be utterly beyond the power of language to express, and not less so for the mind to conceive.

But yet, at the same time, such considerations appear to me to enhance so greatly the deep interest which ever attends the examination and contemplation of the moon's wonderful surface, that I would earnestly urge those who agree with the soundness of these views to bear them in mind next time they have an opportunity to behold the marvellous details of the lunar surface, as I am fain to think that in doing so the interest of what is there revealed to them will be rendered vastly more impressive. Penshurst, Kent: 1864.

JAMES NASMYTH.

THE LIGHT OF THE SUN.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-A moment's reflection will show your correspondent "Readingensis" that the body of the sun may itself be most brilliantly illuminated, and yet appear dark in comparison with the infinitely greater brilliance of his photosphere. Light and darkness are merely relative terms. "The ball of ignited quicklime," says Sir John Herschel, "in Lieutenant Drummond's oxyhydrogen lamp gives the nearest imitation to the solar splendour which has yet been produced. The appearance of this was, however, as described (viz. a dark spot) in an imperfect trial at which I was present.' If the intense glare, then, of the lime-light projected on the solar disc merely shows as a dark spot, is it not almost certain that, could we isolate these solar maculæ, they would be far too brilliant to admit of being observed with the naked eye?

66

With reference to Mr. E. Hopkins, he confuses reflection and refraction so curiously that it is difficult to follow him. How, according to his theory, anybody but a fish can see the apparent bending of an oar in water by refraction it is impossible to conceive. Reference to Brewster's "Optics," chap. 3, p. 20, Galbraith and Haughton's Optics," chap. 2, p. 18, Lardner's "Optics," chap. 4, p. 68, Tomlinson's "Natural Philosophy" (in Weale's series), pp. 106 et seq., Lynn's "Natural Philosophy," sec. 5, p. 75, or, in fact, any elementary book on Optics, will at once show him the mistake into which he has fallen. Mr. Hopkins can surely never have read the details of the observations of the Total Solar Eclipses of 1851 and 1860, nor have seen Mr. De la Rue's photographs of the last-named phenomenon, or he could never deliberately talk about the rose-coloured prominences having anything to do with the moon! While as for the "beads," the serrated lunar limb and the effect of irradiation amply suffice to account for them.

Finally, I ask, in some wonder, can Mr. Hopkins possibly have discovered the radiating streaks from Tycho, and reproduced them in the pages of the Astronomical Register as "beautiful radial lines from the polar focus" (whatever that may mean) of the moon? I am, Sir, your most obedient servant,

December 5, 1864.

OPTICE.

ADJUSTABLE v. ADJUSTIBLE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I should scarcely have thought there was much need of the word "adjustable," which "Another Enquirer" proposes to coin. If it be really wanted, it should be introduced by some one or more of our leading scientific writers, though even then it might never get into common use. As to the spelling of it, as there is not such an adjective in Latin, nor in French or Italian, the connecting vowel of the termination must be got at by a roundabout analogy. Supposing "adjust" comes from ad justum, justus comes from jus, and jus from juro, which, as being of the first conjugation, would give a as the Vowel to connect with the adjectival ending bilis. As the verb is "aggiustare" in Italian, if the verbal adjective existed in that language it would be "aggiustabile." For these reasons adjustable would be better than adjustible. In "combustible," the original Latin verb being of the third conjugation, the vowel is properly i.

À propos de bottes, has no one ever been shocked at the almost exclusively masculine nomenclature of the lunar spots ? Amidst a lot of famous, but some of them obscure and outlandish names (which might well be transferred, if it were possible, to the other hemisphere!) only those of one or two ladies appear. Might not this ungallant procedure be a little rectified now that new names are being introduced into the map?

Notwithstanding bad taste in the choice of the names of a few of the more recent minor planets, the list is an elegant one; the fair sex have it all to themselves there. A sprinkling, at least, of their names would be an agreeable variety in our selenography, and an ornament in future charts of our beautiful satellite.

Yours faithfully,

G. J. W.

December 1864..

VARIABLE STARS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-The catalogue of variable stars, which you did me the honour to insert in a recent number of your journal, was copied into the Astronomische Nachrichten some weeks, and so, coming under the notice of M. Schönfeld of Mannheim, has drawn from that gentleman a communication containing one or two facts which it will be convenient to transfer to the pages of the Register. M. Schönfeld" announces the discovery by himself of a new variable star, whose position he has brought up to my epoch, and which is as follows::

The star varies between the 7th and 11th magnitudes, but its period is uncertain.

M. Schönfeld states that the star which I, following my predecessors, call 21 Virginis, should be U Virginis, but he omits to specify whether the position is correctly given. Assuming this emendation is accepted, the letters appended to other variables in Virgo must be shifted backwards. It is greatly to be wished that some new nomenclature for variable stars were brought into use. What occurs to me as a convenient expedient is the use of Old English Gothic letters; this would give a range of upwards of 40 letters, sufficient for all constellations for many years to come, and would be preferable in many ways to continuing the Roman capitals onwards from R.

Finally, M. Schönfeld expresses his opinion that the variability of Nos. 43, 56, 63, 65, 96, 99, 115, and 116, is too problematical to warrant their retention in a catalogue of recognised variables.

I am, Sir, your obedient servant,

Kensington: Nov. 26, 1864.

GEORGE F. CHAMBERS.

ON THE MOON'S ROTATION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-A few lines will, I hope, suffice to expose the fallacies with which my argument and illustrations have been met on the vexed question of the moon's rotation.

Mr. Perigal asserts that "When a body maintains its parallelism while orbitating with its centre in a circle, as a compass-needle, for instance, when carried in a circle, it rotates on its axis with the same angular velocity in a contrary direction." So we are required to receive it as a certain truth, which is to rectify all previous errors on the subject, that, when the box and card of a compass is rotated under the needle, it is the needle itself (though maintaining its parallelism to itself and to the magnetic meridian) which really rotates in the opposite direction! (See Mr. Perigal's paper appended to the Astronomical Register for this month, page 6.) On page 7 it is also asserted that "That diameter of a moving body which at any instant coincides with a tangent to the orbit, or path of the centre, always continues tangential when there is no axial motion; consequently the deviation of the diameter from the tangent of the orbit, or path described by the centre, is the exact angular measure of the axial movement called rotation."

Such an assertion (for it contains neither argument nor correct illustration) can only arise, as it appears to me, from confounding together two very different ideas, which in such a discussion as the present ought to be carefully distinguished, viz. absolute rotation and relative rotation. When a rotating spherical body is viewed in the direction of its axis, every particle on its surface is seen to describe a circle round the axial line. This is the necessary consequence of its rotation, and therefore proves it. Whether the centre of the body is at rest, or moving in any line straight or curved, does not at all affect this fact. Its rotation is absolute. And this is its meaning when the periods of planetary rotation are stated. The absolute rotations of the

earth, for instance, are 3661 in the period of one revolution round the sun.

But the non-rotationists prefer to refer the question of rotation to a line which is itself undergoing a constant change of its direction in space! This line is the radius-vector of a planetary orbit. In the case of the moon it is asserted that, because a certain diameter of the moon preserves the same position relatively to the radius-vector, therefore that diameter has no movement of rotation. Granted that, with respect to the radius-vector, it has no relative rotation. But that very fact confutes the conclusion erroneously drawn from it. Put it briefly thus:-"A certain diameter of the moon is always at right angles to the R.V. But the R.V. itself rotates round the centre of the orbit (as a spoke of a wheel rotates round the axle), through 360° in about 27 days. Therefore the said diameter, having throughout maintained the same relative position to the R.V., has also rotated through the same arc with the same angular velocity."

Mr. Perigal's diagram on page 6 well represents my argument and illustration, by means of the compass-card and needle. The line join-. ing the centre of the circle, and the middle of the arrow (a radiusvector) is supposed to rotate in one direction, while the arrow continues parallel to the horizontal diameter of the orbital circle, which may represent the magnetic meridian. Yet it is strenuously asserted that the variation of angle made by the arrow with this line arises, not from the rotation of that line, but from the rotation of the arrow (representing the compass-needle) in the opposite direction, and, therefore, of the horizontal diameter of the circle, or magnetic meridian, to which it has continued parallel! Such an assertion seems to me to be so contrary to the testimony of our senses, and to the obvious facts of the case, that it appears quite useless to continue the discussion with those who can seriously receive it as a truth.

I remain, Sir, your obedient servant,

Hopefield Observatory, Haddenham, Bucks:

December 15, 1864.

W. R. DAWES.

P.S.-To Mr. Little's question respecting the peas in his hummingtop, there can be no hesitation in replying Yes. A pea in the axis of the top would be easily satisfied of its rotation; and as all the rest continue in the same relative position to it, they must partake of a similar movement. It is a case of forced rotation as a part of a rotating body, but does not truly represent the case of the moon.

"Nauticus" justly considers himself "at sea." I have provided him with a compass to steer by.

W. R. D.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir, I have no wish to prolong unnecessarily the discussion on the moon's rotation; but as the consideration of the subject from a physical point of view has been overlooked, I trouble you with another short letter. There is this essential difference between the