Sir Thomas Maclear has favoured your readers with his Cape observations for the winter and summer solstices of 1859-60; but I should hardly have presumed to address you had I not consulted before his Cape observations (but of an earlier date), and considered their bearing on the question. The vertical solar diameter of the Cape is very nearly the horizontal polar diameter of Greenwich. To justify the conclusion that one diameter may be taken for the other in measuring the sun, we must first assume that the true form of the sun is spherical, which brings us face to face with the Newtonian hypothesis that the form of all rotating bodies must of necessity be spheroidal. We must next assume that the sun radiates with equal force on all sides, which is denied by Father Secchi, of Rome, who has proved from observation what might be inferred from known dynamical lawsthat the solar force has a maximum and a minimum, and is greatest on the plane of the sun's equator. If so, the sun's figure must vary from this cause; for unequal radiation involves unequal refraction. And, lastly, we must assume that our rules of refraction for different altitudes (which, since Mr. Glaisher's ascents, everyone is learning to distrust) are not only perfect for our northern hemisphere, but equally perfect for a southern hemisphere, the atmosphere of which is known to hold in suspension a much larger amount of aqueous vapour than is present in our latitudes.

These remarks may in part, but in part only, help to account for the remarkable difference between the observations in both hemispheres made by one of your correspondents, Mr. Evan Hopkins, with a repeating sextant, and those of our Royal Astronomers with far better instruments; but I have no theory to defend in the matter. I write only to show the difficulties by which the subject is surrounded, and with a desire to assist in their removal. Yours obediently,

Fairseat, Wrotham: Jan. 7, 1865.

THE SOLAR DISC.

W. E. HICKSON.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In examining recently the Report of the British Association for 1861, I met with the following passage in a paper read before the Mathematical Section, entitled Cases of Planetary Instability indicated by the appearance of Temporary Stars, by Daniel Vaughan :-"From the imperfect accounts which I have seen of Mr. Harrington's observations on September 1, 1859, I have concluded that the meteors which he saw falling to the solar disc must have been self-luminous about 100,000 miles above the boundary of the great ocean of flame." I have no idea what the observations here referred to may have been, or where they may be found, and I should feel especially obliged to any of your correspondents who would favour me with additional particulars respecting so remarkable a phenomenon. I remain, yours faithfully,

Hardwick Parsonage, Hay:
Jan. 5, 1865.

T. W. WEBB.

ROTATION OF SATELLITES.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Numerous and varied as the illustrations on both sides of the Lunar controversy have been, I do not recollect that, with the exception of a remark of Mr. Bird's in your last Number, any reference has been made to the other satellites of our system, although they furnish what is so desirable in most controversies," an outside view of the subject."

Any evidence they afford has, it is true, the weight of analogy only; but their conditions in other respects are so similar to those of our own satellite, that it is highly probable their motions are so likewise.

My object, however, is not to enter into the Lunar controversy, but to draw the attention of your readers to what is in itself a very interesting enquiry, and with regard to which some of them are able to give us valuable information.

It is well known that the diminution in the light of Iapetus, the outer satellite of Saturn, when near and at his eastern elongations, is so great that it can then only be perceived by powerful telescopes, and my impression, although I have never made notes of the times and places, has long been, what Titan and the other satellites usually seen exhibit-periodic variations in brightness. None of these bodies, I believe, present any disks, even when viewed by large instruments, and direct observation of their surfaces is of course impracticable. But it is difficult to account for this phenomenon but on the supposition that in one part of the orbit a different side of the satellite much less reflective is presented to our view.

The satellites of Jupiter, however, show disks even with instruments of moderate aperture, and are susceptible of micrometrical measurement, the third being about "48 in diameter; and in one of the recent astronomical notices which I have not at hand, there are drawings of the disk of one of the satellites in transit, showing large obscure portions upon its surface.

I am not aware whether any periodic variation in the light, or difference in the aspect of their discs when in different parts of their orbits, has been observed in the satellites of Jupiter; but I do not doubt that this subject has been one of the many which have engaged the attention of the Rev. W. Dawes, and am sure you will concur with me in thinking that his observations or those of others of your correspondents who have enquired into these phenomena would be a very acceptable contribution to your Register, whilst I venture to believe that the combined observations of some of us whose appliances are more limited might, nevertheless, if carefully and perseveringly made, conduce to their further elucidation.

Ealing: Dec. 19, 1864.

I am, Sir, your obedient servant,
W. L. BANKS.

P.S.-Since writing the above, I find that the Rev. T. W. Webb's useful little work Celestial Objects gives some interesting details of the appearances of Jupiter's satellites when in transit, accompanied with some curious figures of the disc of No. 3; but Mr. Webb does not state whether any periodic variation in brightness has been observed, especially in elongation.

THE REGISTER, THE SUN AND THE MOON.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I venture to congratulate you and your subscribers upon the improved prospects of the Astronomical Register at the commencement of its third year. Its appearance is also improved by being printed on toned paper, thanks to you for all; and it may be interesting to yourself and readers if I state (what I believe is not generally known) that reading what is written or printed on toned paper is less fatiguing to the eyesight than when the paper is either "cream-white or (which is still worse) "bluish-white." This is so well ascertained, that in the Ophthalmic Hospital only toned paper is now allowed to be used and as astronomers' eyes are subjected to more straining than most other men's, they especially are deeply interested in the general use of toned paper for all their literary purposes.

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THE MOON.-Giving "the lady" the preference, I beg only to be allowed to say to Mr. Dawes, that if I am" at sea "in his sense, it is only to understand how so earnest an advocate as he is should in his lengthy epistles blink all the arguments I have advanced, and be content, when "the correspondence is closed," with a "last word," based upon a mere phrase! I must also express astonishment that Academicus" should be hazy upon what is, ex hypothesi, a demonstrable fact connected with the Moon question. Poor "Cambridge" and poor Academies of Astronomical Science, if this be your teaching! But I forget: "An Enquirer" at Cambridge is a convert. I refer "Academicus to him, and to last year's Register therefore, and advise him and all new subscribers to procure both Volumes I. and II. while they may.

THE SUN.-I am glad to find from the last Register that Mr. Brayley gives up all the extravagant speculations about the sun's heat, which he ventilated in last year's Companion to the British Almanac. Allow me to remind him, Professor Thomson, and others, who have been overheating the sun for the last few years, that, as its size is a definite thing (850,000 miles in diameter) upon the now received parallax of 91 million miles, if they heat it up inside and out, they must reduce its density and mass pro tanto. When the earth was (as Laplace supposed) in a hot, nebulous, or gaseous state, it was then from 1,500 to 1,800 times less dense and larger than now. If the sun, therefore, is as hot as the earth was (its size being unaltered), how can its mass be any longer 350,000 times greater than that of the earth now? Taking the relative density and masses of the earth and sun upon modern Newtonian bases, and then heating up the sun (in more recent style), the sun's density and mass will be reduced to about 1-400th of what they once were calculated to be. In other words, instead of having a mass 350,000 times greater than the earth, the mass of a hot nebulous sun will not be even 1,000 times greater! And I should like Professor Adams or Mr. Brayley to tell us, on Newtonian principles, if the mass of the sun as the centre of our system is so reduced, "What then 'would make the world go round?'" I am, Sir, yours faithfully,

NAUTICUS.

Still at Sea: January 16, 1865. In my letter in the Register for December, instead of "from west to east," read "from east to east."

THE MOON CONTROVERSY.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Dear Sir,-If you think the remarks on the other side worthy of notice, and are not thoroughly tired of the Moon controversy, will you kindly insert them in your Register?

December 13, 1864.

I am, dear Sir, yours truly,

W. M.

Fig. 1 (see plate) represents a globe C D firmly fixed to a straight rod A B. While the rod is straight the globe cannot rotate. Here

is only progression.

Fig. 2. Let the rod be a long one, and let it be bent into a large circle (the globe of course firmly fixed to rod). Here we have progression, but also rotation the former most obvious.

Fig. 3. Let the rod be more bent, and consequently shortened, so that circumference of A в does not greatly exceed that of globe C D. Here we have progression and also rotation, but the latter more obvious.

Fig. 4. Now let the rod be shortened and more bent, until the circumference of rod does not exceed that of globe. Here we have rotation alone.

It is plain that in figure 2 (which always turns the same face or surface to the centre of its orbit) the motion must be that of rotation as well as progression, for it is clearly shown by figures 3 and 4; for we have only to decrease circumference of A B in proportion to that of C D to make the matter quite obvious. Figure 2 seems quite as closely connected with figure 4 as with figure 1.

с

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-I do not wish to occupy your valuable space by extending the Moon controversy uselessly, but you may possibly think the following pretty and simple experiment as bearing on the case; and although it may be interesting to many as "science in sport," yet it is really "science in earnest."

The accompanying figure (see plate, fig. 5) represents a glass tumbler, three-parts filled with water; on the surface of the water is floated a piece of round paper, with an arrow drawn across its diameter, to better exhibit its motion; a pin passes through a piece of paper pasted across the top of the glass and through the centre of the round disk, to keep it from touching the sides of the glass. If the glass is now taken with both hands, and the person holding it turns slowly on his own axis, the round disk will appear to rotate; but that it does not rotate will be evident from the head of the arrow always pointing in the same direction. After one or two turns the water will acquire a rotatory motion from its contact with the sides of the glass, and thus the paper disk will have the same motion.

Now, what does this scientific toy show? Why, that the glass does rotate on its own axis, the pin, once during each revolution, and by friction gradually communicates its own rotatory motion to the