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the whole of that time; and further that its next perihelion passage would occur on April 3, 1851.

It was rediscovered by Challis, at Cambridge, on November 28, 1850. 0. Struve described it, under the date of January 24, 1851, as having a diameter of 24". It was, more or less, found the whole time without any nucleus or tail. This comet returned in due course to perihelion on September 12, 1858, having been detected four days previously by Bruhns, at Berlin.

No 8 was detected by Méchain, on January 9, 1790. It was only followed for a fortnight. On January 11, Niessier could see but å confused nebulosity, without any indications of a nucleus. not re-observed until its return, at the commencement of 1858, on January 4 of which year it was detected by H. P. Tuttle, at Harvard College Observatory, Cambridge, U.S.A. Sydenham, Kent.

G. F. C.

It was

CORRESPONDENCE.

N.B.- We do not hold ourselves answerable for any opinions

expressed by our correspondents.

THE TRAPEZIUM IN ORION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-A good deal has been said lately about the variableness of the small stars in and about the trapezium in Orion; so much, that it is beginning to be accepted as an all but established fact. Mr. Dawes, indeed, speaks of it as almost certain that the sixth star is variable. Still there are so many good observers, with ample optical means, who have not yet given the results of their experience on the point, that some discussion in your pages would be very desirable; and it is with the hopes of eliciting their opinions that I now address you.

To draw any definite conclusion from one year's apparition of the constellation would be, of course, absurd; still I cannot help putting on record that, as far as careful observation has gone this season, with this express object in view, and from more cursory observations in former years, I have as yet seen no indication of variableness in any of these six stars. Except on extremely bad nights, the sixth star never fails to be plainly visible with both my 12-inch silvered-glass specula; the degree of visibility varying according to the steadiness of the air. I invariably find that on good nights it is perfectly steady, bright, and very little inferior in magnitude to the fifth star; and, under the same circumstances, its distance from its larger companion is much greater than anyone would suppose who has seen it only in an unfavourable state of the air. I have repeatedly estimated this distance, with confidence, as equal to nearly three diameters of the larger star.

any of the other planets; and (2) the fact that the aphelia of all these comets lie very close to the orbit of Jupiter; so that when at their greatest distance from the sun, they are constantly liable to rencontres, more or less intimate, though by no means friendly, with the colossal planet.

I quite agree with Mr. Brodie in his estimate of the magnitude of the sixth star-viz., that it is just half a magnitude less than the fifth, although this is perhaps not the generally received opinion. I have never on any occasion seen it of greater magnitude than this, or anything approaching Mr. De la Rue's experience, who has seen it "very much brighter than the fifth.” With my aperture limited to 6 inches, I can always glimpse this star on a fair average night, when it is within two hours of the meridian; but with a small aperture like this, its visibility seems to depend entirely on the state of the air, and that alone; at least, such is my experience.

There is, however, something peculiar about this sixth star. On the same evening, when the air has been tremulous for a time, its light, with the full aperture of my reflector, has been unsteady, twinkling, disappearing altogether for an instant, feeble, and apparently much inferior to that of the fifth ; while as the night improved, it has become steadily visible, and very little inferior in light to the fifth star. It will be said that this is owing to the false light playing about its larger companion, but in my instrument this never extends so far as the small comes, but (unless when the air is very bad indeed) is “well brailed up," as poor Admiral Smyth would say.

It appears to me that the only way of accounting for this peculiarity is to suppose that the light of the sixth star is intrinsically weaker than that of the fifth, but that its body is larger; and this seems to be confirmed by the fact of its not bearing magnifying so well as the latter. We constantly meet with stars which loom large with a faint light, and others again very vivid, but contracted apparently to a point.

It is worthy of note how readily this star is seen in the strong twilight.

Since Mr. Huggins' announcement of his discovery of three minute stars with the trapezium, I have examined it most carefully, but hitherto without the smallest suspicion of any of them; which circumstance seems to imply that the two nights on which he saw them with his 8-inch refractor must have been extraordinarily fine, unless we suppose all of them to be variable stars of short period.

It is worthy of note that the new stars within the trapezium, seen by Lamont, De Vico, Porro, Bond, Mr. Lassell, and Mr. Huggins, to the number of nine, are all, with perhaps the exception of De Vico's and Mr. Huggins' faintest (9), distinct stars; and that Mr. Lassell's enormous speculum has not revealed any of those seen by the other observers with far inferior optical means.

At present, therefore, the subject appears to be involved in mystery; and I trust you will invite communications upon it. I will conclude by adding two or three queries, viz. :

1. Is the sixth a nebulous star?

2. Is the region within the trapezium itself quite free from the general nebulous matter?

3. Is it not possible that these various evanescent points of light, seen by so many observers, may be portions of the nebulæ itself, which some eminent observers now suppose to be in a state of continual change ?

The first query, I should answer myself in the affirmative; the second, in the negative.

I am, Sir, your obedient servant, Stretton Rectory, Hereford:

HENRY COOPER KEY. March 14, 1866.

VENUS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—Noticing! some time ago a suggestion that amateurs should try how near to superior conjunction Venus could be observed, I may state that, as anticipated, I saw it very satisfactorily a few hours after, in the forenoon of same day, as soon as sufficiently above the horizon, with a lightish-tinted shade on eye-piece, power 54; and 4 days after, when again turning the instrument upon her at 11.45 A.M., 10 minutes after her conjunction with Mercury, I observed them both simultaneously, in same field of view, when differing in A.R. by only 5s., and in Declination by 14' 57'. This was a most agreeable surprise; Mercury differing then only 6m. 258. from Sun, and not having before observed them nearer in A.R. than 39m., I believe, when presenting a crescent form, and now, as might be expected, the all but circular outline of the disc was not very clearly defined. Equatorial instrument used in the above, 44-in. ap.

Turning very abruptly to another subject, may I suggest whether some account of the revised solar parallax would not be a subject interesting to many readers, if introduced into the Astronomical Register, -as by whom the observations were made, and where the mode by which the amended parallax has been arrived at, &c. ?

Respectfully, &c., Thornton-in-Craven :

THOS. WILSON. March 14, 1866.

ARE ALL LARGE CELESTIAL BODIES HOLLOW?

5:6

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—Grant me the opportunity of asking your readers the following question :-On what theory, except that of the extensive hollowness of the Sun and Planets, can the following facts relative to their mean densities be accounted for? The following are the approximate estimates of the mean densities of the principal bodies in the solar system :

Diameters.

Mean Densities.
The Sun.
882,000 miles.

I'42
Mercury
3,140

6.6
Venus

7,700
Earth
7,912

5.5
Mars
4,100

5-6
Asteroids
Various.

Unknown
Jupiter
89,069

1'37
Saturn
76,086

0-5
Uranus
34,500

I'O
Neptune.
37,000

0-5 It will at once be seen that all the smaller bodies have high and almost uniform mean densities, and the Sun and larger planets low mean densities. Does that contrast arise from differences in the absolute specific gravities of the materials of which each body is composed; or does it arise from the fact that they are shells, and that, therefore, the larger the body the less its mean density ?

I am, Sir, your obedient servant, Newcastle-on-Tyne:

T. P. BARKAS, March 5, 1866.

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32 ORIONIS: DIAMETER OF CERES.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, — In reference to Mr. Matthews' remarks on the double star, 32 Orionis, in the April number of the Register, I may just say that I have

Ι always found this star, for some reason or other, a troublesome object, and generally ill-defined. The distance is evidently less than it was some years ago. The Baron Dembowski, using an equatorial by Merz of 7 French inches aperture, describes the pair in 1863 as in contact or just separated; and I have usually noted them as notched or in contact with my 7-inch refractor, which is, of course, inferior in separating power to the reflectors employed by Mr. Matthews.

I was in hopes that I should have succeeded in getting a series of measures of the diameter of Ceres at the recent opposition with a spherical crystal micrometer, but, owing to unfavourable weather, I was only able to secure one set—viz., on January 24th—the resulting diameter being o'.885, a result which, as depending on one night's measures only, is of course open to considerable uncertainty.

The disc on this occasion was well defined, and of a ruddy yellow hue, and I failed to detect any certain appearance of a nebulous envelope. On the 6th, 9th, and 24th of January I gauged the planet's magnitude by the method of limiting apertures, the three determinations, which were very fairly accordant, yielding 6.9 mag. as the mean result.

I am, Sir, yours faithfully, Woodcroft Observatory, Cuckfield :

GEORGE KNOTT. April 16, 1866.

COMETS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,--I must say that the perusal of the letter of Mr. E. Lawrence, in the April number of the Astronomical Register, involuntarily induced in my mind the recollection of that fine dictum of the American philosopher, "There's nothing new, and there's nothing true, and it don't much signify." Twelve years ago, a Professor A. Crestadora propounded the same wild theory in an Astronomical Annual (published by Weale, and so called, I suppose, on the lucus a non lucendo principle, because it only appeared in the one year, 1854). Were comets really lenses they would be invisible when turned edgeways to the earth, and only seen as bright nuclei surrounded by a halo when exactly between the earth and sun; or, so to speak, in inferior conjunction. As for what your correspondent says about the tail, he must bear in mind that it is by no means “always in the direction of a ray from the sun," although it is generally turned away from it; and, moreover, if he is old enough to recollect that superb object, Donati's Comet of 1858, he must admit, from the marked curvature of its tail, that the “lens" of the comet must have refracted the light in a curvilinear direction : a perversion of physical law only paralleled by the Irishman's gun, which shot round a corner. Finally, several comets are recorded with more than one tail, and their appendages have been visible at most varying angles with each other ; which involves, as it seems to me, another and an insuperable objection to Signor Crestadora's theory as rev by Mr. Lawrence.

I am, Sir, yours, &c., April 9th, 1866.

A NORRIBLE TAIL.

ARE THE STARS VISIBLE IN THE DAYTIME?

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TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—I ask the above question after reading an interesting letter in your last number, from Mr. F. Penrose, of Wimbledon, upon a question of the variation of focus being necessary when observations are made of the solar spots and stars. The fact I wish to ascertain is, “ whether any star less in magnitude or brilliancy than the planet Venus can be seen whilst the Sun is above the horizon ?". I have a telescope of the size and perhaps more power than that described by Mr. Penrose, but I should as soon think of looking for a needle in a pottle of hay as for a star in the daytime) with it. Where is my error? Is there any extra tubing requisite, or is getting the exact spot the solution of my difficulty ? We are all aware that, even in the case of total solar eclipses, only a few first magnitude stars are visible to the naked eye. A little illumination upon this point will agreeably dissipate the mist of

Your obedient servant, Reading: April 10th, 1866.

H. H. COWSLADE. [Stars of the first, and in many cases of the second magnitude may be readily seen in the daytime, in the full sun-light, with a telescope of 3 inches aperture or even less. Our correspondent must, however, get the exact spot, which is to be done by means of an equatorial, properly adjusted. In the same way, the planet Venus may be seen with the naked eye in the middle of the day if the exact spot is known.-See Astro. Reg., vol. ii. pp. 205 and 265.--Ed.]

THE PLANET JUPITER.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Mr. D. A. Freeman notices the visibility of Jupiter's equatorial belts by day, his instrument being of 47-inch aperture. The following extracts from my note-book will show that not only his equatorial belts, but at least one of his satellites, may be seen in full sunshine with a much smaller instrument-viz, 2-inch aperture :

1863, June 9th.—The 3rd satellite of Jupiter visible 12 minutes before sunset ; the planet's equatorial belts well seen; power 80.

June 30th.—The 3rd satellite and equatorial belts well seen 45m. before sunset.

July 2nd.-Sun shining brightly; Jupiter's belts plainly seen.

From the fact of the 3rd satellite being occasionally visible by day with this small aperture, I should suppose that larger instruments will show all four, though I have seen no mention of such observations.

I am, Sir, yours obediently, Red Lion Street, Chesham, Bucks :

CHARLES GROVER. April 8th, 1866.

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