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whilst the dates of their perihelion passage could not in some instances be certainly determined with promptitude.

In the general indexes to the first twenty volumes of the Astronomische Nachrichten. The designations of the Comets were first introduced by means of Roman numerals, and they were arranged in the order of discovery. The compiler of the index of Vol. XXVI. made the order dependent on the epoch of perihelion passage, and this system has been uniformly adopted since then in the pages of this publication. In 1863 the numbers of the Comet had continually to be altered, and alterations had also to be made in the accounts sent in by the observers. These various inconveniences are very generally admitted, and M. Oppolzen suggests a system based entirely upon dates of discovery, arranged in the following fashion :

The first Comet of 1871 having been discovered on April 7, 1871, he would symbolise the Comet as :

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The second Comet of the same year having been discovered on June 14, he would thus indicate it :

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VI.

and so on. A Comet known by a specific title he would allude to as:Comet of Encke, 1871. How far this system would work is, I think, open to serious question. G. F. CHAMBERS.

JUPITER AND SATELLITES.

Sir, The transits of Jupiter's satellites are now taking place at lower Jovian latitudes, and will for some time continue to approach his equator, so that we may be able to collect observations sufficient to clear up some of the unsettled points in this system. It is at least doubtful whether succeeding transits really present 3 and 4 under a markedly different aspect. Permit me to invite the assistance of every observer, having sufficient interest and leisure, and to suggest two or three essential points in the inquiry

1. The path of the satellite referred to zones or belts. (Mr. Gledhill's Nos. will be useful.)

2. Particulars of distance from limb (which may be estimated by the diameters of planet or satellite), or time from ingress to egress, when any change in tint or visibility occurs; or,

3. Peculiarities in shape or markings.

All sizes of telescopes will be useful, and if a good number of amateurs will help during the next three years, I hope we shall be furnished with sufficient evidence to determine whether the light and markings are permanent, and the rotation analogous to that of our moon.

As a small contribution, allow me to add what my 9-in. "With-Browning" showed of the Tr. of 3, December 29, with power 212.

The satellite traversed Gledhill's No. 5 a little below the centre of the belt. When about 3" clear from the limb I completely lost it, and at 6" it was distinctly coming out as a dark spot, very much smaller than its shadow (then on the disc). It became plainer and larger up to mid-tr., when the dark spot was elongated N.E., S.W. I thought in steadier moments I could see a brighter segment S.E. Its tint was dark grey, very

different from the black umbra.

The latter was on the same belt, slightly S. of the path of 3. The air was not sufficiently good to allow the penumbra to be certainly seen until approaching the middle, and then it was not quite so easy as in Nov., 1869, when near the pole, but its breadth seemed about the same. At this time a bright speck just f it at least kept pace with the shadow for some time. Had this shadow been that of the 4th, the spot would have passed underneath and left it behind, causing, to an observer on Jupiter, the remarkable phenomenon of a solar eclipse, commencing on the E. of the sun. Under certain circumstances this might be nearly intermediate between two normal eclipses occurring near sunrise and sunset. The observation of this local retrogression will be interesting to those whose optical means are adequate. (Fuller details may be found in Eng. Mech. xiv., p. 387.)

Some changes on Jupiter would seem to have taken place since Mr. Gledhill's observations earlier in December. On the 29th, about 11h., there were two large (E. and W.) bright masses on the same meridian of belts 3 and 4, having dark borders and darker ends, about 15" from the W. limb, with others in a similar position towards the E. A few quieter minutes, about 13h., the whole disc was covered with markings. The boundaries of the equatorial zone contained large white masses. White zone a had a large round brighter mass, with several f it. Zone ẞ was of a similar character. Belt 2, ruddy chocolate, was irregular and knotty. No. 6 was broken about, with a curved dark line enclosing a large bright mass on B. The colour of the ground of the equatorial zone seemed to me a pure deep yellow, not ochre as in 1870, but having the darker portions usually seen hanging down from No. 4, between the bright ovals. I am, Sir, yours very truly, Bonner's Road, Victoria Park:

Jan. 12, 1872.

T. H. BUFFHAM.

P.S.-Allow me to add, under the third point, that the position of markings, detected on a satellite, should be referred to the equator of Jupiter, and not to the p and ƒ motion through the field.

I have obtained no observation worth mentioning since Dec.

Feb. 14, 1872.

Yours truly,

JUPITER.

T. H. B.

SIR,-On the night of Feb. 20, at 11th., the face of the planet was a magnificent picture. Something, doubtless, was due to favourable circumstances, but more to an unusual display of the features, which lately have increased our interest, and attracted our attention. At this time, the whole of the disc was covered with spots, or other markings. The streak between No. 2 and No. 3 was narrow, and the space between it and No. 3 was filled with bright masses, interspersed with marbled markings. The whitest part of the disc was below No. 2, and even this was divided by an exceedingly fine line,-really another belt between No. I and No. 2. No. 2 was knotted, and also had on it one or two bright beads. The upper half of Jupiter was also full of detail. The three principal belts were very irregular, the general character being a dark stratum, on which were superposed lighter small elongated masses. One of these, in the latitude of No. 5, assumed large proportions-perhaps 6" in length-its upper very dark boundary bulging out of the S. of the belt, while its fainter lower border dipped into the bright zone between No. 4 and No. 5.

The colour of the central part of the equatorial zone is still, to my eye, of a deep yellow, without any warmth of tint. Casual notice shows it extending across fully three-fourths of the planet. Careful watching,

however, reveals it almost to the limb on the western extremity; on the other end, where the terminator is, it cannot be traced quite so far, (with power 212.)

The bright masses on the upper side of the zone I usually see of less regular sizes than they are generally drawn by your correspondents. At Ioh. on the 21st Feb. one was observed having three or four times the area of its companions.

Bonner's Road, Victoria Park :

March 15, 1872.

I am, Sir, yours truly

T. H. BUFFHAM.

THE SHADOW OF JUPITER'S FOURTH SATELLITE.

SIR, I have been enabled to scrutinise this shadow until the transit of March 6. When well on the planet, the umbra and penumbra were certainly distinguished with 212 and 320 on my nine inch "With Browning." The umbra was about equal to half the diameter of IV. outside and the breadth of annulus of penumbra about half the diameter of the umbra. Still both parts of the shadow-which traversed the disc of Jupiter half way from the equator to the S. pole-were less clearly defined than in the shadow of III. The size of the umbra would appear to indicate that its exterior was merged with the denser part of the penumbra. Near mid-transit the shadows seem smaller and darker; near egress its compound character was again visible.

Soon after ingress, two small light spots were noticed, just preceding, and a little below the shadow. When the latter had reached the middle of its course, the two spots had gained some 1" on it. Nearer the equator, the local retrogression would have been more marked. I am, Sir, yours very truly,

Bonner's Road, Victoria Park:
March 15, 1872.

JUPITER.

T. H. BUFFHAM.

As well as I could see in weather that gives the worst telescopic definition, the slant streak that I noticed in Jupiter, on Jan. 11, has now become faint, but it has got additional features, which were first noticed on March 17. Referring to my sketch in the February number of the Register, an imperfect belt will be seen ending about half-way across the disc. This is Mr. Gledhill's No. 6, and above it is a belt which I will call No. 7, while still further I can sometimes distinguish another, which may be marked as No. 8. The slant streak, when first observed,* reached no higher than 7, but now it appears to extend to 8, or the dark region about the pole. In the angular space between it and 5, and reaching quite across, is a large dark spot, and under the spot a bright gap seems cut out of 5. No trace of these was noticed before March 17.

On Feb. 16, I remarked another slant streak rising in a similar direction from 5, in a place about two hours east of the first one, and, at the same time, a great thickening appeared above it in 7. From this thickening I perceived, on Feb. 24, a slant streak descending, and both streaks met at an angle at No. 6. On the same occasion I saw, for the first time, two slightly inclined, and very faint, dark, conical forms, with their bases on

In the sketch the streak rises at too great an angle, and the space between 4 and 5 is too wide.

belt 3, and crossing the equatorial zone to 4, where they seemed to touch with their apexes two of the spots on the latter.

During February, No. 3 appeared to me not straight, but with three or four slight curves, and interrupted by bright spots in two places. This I have latterly not been able to observe.

Mr. Gledhill's new belt, which may be called 2b, never seemed complete. It distinctly terminated at two bright spots separated from each other about a tenth of the circumference. My failure to detect this belt during the present month, as well as the curves of No. 3, must be, most probably, attributed to the badness of the definition on the few occasions that the heavens were at all visible.

March 19, 1872.

LUNAR TINTS.

J. BIRMINGHAM.

SIR,-May I ask for a small space wherein to say a few words on the conversation at the meeting of the Royal Astronomical Society, which ensued on the reading of my paper relative to the floor of Plato. Although your report of that conversation is perfectly accurate, it is, I think, calculated to lead the general reader to conclude that the appliances of modern science were not employed in obtaining the results which formed the subject of my communication, and, consequently, a doubt may rest on their accuracy. It is quite true that instrumental determinations carry a much greater weight with them than estimations, but no one, except a selenographer who has had considerable practice, can form the least idea of the difficulty of determining instrumentally numerical values of lunar tints. A telescope is required which is furnished not only with a perfect clock motion, adjusted to the moon's diurnal motion, but a diaphragm of the nature of Dawe's eye-piece, with exceedingly small perforations to shut off every portion of the moon's surface, except that under examination, which must be kept steadily in the field while a number of graduated tints are compared, in order to select the one which agrees best with it. The wedge of coloured glass, of which I said "nothing could be better," would determine rather the intensity of the light reflected from the object than the colour presented by it. Up to this time "estimation" has been used in determining brightness, and in the lower part of the scale colour. Schröter, Lohrmann, and Mädler, adopted this plan, and although the scales of these selenographers are not identical, we do not question their determinations. I may remark in passing, that, however numerous the collected instances of brightness as determined by them may be, the observations of any particular object are but few; and as regards Lohrmann's and Mädler's, the dates are not given. In the case of Schröter, the date of each observation is recorded. The paucity of observations in a given instance is a strong inducement for re-determining brightness on every available opportunity. A well-trained eye can readily estimate minute shades of difference between the colours and brightnesses of neighbouring objects, and constant practice will enable two or more observers to come very close indeed in their estimation of colour and brightness. In the case of Plato, the differience between the lightest and the darkest floor is sufficiently great to preclude the possibility of the same observer mistaking dark for light, and the contrary, and when three observers, during a period of two years not in communication with each other, and using telescopes of different apertures, so closely agree as to furnish the materials of a curve which rises and falls with that of solar altitudes, there can be little room to doubt of the accuracy of a result which incontestably proves that the position of the sun in the lunar heavens affects the appearance of objects on the moon's surface, a principle long held by

selenographers, but, so far as I am aware, never proved numerically before. Regarding a very light floor as o'10, and a very dark floor as o'90; a curve deduced from the observations gives for each interval of 12 hours from sun-rise to sun-set on Plato, the following values :-Interval o to 12 hours sun's altitude o° o'o to 3° 54'8. Tint of floor o'30 to 0.33. Interval 12 to 24 hours. alt. 3° 54'9 to 7° 48'1. Interval 24 to 36 hours. alt. 7° 48'1 to 11° 38' 2. Interval 36 to 48 hours. alt. 11° 38' 2 to 15° 23'3. Interval 48 to 60 hours. alt. 15° 23'3 to 19° 2'0. Interval 60 to 72 hours. alt. 19° 2.0 to 22° 31'3. Interval 72 to 84 hours. alt. 22° 31'3 to 25° 49'5. Interval 84 to 96 hours. alt. 25° 49'5 to 28° Interval 96 to 108 hours. alt. 28° 54'3 to 31° 42'7. Interval 108 to 120 hours. alt. 31° 42'7 to 34° 11'5. Interval 120 to 132 hours. alt. 34° 11'5 to 36° 17'5. Interval 132 to 144 hours. alt. 36° 17'5 to 37° 57'8. Interval 144 to 156 hours. alt. 37° 57'8 to 39° 9'2. Interval 156 to 168 hours. alt. 39° 9'2 to 39° 50'5. Interval 168 to 177, or meridian passage alt. 39° 50'5 to 40° 0'o. Tint o'66 to 067. The declining sun gives the same altitudes and tints from meridian passage, reckoning in the reverse order,

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Tint 0.33 to 0.36. Tint o 36 to 0.39. Tint 0.39 to 0:42. Tint 0'42 to 0'45. Tint o 45 to 0'49. Tint 0:49 to 0.52. Tint 0'52 to 0'54. Tint o'54 to 0.57. Tint o 57 to 0.60. Tint o'60 to 0.62. Tint o'62 to 0.64. Tint o'64 to 0.65. Tint o'65 to 066.

I am, Sir,

Your obedient servant,

W. R. BIRT.

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Sir,--The want of a comfortable seat suitable for observing with reflecting telescopes has been long felt. I beg to send you a photograph of an Observing Seat I have just constructed for my 84-inch "BrowningWith" Reflector, which I find to answer its purpose very satisfactorily, and which may, therefore, be interesting to your readers.

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