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INSTRUMENTS, &c. FOR SALE. These Notices, which are restricted to two lines each, are inserted free of charge to subscribers : applications respecting them to be made to the Editor, with a stamped envelope for reply, without which no answer can be sent.--For Advertisements with more complete details a small charge will be made.--N. B.-It is particularly requested that we may be informed when instruments are disposed of, in order that the notice may be withdrawn, and fruitless applications prevented. Achromatic Refractor, 7in. ap. 9.ft. focus, by Troughton & Simms ; 6 eye-pieces ; on garden Equatorial, steadying rods, &c.
 Achromatic Refractor, 6-4 in. ap. 8} ft. focus, by Merz, Equatorial and clockwork by Troughton & Simms.
 Achromatic Refractor, 2} in. ap. 4 ft. focus, 3 pancratic eye-pieces,
powers 60 to 200, pillar and claw, and garden tripod stand. (61) Achromatic Refractor, 27 in. ap. 42 in. focus, one day and one night eye-pieces, on tall tripod stand.
 Gregorian Reflector, 41 in. ap. 2 ft. focus, by Dollond, 4 powers, 50 to 200, finder, &c. on brass table stand.
 Newtonian Metal Speculum, 8 in. ap. first class quality.  Transit Instrument, 34 in. ap. 5ft. focus, Ys fitted with agates for mounting on stone piers.
 Equatorial Stand, for a 5 or 6 ft. telescope : 13 in. circles, divided on
silver, to 10" of space and 4s. of time.-Has never been used.  Varley Stand, improved, rackwork movements, for a telescope of 5 or 6 ft. focus : on circular turn-table, to point in any direction.
 Polar Axis of mahogany, 7 ft. long, adapted for a 4ft. telescope, with 12 in. circles, reading to minutes.
 Speculum Grinding Apparatus, capable of working a 7 ft. speculum : with tube and fittings for a gin. Newtonian.
 Recreative Science : complete in numbers, half-price. 
INSTRUMENTS, &c. WANTED. Astronomical Clock: not very first-class ; price low.  Equatorial Stand, for a 5 ft. Refractor : graduated circles.
 Transit Instrument, 2 in. ap. 18 or 24 in. focus.
 Monthly Notices of the Royal Astronomical Society, Vol. 3.  Monthly Notices : Vols. 3, 4, 5, and
 Bartholomew Prescot on the Universe, published 1823 : a copy wanted, in good condition.
FIREBALL OBSERVED AT YORK.-Mr T. H. Waller communicates particulars of a fine fireball observed at York, April 17th,
at 8h. 47m. G.M.T. about the size of Jupiter, and of a fine orange colour. The appearance was close to Ś Cassiopea, then at an altitude of 23° 40', and an azimuth of 18° 14' W. of North. The disappearance was, azimuth, W. from North, 23° ; altitude, 10°. It burst into many fragments which were hidden by a cloud; there was no noise: the time of flight was about one second.
April 18 is given as a maximum in Mr Herschel's “Dates preferred by Fireballs,” in the Astronomical Register for January, 1865.
DIETRICHSEN & HANNAY'S ALMANAC. --Mr Kincaid has pointed out to us two errors in the last issue of this useful work: 1st, the mean time of transit of a Andromedæ, Nov. 14, is given as 9h. 26m. 33s.: it should be 8h. 26. 435.—2nd, for the same month, Mars is stated to rise " after the Sun”; it should be “after the setting of the Sun.”
ASTRONOMICAL OCCURRENCES FOR MAY, 1866.
Meridian DATE. Principal Occurrences.
h. m. s. h. m. Sidereal Time at Mean
Saturn. Tues 1 Noon, 2 36 44-7
15 37 Occultation x Ophiuchi 1st Oc. R. 14 O II 5006
16 37 Reappearance of ditto (6) Wed 2
Sat. 12 8 29 Conjunction, Moon and 3d Sh. E. 14 54
Mercury, 20° 0' S.
Tues 15 11 44 Conjunction of Moon and'Ist Ec. D. 14 11 13 10 516
Venus, 5° 23' N.
Meridian DATE. Principal Occurrences,
h. m. s. h. m. Sidereal Time at Mean 1st Tr. I. 12 39 Saturn. Wed 16 Noon, 3 35 530
2nd Oc. R. 13 24 16 13 Conjunction of Moon and 1st Sh. E. 13 45
10 474 Uranus, 5° 21' N.
Tr. E. | 59
3 m. 51 8. before 4th Ec. R. 15 53 47 10 43*2
1911 39 Occultation disappearance 3d Sh. I. 15 28
of 60 Cancri (6)
26||11 11 Conjunction of Moon and 4th Tr. E. 12 27
Saturn, 0° 21' N.
3d Ec. R. 12 41 13 Wed 30|| 0 23 Conjunction, Uranus and 2nd Ec. D. 13 17 19 9 48-8 Venus, 1° Ö N.
3d Oc. D. 13 34
Oc. R. 15 48 9 44o7 Thur 31
BINOCULAR TELESCOPE.-Mr Troward of Doncaster informs us that he has used the microscope as a binocular eye-piece to a telescope for three years, and considers that a telescope with a large proportion of aperture to length is desirable for this purpose. He does not consider that any stereoscopic effect is gained by its use. Mr Troward has forwarded to us photographs of his reflecting telescope, fitted with a binocular eye-piece.
ASTRONOMICAL REGISTER–Subscriptions received by the Editor.
To Dec. 66.
Potter, Rev. T.J
Troward, C April 23, 1866. In order to save the expense of Postage, Subscriptions, as a rule, are acknow. ledged in the number of the Register for the month following that in which they are received, if not sent later than the 20th, after which they will be acknowledged in the next following number.
LIST OF SUBSCRIBERS—Names received since our last number.
30 s. Page 101, line 9, for latitude read longitude.
LONDON.Object Glass, 9} in. aperture, and 12 ft. 4 in. focal length. This Telescope has been used in a private observatory three years, and its performance on close double stars guaranteed: it has been removed in consequence of its late owner having purchased of the same maker an object glass of 16 in. diameter.  TO BE SOLD, a great bargain, a 7-inch ACHROMATIC TELE
SCOPE, 9ft. 6 in. focal length, by Troughton & Simms: 2 in finder, iron tube, brass-mounted.-Eye-pieces, eight negative, 90 to 600; four positive, one day, one nebula, one comet, small diagonal, large diagonal adapter, sun prism, transit, three Barlow lenses : Ramsden's micrometer by Berge, two reading micrometers, a position micrometer with two eye-pieces, graduated on platina, by Troughton & Simms, with slipping piece and illuminating apparatus. -On large garden Equatorial Stand, without circles; with steadying rods, and 5 in.
circle on silver with level,
adapted to the telescope to find declination when on the meridian.
 NO BE SOLD, a 12 in. SILVERED GLASS SPECULUM, greatest ease.
FOR ILLUSTRATION Kingsdown, Bristol.
 Now ready, in 8vo. pp. 812, price 258. cloth, LEMENTS of QUATERNIONS. By Sir WILLIAM ROWAN
WANTERORINSTRUMENTES. Address, ASTER, 35, Somerset Street,
HAMILTON, LL.D., M.R.I.A., D.C.L. Cantab. &c., late Royal Astronomer of Ireland. Edited by his Son, W. E. HAMILTON, B.A., C.E.
LONDON: LONGMANS, GREEN, and CO. Paternoster Row.
The Astronomical Register is intended to appear at the commencement of each
month: the Subscription, (including Postage) is fixed at Three Shillings per
Quarter, payable in advance, by postage stamps or otherwise. The pages of the Astronomical Register are open to all suitable communications: Let.
ters, Articles for insertion, &c., must be sent to the Editor, MR.S.GORTON, Stamford Villa, Downs Road, Clapton, N.E., not later than the 15th of the month,
The Astronomical Register.
A DESCRIPTIVE ACCOUNT OF
MERCURY. Mercury is not known to be attended by any satellite, and the determination of its mass is a difficult and uncertain problem. However, the small comet of Encke has furnished the means of learning something, and from considerations based on the digturbances effected in the motion of this comet by the action of Mercury, it has been calculated by Encke that the mass of the latter is 786675T that of the Sun. Le Verrier gives 2343000; Littrow
2026870; and Mädler 787333. The best time to see Mercury is at its greatest easterly elongation in the spring, when it is visible for 1 hours after sunset, and at the opposite elongation in the autumn, when it is visible for a similar period before sunrise. A good telescope, equatorially mounted, will frequently enable it to be found in the daytime.
The ancients were not only acquainted with the existence of this planet,* but were able to approximate with considerable accuracy its period, and the nature of its motions in the heavens. “The most ancient observation of this planet that has descended to us is dated in the year of Nabonasser 494, or 60 years after the death of Alexander the eat, on the morning of the 19th of the Egyptian month Thoth, answering to November 15 in the year 265 before the Christian era. The planet was observed to be distant from the right line, joining the stars called B and d in Scorpio, one diameter of the moon; and from the star f two diameters towards the north, and following it in right ascension. Claudius Ptolemy reports this and many similar observations, extending to the year 134 of our era, in his great work known as the Almagest."
We have also observations of the planet Mercury, by the * Pliny, Hist. Nat. lib. ii. cap. 7; Cicero De Naturâ Deorum, lib. ii.