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the former paper these numbers were 20'7 and 14'3; so that there is some reason for regarding A as better determined than B, and also for regarding the former results as entitled to greater weight than the present. The best provisional determination of the errors of Carrington's axis, assuming that it remained fixed in space during the period 1874-1901, would therefore be

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giving double weight to the results of the former paper.

(c) As regards the discordances between the different zones, we may compare the results of the former paper and the present as follows:

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double weight being given to the second series in taking the mean.
(d) We now come to the constant C, the mean value of which
is positive (as already remarked in a note on p. 103 of the last
paper), instead of negative as before. Rearranging the material in
consecutive years, as in the former paper, we get

TABLE IV.

Drift in Latitude between +65° and -65° from central meridian.

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In Table V. the results of the former paper are repeated, so as to show the annual results for the whole series of years.

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The unit of the table is o°01, and the drift tabulated takes place in 130° of longitude. The figures have not yet been corrected for error of the Sun's axis, so that the results for years with few spots are subject to sensible corrections, depending on the particular months in which the spots were visible.

But the main feature of the table seems to be an oscillation from a drift of about +20, or +33' per rotation, in 1880 to an opposite drift of 25, or 40' per rotation in about 1893.

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Now one possible explanation of these figures is that the Sun's axis is not stationary, but is describing a cone in space, in a period which may be one of several.

(a) If the period were about 26 years, the effect would be reversed in 13 years, but it is easily seen that the angular radius of the cone would have to be too large to fit the facts.

(B) If the period were nearly a submultiple of a year, say six months, Tables I. and II. would show it. Analysis does not yield a suitable result.

(y) It seems most likely that the period is nearly a year. If it were exactly a year, and we had adopted a wrong axis so as to get an apparent drift in latitude, the error and its effects would travel round with the Earth, so that we should get a spurious constant drift. If the period differs from a year, the spurious drift would slowly change, going through a complete cycle in the L.C.M. of the period and the year. For instance, if the cycle is about 26 years, the period of the wobble of the Sun's axis must differ from one year by about half a month.

(8) The drift may, however, be a physical drift on the surface of the Sun; and we can distinguish between these rival hypotheses by examining spots which return after disappearing. If the drift is physical, it will persist on the side of the Sun remote from the Earth; if due to the rotation of the axis, it will be reversed on the other side. Since the amount per rotation is large near a maximum (say 40' per rotation), even a cursory examination should settle the point. Carrington's observations show no tendency to physical drift, as may easily be seen from the results 130 to 213, or the table pp. 213-219, or the summary on p. 220. Nor does an examination of the spot returns tabulated by Father Corte (Mon. Not., vol. lx.) give much support to the hypothesis of a physical drift.

On the whole, hypothesis (7) seems best worthy of further study.

But before deducing definitive elements for the axis, we await the results deduced from the measures of C. H. F. Peters, and recently published by Professor Frost. These are nearly completed, and can probably be given in the next number of the Monthly Notices.

Magnitude of n Argus, 1908. By R. T. A. Innes.

Comparison star used, Gilliss 1332 of 76 mag., yellowish or reddish, 4 on Chandler's colour-scale. This star is 8' N.pr.

n Argus.

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A low-power field, with both stars in view, on the 9-inch.

refractor.

Johannesburg: 1908 June 3.

Photometric Measurements of Neptune, January to April 1908. By J. M. Baldwin, M.A., 1851 Exhibition Scholar (Melbourne).

(Communicated by Sir David Gill, K.C.B., F.R.S.)

1. The observations which are recorded in this paper were undertaken chiefly to ascertain whether it were possible to recognise a short-period variation in the brightness of Neptune such as that which Maxwell Hall believed he had found in 1883 and again in 1884.

The first announcement of a variation was made in the Monthly Notices (xliv. 257, 1884). The observations made by Mr. Hall were estimations, generally in fourths of a magnitude, of the difference between Neptune and a star in the same field, assumed as of magnitude 8.5 (B.D. +15°, 453, mag. 8.5). On the night of 1883 November 29 Neptune seemed to him fainter than on the two preceding evenings, being "as nearly as possible" of the same brightness as the comparison star, but later in the evening Neptune appeared somewhat the brighter. From then till December 14 the estimated magnitude varied between 7.5 and 8.5, but during the nights of December 15, 16, and 18 no variation could be observed. From his observations he deduced a period of 7'92 hours. On December 27 and again on 1884 January 8 Neptune was compared with another star (B.D. 15°, 446, mag. 7*5), but " no variation was noticed on either night, yet Neptune was undoubtedly brighter on January 8 than on December 27" (Observatory, vii. 73, 1884).

He observed Neptune again at the next opposition, and in a short note in the Observatory (viii. 26, 1885) says: "Careful photometric measures gave November 29d 9h 44m G.M.T. as an epoch of maximum brilliancy, but the variation was only 0'4 magnitude between max. and min. The period of 792 observed last year has been fully confirmed." I have unfortunately been. unable to find any information as to the observations beyond that contained in this brief paragraph.

2. A considerable number of measurements of Neptune with the meridian photometer have been made at Harvard College Observatory, almost all by E. C. Pickering, A Ursa Minoris

*

* Observatory, vii. 134, 1884, and viii. 111, 1885; Harvard Annals, xxiv. p. 265 and xlvi., part ii., p. 203. In this paper the early comparisons, 4 in number, of Neptune with Uranus, made by Zollner (Photometrische Untersuchungen, p. 150) and the 8 Harvard comparisons with a Arietis in 1878 (Harvard Annals, xl. p. 224) have not been made use of.

being used as standard. The number of observations and results obtained are given in Table I.

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3. The observations of Müller at Potsdam are more numerous. These are measurements with the Zöllner photometer of the difference in magnitude between Neptune and a suitable comparison star or stars, the same throughout any one opposition. The mean results, etc. for the separate years will be found later on in this paper (Table IV). The mean of all the 138 observations is 7.66 on the scale used in Band viii.; this is exactly the same as the mean of Pickering's 56 observations between 1882 and 1898.

4. In the observations given below, a Zöllner photometer attached to the Steinheil refractor (13'5 cm. aperture and 216 cm. focal length, called photometer D in the Potsdam Publications) was used throughout. Two comparison stars were chosen near Neptune, namely B.D. + 24°, 1457 and +22°, 1531, the magnitudes given in the Potsdam General Catalogue being respectively 7'97 and 7.18. As usual, four readings were taken for each object, one in each quadrant of the intensity circle (which was always turned in the same sense), the star being respectively to the left of, above, to the right of, below, the artificial star. The complete observation consisted in observing first one comparison star, then Neptune, then the other comparison star. For the next observation the stars were taken in the reverse order, and so on throughout the whole series. The complete results are given in Table II.

*A.N., 2600, Bd. cix. 121, 1884, or Observatory, vii. 264, 1884; Pots. Publ., Bd. viii. It is to be noticed that the magnitudes as given in the A.N. are not referred to the same scale as those in the collected results in the Pots. Publ.

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