Page ADDENDA ET CORRIGENDA. 3, note e. Add:-A further description of the principle of the method will be found in Challis's Lectures on Practical Astronomy, p. 301. 16, Fig. 7. The dotted lines on these 4 discs have been somewhat exaggerated. The curves should neither be quite so sharp, nor the inclination of the straight lines quite so great, as the engraver has made them. 17, line 18. A good description of the details of the structure of a sunspot is given by Janssen (Comptes Rendus, vol. cii. p. 80. 1886). 56, line 3, for "Eastern" read "Western." line 6, for "motion" read "the apparent motion of revolution round the Sun." 68, line 15, for "appendix " read "Book VI." 78, line 15. For 0.132′ read o.132. 126. In connection with Sir W. Herschel's supposition that he had seen a volcano in action, on the Moon, attention may be called to some remarks by Prof. Holden in The Observatory, vol. xi. p. 334, Sept. 1888. 165, line 8. The minor planet Thule (279) is now the most distant one known. line 16, for "Massalia" read "Massilia." 186, line 8. Add:-Lord Stratford De Redcliffe relates that on his voyage to America in September 1820 one night "at anchor on board ship I had occasion to observe the wonderful clearness of the atmosphere. From the Spartan's deck I saw with my naked eye the satellites of Jupiter." (Life of Stratford Canning, vol. i. p. 299, Lond. 1889.) 189, note f. Add:-Some useful information relating to the physical features of Jupiter's satellites will be found in R. Engelmann's Uber die Helligkeitsverhältnisse der Jupiterstral anten. Leipzig, 1871. 200, line 5 of Chapter Contents, for "the brothers Ball" read "Cassini." Page 233. For an account of some curiously mysterious circumstances connected with the discovery of the satellite Titan see a letter by Lynn in The Observatory, vol. xi. p. 338, Sept. 1888, and other letters in the numbers of that Magazine for March and April 1889. 250. Newcomb's mass of Uranus is 20. 259. Newcomb's mass of Neptune is 1350. 320. The Total Eclipse of the Sun of Jan. 1, 1889 was successfully observed in America. Professor Pickering noticed the corona to be longer and more irregular in its shape than usual, and that it exhibited great detail in its filaments. 367. With regard to Wicklow Head, there is another reason why the rise and fall of the tide there is so small. That Head is only about 22 miles N. of Courtown, where the tide waves entering the Irish Sea by the South and by the North of Ireland nearly cancel each other. At Courtown the range of the tide is only 18 inches, and that place is at the head of a bay, though a wide and shallow one. 375, line 16, dele “periodical." 376, lines 8 and 10, for "ecliptic" read "zodiac." 377, line 10 from bottom, read Aristillus. line 3 from bottom, for "effect of" read "solar." 385, line 14, after "these "insert "latter." * THE GREEK ALPHABET. ** The small letters of this alphabet are so frequently employed in Astronomy that a tabular view of them, together with their pronunciation, will be useful to many unacquainted with the Greek language. BOOK I. THE SUN AND PLANETS. CHAPTER I. THE SUN. O "O ye Sun and Moon, bless ye the LORD: praise Him, and magnify Astronomical importance of the Sun.—Solar parallax.—The means of determining it.—By observations of Mars.—By Transits of Venus. Numerical data.—Light and Heat of the Sun.-Gravity at the Sun's surface.-Spots.-Description of their appearance. How distributed.-Their duration.-Period of the Sun's Rotation.-Effect of the varying position of the Earth with respect to the Sun.Their size. Instances of large Spots visible to the naked eye.-The Great Spot of October 1865.—Their periodicity.—Discovered by Schwabe.-Table of his results.-Table of Wolf's results.—Curious connexion between the periodicity of sun-spots and that of other physical phenomena.—The Diurnal variation of the Magnetic Needle.—Singular occurrence in September 1859.— Wolf's researches.— Spots and Terrestrial Temperatures and Weather.—Ballot's inquiry into Terrestrial Temperatures.-The Physical Nature of Spots.-The Wilson-Herschel Theory.-Luminosity of the Sun.-Historical Notices.-Scheiner.-Facula.— Luculi.-Nasmyth's observations on the character of the Sun's Surface.Huggins's conclusions.—Present state of our knowledge of the Sun's constitution.-Tacchini's conclusions. IF F there is one celestial object more than another which may be regarded as occupying the foremost place in the mind of the astronomer, it is the Sun: for, speaking generally, there is scarcely any branch of astronomical inquiry with which, directly or indirectly, the Sun is not in some way associated. It will be only appropriate therefore to deal with this important body at the very commencement of a treatise on Descriptive Astronomy. By common consent, the mean distance of the centre of the Earth from the centre of the Sun is taken as the chief unit of astronomical measurement. The most approved method of determining the value of this was at one time believed to be by the aid of observations of transits of the planet Venus across the Sun' (as was first pointed out by Halley). The problem is, for various reasons, an intricate one in practice, but when solved places us in possession of the amount of the Sun's equatorial horizontal parallax; in other words, gives us the angular measure of the Earth's equatorial semi-diameter as seen from the Sun's centre, the Earth being at its mean distance from the Sun. With this element given, it is not difficult to determine, by trigonometry, the Sun's distance, expressed in radii of the Earth; reducible thereafter to miles. Encke, of Berlin, executed an able discussion of the observations of the transits of Venus in 1761 and 1769, and deduced 8.571′′ as the amount of the angle in question. From this it was found that the mean distance of the Earth from the Sun is 24065.1 times the equatorial radius of the former (3963 miles), equal to 95,370,000 miles; but these results, excellent as they were once thought to be, have long ceased to command the acceptance of astronomers, the fact being that modern experience has discredited Halley's method. d At a meeting of the Royal Astronomical Society, on May 8, 1857, Sir G. B. Airy proposed to adopt a suggestion of Flamsteed's for determining the absolute dimensions of the solar system, founded upon observations of the displacement of Mars in Right Ascension, when it is far E. of the meridian and far W. of the meridian, as seen from a single observatory; such |