NAUTICAL TABLES DESIGNED FOR THE USE OF BRITISH SEAMEN BY THE REV. JAMES INMAN, D.D. LATE PROFESSOR AT THE ROYAL NAVAL COLLEGE, PORTSMOUTH New Edition, Revised by the REV. JAMES WILLIAMS INMAN LATE FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE, AND HEAD MASTER B RIVINGTONS London, Oxford, and Cambridge 1870 Augmentation of Moon's Horizontal Semidiameter Reduction of Horizontal Parallax, and Latitude of Place, for Figure of Earth Correction of Sun's Apparent Altitude for Refraction and Parallax Correction of a Star's Apparent Altitude for Refraction Time of Sun's passing Prime Vertical and Azimuth Tables: Increase of R.A. of Mean Sun: Distance of Objects at Sea: Eq. of 2nd Difference Bearing Amplitude and Time Amplitude at the Rising and Setting of the Sun Time from Noon at which the True Bearing of the Sun is East or West EXPLANATION AND USE OF TABLES. Table (a), p. 1.-This table contains the whole correction of the sun's altitude, involving dip, refraction, semidiameter, and parallax. The lower limb is supposed to be observed. When the upper limb is observed proceed as in the Table, and deduct 32'. Table (b), p. 2. Parallax in Altitude for Planets. Enter with the hor. parallax of a planet (supposed to be known from the Nautical Almanac) at the top, and the app. altitude of planet at the side. Thus will be found the parallax in altitude, to be added to the app. alt. to get the true alt. The app. alt. is first supposed to be corrected for refraction, as for a fixed See Tab. (n.) star. Table (c), p. 2. Correction of A for Planet in Clearing Distance.For explanation of this table, see that of Table (w). Table (cc), p. 2. Sun's Parallax in Altitude.—The effect of parallax in alt. for the sun, is included in correction of Table (m): it is put down here, as it is occasionally of use considered separately. (See Navigation on Occultations, p. 180.) Table (d), p. 3. Points of Compass, &c.—This table is of use in turning the points of the compass into degrees, and conversely also, in finding at once the log. sines of points and quarter points. Table (e), p. 3. Dip of the Sea Horizon.-The altitude of a heavenly body is taken in the open sea with Hadley's quadrant, in doing which the image is brought down to the apparent sea horizon, which is below the level of the eye. The observed altitude is on this account something too great. The necessary correction is put in Table (e), which is entered with the estimated height of the eye above the sea. The correction or dip taken out is subtractive.* In computing this table, it is necessary to consider the effect of horizontal refraction in raising the visible sea horizon, which has been found to be about 8-100ths of the dip independently of this correction. But this effect varies with the state of the air near the sea horizon. Hence the correction applied for dip may frequently be erroneous; and this is one cause why altitudes taken at sea, particularly those taken near the horizon, are not to be depended on where great accuracy is required. |