NAVIGATION

This may be done by a geometrical projection, but it is generally effected in practice by the aid of the traverse table. From this table the diff: lat. and dep. corresponding to each course and distance is taken, and entered in an appropriate table, having columns headed N. S. E. W.: namely, N. and S. for diff. lat., and E, W. for departure. The difference between the sums under N. and S. shows the diff. lat., as does the difference between the sums under E. and W. the departure; and in either case the difference is of the same denomination as the larger sum. The course and distance required are then either found by inspection in a traverse table, or by the formulae (A.). When a ship makes considerable way through the water, and the wind is on the beam, abaft it, or even a little before it, she generally moves forward in the direction of the fore and aft line; but in rough weather, with the wind forward, she will generally be driven more or le to leeward, as will be shown by the direction of the wake, or the ripple formed by the waves closing behind her. The angle which this ripple makes with the direction of the keel is called the leeway; and it must be applied as a correction to the course shown by the compa , and always allowed /ro?w the wind, — that is, to the left, if the wind is on the right-hand side of the ship, and to the right, if on the left. See Leeway. All matters relating to the navigation of a ship are entered in a systematically ruled book, called the log. book; and what day after day is so recorded is called the ship's journal. The principal columns in the log-book are for the hour of the day, the course, rate of sailing, leeway, and winds; one for general remarks, and for entering the particulars and results of celestial observations, for notes on the weather, and memoranda as to all important points of duty in the ship, the sails set, and the manner in which the crew are employed. To this is daily appended the latitude and longitude of the ship at noon, both as deduced from celestial observations, and as computed from the course and distance since the time when the place was last ascertained. The place determined from the course and distance is called the place by dead reckoning. The bearing and distance of the land first expected to be seen, and the course and distance made on the whole, during the day, are also added. If the course and distance could always be accurately determined, the place of the ship could be computed with corresponding exactne from the principles of whicii we have above given a concise account. But these data can only be obtained in a roughly approximative form. The effect of unknown currents, unavoidable imperfections in steering, and numberle other sources of error, render the place of the ship, as estimated from the reckoning, very doubtful; and, in fact, when the mariner is obliged to rely for several days on these data only, he often finds that his expected and his true place are considerably distant from each other. In the modern practice of navigation, therefore, the course and distance are only used to enable the seaman to a ign approximately the place of his ship between the times at which it is determined, independently, by celestial observations. This branch of nautical knowledge, which is generally and properly included in every system of navigation, is called nautical astronomy j and the improvements which have been introduced in its modern application constitute the chief difference between navigation as practised in our own and former times. For a minute explanation of the proce es by which the place of a ship on the wide ocean maybe determined, from the observed situation of celestial objects with respect to each other and to the horizon, we must refer to works expre ly devoted to the subject. But we shall give a short account of the most useful practical methods of finding the latitude, the longitude, and the variation of the compa , which are the three principal problems in nautical astronomy. Rcdu£tion of Altitudes Before the altitudes of celestial objects as observed at sea can be employed in the solution of astronomical problems, they must be corrected for the effects of dip and parallax; and for semidiameter, when the altitude of the upper or lower border, instead of that of the central, has been observed, as in the case of the sun or moon. If A = the altitude of the upper or lower border, s = the semidiameter, d the dip of the horizon (that is, the angle through which the sea horizon appears depre ed in consequence of the elevation of the observer), r the refraction corresponding to the alt. A, and/? the horizontal parallax taken from the Nautical Almanac for the time of observation, and A' = the true altitude: then_ ' A' = ^. — d+s-Vp [s. 826]