referring our readers for more technical information to the
professional text-books enumerated at the end of this article.
The great development in both size and speed of modern ships
enormously increases the responsibilities of those who command
and navigate them, and has led to a careful examination of
the existing modes of determining a ship’s position at all times
by day or night, both when in sight of land and on the open ocean.
An examination of the present text-books on the subject of
navigation shows how problems and methods which were formerly
considered chiefly as theoretical exercises have now, from the
altered conditions of the navigation of very fast ships, become
methods of frequent practice, while corresponding improvements
have been made in the instruments, such as compasses, charts
and chronometers, by the aid of which more satisfactory results
are now attained. Much has also been done to advance the
study of this and its numerous allied subjects by the development
of the Royal Naval College at Greenwich and the United Service
Institution; also by the establishment of shipmasters’ societies
(of which the well-known society in London is typical), where
during the year valuable papers are read and useful discussions
take place among those actually carrying out the practice of
navigation.
In planning out in advance a long ocean voyage the experienced navigator would first, by laying down the track from port to port on a great circle chart, ascertain the shortest route between them, remembering that the greatest saving in distance over other routes is when the ports are far apart in longitude and both in high latitudes of the same name. On examining such a track in conjunction with the wind and current charts it will be seen what modifications the intervention of land, unfavourable currents or winds, ice or unduly high latitude render necessary, and such modified route would be finally adopted subject to possible change as the voyage progressed. The judgment formed on the best route to follow would also be largely influenced by the remarks in the volumes of Sailing directions or “Pilots” relating to the region about to be traversed, while among the many excellent modern publications of the Hydrographic Office of the Admiralty perhaps the Ocean Passage Book is one of the most generally useful, since, when used in combination with the admirable charts of suggested full-powered and auxiliary tracks, it very greatly assists all navigators in planning out a successful voyage. Finally the intended route would be transferred from the great circle chart to one on Mercator’s projection, which is the more convenient for purposes of navigation since in constructing the former for the sake of simplicity a projection of the coast’s surface is adopted on which great circles are correctly shown as straight lines (gnomonic), while for practical purposes in navigation such a representation on which a ship’s track when steering a continuous course (technically termed a rhumb line) is truly shown as a straight line (Mercator) is the most convenient, although in high latitudes giving a very distorted representation of the surface depicted. It is well to remember that on great circle charts rhumb lines become curves and great circles straight lines, and, vice versa, on Mercator charts, the rhumb line on each projection being that nearer to the equator, all meridians and the equator on both projections are shown as straight lines.
Ships rarely steer on great circles, which would generally theoretically involve continually altering course, but a series of chords of such circles are described of lengths such as involve a practical change of course of one or two degrees on the completion of each.
Great circle charts are very useful for drawing what is known as a composite track where if the great circle route would lead into too high a latitude the shortest route to and from the highest desirable parallel is readily laid down, the intervening track being pursued on that parallel.
A method of drawing approximate great circles directly on Mercator charts was proposed by Airy in 1858, and is sometimes very useful. The excellent idea, originally suggested by M. F. Maury, of establishing steam “lanes” in localities where there is much ocean traffic, so as to minimize the risks of collision between outward and homeward bound ships, has been successfully carried out in the North Atlantic. The leading transatlantic steamship companies now agree to follow great circle routes from the Irish coast to points on the Banks of Newfoundland, which vary somewhat in position with the season of the year, but are published in advance. These “lanes” being avoided by sailing vessels, risks of collision are materially lessened.
Having thus planned the most desirable general track to pursue, three methods are employed to ascertain the position of the ship at any time during such voyage: these are (1) projecting the track on charts; (2) simple trigonometrical calculations where the data are the course steered and distance run; and (3) astronomical observations, which form an entirely independent method.
Of these the first is the least trustworthy, owing to the usual difficulties attending accurate graphic methods and the small scales on which ocean charts are necessarily drawn. When near the land the larger scale coast charts are used, and in the approaches to harbours still larger scale plans give increasing accuracy to this record of a ship’s position. Index charts of all parts of the world are provided, by referring to which the navigator ascertains which chart or plan to employ, always preferably using that on the largest scale.
On leaving harbour, and while near the coast, the position is not found by calculation but by frequently observing (when a variety of objects is in sight) (1) simultaneous sextant angles between suitably situated objects subsequently laid down on the chart by a station pointer; (2) simultaneous compass bearings of two or more objects (technically known as cross bearings); or (3) a combination of both methods by employing one bearing and one angle. All such methods are capable of considerable accuracy if the observations are made simultaneously. Should only a small number of objects, or sometimes only one, be visible (as frequently occurs at night) other and rougher methods are practised, depending upon the change of bearing of an object while a certain distance in a certain direction is traversed by the ship, such knowledge being based in many cases on an estimate of the action of the tide. When a ship is steaming at the rate of 20 knots the navigator remembers that a mile is passed over in three minutes, and that if in sight of land and fixing positions by objects on shore, it is essential to adopt some rapid method; otherwise when laid down on the chart the position shows where the ship was, and not where she is. This difficulty has led to the more general use of methods of obtaining positions by angles instead of bearings, and laying them down on the chart by the aid of the station pointer. Many advantages accrue from this, as the observer is not restricted in position on board, as is the case when using the compass, and especially if a double sextant (having two index glasses and one horizon glass) is employed two angles can be measured simultaneously, the result on the chart being very rapidly arrived at. An ingenious combination of sextant and station pointer in one has been proposed, and most simply carried out by attaching vertical sights to the legs of a station pointer, which is put on a suitable horizontal stand, and the legs moved until the sights are in line with the objects observed. To assist the navigator in the choice of suitable objects between which to measure the angles, a very useful pamphlet is issued by the Admiralty, from the diagrams in which it can be seen at a glance which combination of objects in sight gives the most favourable result, always remembering as a broad principle that nearer objects are more suitable than distant ones, and that the accuracy of position determined depends on the relative distances of the objects as well as on the magnitude of the angles between them.
In these circumstances which render these rougher methods those only available, and especially in hazy weather in many known localities (such as the English Channel), a continuous line of deep sea soundings at fairly even distances apart affords an additional verification of position, remembering that only an occasional sounding might prove very misleading.
The chronicle of progress in the art of navigation would be very
