To conduct a ship safely across the high seas to a distant port calls for skill of the highest order and an intimate knowledge of the science of navigation. This chapter, written by a retired navigating officer of the Royal Navy, gives a clear account of the instruments required
A MODERN LINER’S CHART HOUSE. Located on the forebridge, the chart-
THE navigation of a ship is often a mysterious business to a landsman. If he is in a ship there is nothing to be seen except the ocean; yet day by day the ship speeds onwards, heading apparently straight as an arrow for some distant port perhaps thousands of miles away. Occasionally he may see one of the officers “shooting the sun” with an odd-
There is no other indication of how the ship is being navigated. He may be almost anywhere on the Seven Seas; yet day by day he finds, posted up, the ship’s exact noon-
To conduct a ship safely across the high seas to a distant port, the navigator requires first a chart, embracing his starting-
There are, of course, many other instruments which, although not essential, contribute an additional factor of safety which no master of a large ship would reject. But the foregoing, which will be briefly described in turn, are the navigator’s everyday tools.
Let us describe first the chart, since this is the navigator’s primary requirement. The capacity to construct and use marine charts is not confined to civilized man. The Eskimo, for example, have long been noted for their skill in map-
Nowadays, almost every sea-
A chart differs from a map in being designed for use at some distance from the land, rather than upon it. Thus it devotes as much attention to the configuration and depth of the sea-
Wrecked in Charted Waters
The outline of this, indeed, is shown with extreme accuracy, as are the positions of all conspicuous objects visible from seaward; but much of the inland detail is omitted. On the other hand, every care is taken to show not only off-
At the same time, tidal observations must be made, and the range of the tide determined, for all soundings are reduced to the level of low-
Even when the results of the survey have been reduced to chart form, and published, the navigator’s troubles are not over. Every chart is out of date before it is published. Incessant and inevitable change is the rule governing everything connected with the sea. Channels silt, or scour; new rocks and shoals are discovered, those already known change their depth and extent; buoys and lightships go adrift, or are moved to new positions; new lights, beacons and the like are established.
All such changes are, of course, notified by “Notices to Mariners”, but it is never possible to embody all of the most recent information in a chart before its publication; the most that can be done is to keep its users informed as far as possible, of all changes affecting it. Most nations, too, issue volumes of “Sailing Directions” which serve as a running commentary on their charts and contain much useful information for which no space is available on the charts themselves.
A feature of almost all charts, which stamps them at a glance, as differing from maps, is that they are generally drawn on Mercator’s Projection, in which all the meridians are parallel. This method involves a good deal of distortion: for example, in the figure of the world (shown below) Greenland appears almost as large as Africa, the area of which is, in fact, rather more than twelve times as great. But while this distortion renders Mercator’s Projection almost useless for maps, it is most valuable to the navigator. All the meridians being parallel, a line ruled from his starting-
THE WORLD (80° N. to 60° S.) AS SHOWN ON MERCATOR’S PROJECTION. The small dials above indicate local time corresponding, for every 30° of longitude, to noon at Greenwich.
Next comes the compass. The introduction of the magnetic compass was the greatest single advance ever effected in the art of navigation. The date of this introduction cannot be accurately fixed, but the compass was undoubtedly known to the European navigators of the twelfth century. That its use did not immediately become general is probably due to the suspicion of sorcery which this method entailed. In the early days of wooden ships the compass in its simplest form -
It was the rapid development of iron shipbuilding in the first half of the last century which really directed public attention to the defects of the magnetic compass. An iron ship, while being hammered on the stocks, becomes a permanent magnet -
That is why every ship, soon after launching and at intervals during her career, is “swung for adjustment of compasses” -
Whatever pains were taken, adjusting the old pattern of magnetic compass in an iron ship was always a difficult business -
In 1876, however, Sir William Thomson (afterwards Lord Kelvin) introduced a greatly-
AT INTERVALS DURING HER CAREER a ship is “swung for adjustment of compasses”. She is headed to each point of the compass in succession, while a bearing of some prominent object (whose magnetic bearing is known) is taken at each point. The difference in each instance between the compass bearing and the magnetic bearing gives the deviation of the compass for that particular direction of the ship’s head. These deviations are tabulated and applied whenever a course is set or a bearing taken. Swinging for adjustment of compasses is necessary because, although the “induced” and permanent magnetisms are corrected by magnets and soft iron, these corrections are neither complete nor lasting.
The last thirty years have seen the evolution of an entirely novel type of compass -
Its inception is due to a German scientist, Dr. Anschutz-
A SHIP BEING STEERED AUTOMATICALLY by a gyro-
The theory of the gyro-
In the gyro-
A STANDARD MAGNETIC COMPASS mounted in a binnacle. On top of the compass bowl is the “azimuth mirror” for taking bearings. The two large spheres are of soft iron and partly correct the ship’s “induced” magnetism. This induced magnetism is from the Earth’s magnetic force which causes temporary magnetism in the soft iron portions of the hull. In the binnacle itself are small permanent magnets, which counteract “permanent” magnetism. An iron ship, while being hammered on the stocks, becomes a permanent magnet just as a poker can be magnetized by holding it approximately N. and S. and tapping it with a hammer.
The earliest method (except that of simple estimation) of determining how fast a ship is moving through the water was the “Dutchman's Log”. A piece of wood or an empty bottle was thrown over the bow, and the time of its passing along the ship’s side, and going clear of the stern, was noted. The ship’s length being known, her speed could be calculated. Oddly enough, this obviously rough method is still used (but with many additional refinements) when a ship is running her trials in the open sea, and a “measured mile” is therefore not available. The “Dutchman’s Log” gradually gave place to the “hand-
AN EARLY METHOD OF DETERMINING A SHIP’S SPEED was the hand log. A small float shaped to offer considerable resistance to towage and with a line attached was thrown overboard, the line -
When navigating in shallow waters it is always necessary, even if they are well charted and one is reasonably sure of the ship’s position, to keep informed as to the depth of water underneath her. The simplest method of doing this is to use the “hand-
TO FIND THE DEPTH OF WATER under a ship when she is being navigated in shallow waters, the leadsman uses the hand-
Standing in the “chains” -
The majority of soundings, outside harbour waters -
An isolated attempt to produce such an instrument was made by Dr. C. W. Siemens, who devised his “bathometer” in 1876. It measured very small variations in the earth’s total attractive force, and could therefore be used to determine by inspection the actual depth of water under a ship. It was tried in H.M. Surveying Ship Fawn with fair success -
The modern “echo-
Such observations will not, of themselves, give the ship’s position. They will give only her latitude and her local time -
The navigator cannot, as the astronomer does, refer his observations to the vertical -
LATITUDE AND LONGITUDE. Latitude is measured by the angle subtended at the earth’s centre between a point on a given parallel and the Equator. Longitude is measured by the similar angle between any given meridian and that of Greenwich.
The early ocean-
To be an expert with-
A SEXTANT FITTED WITH OBSERVING TELESCOPE AND SHADES TO MITIGATE STRONG SUNLIGHT. The angles measured are read on the graduated arc by a small microscope. The sextant was rediscovered simultaneously by John Hadley, of London, and Thomas Godfrey, of Philadelphia. It cannot be said to have been discovered by these men, because afterwards it was learnt that a precisely similar instrument had been invented by Sir Isaac Newton in 1699, although no account of it had ever been published. In essentials Hadley’s sextant is substantially that of to-
In using the sextant the observer sights the sea-
As Newton pointed out in an unpublished MS. description of his instrument, the double reflection ensures that, once the two objects have been brought into apparent coincidence, they will remain so in spite of any motion of the ship or the observer, appearing as if glued together. It is this most valuable property which makes the sextant the one instrument, above all others, for taking astronomical observations on board ship.
In essentials, Hadley’s sextant of 1731 is substantially that of to-
It seems unlikely that the present form of sextant -
“SHOOTING THE SUN”. Observing, with the sextant, the sun’s altitude above the sea horizon. Two small mirrors give the sextant a double reflection, and it is this most valuable property which makes the sextant the one instrument, above all others, for taking astronomical observations on board ship. It will be seen that the two men cover their own shadows. This is possible only at noon in the tropics when the sun is overhead.
Sometimes known as “The Sailor’s Bible”, the Nautical Almanac, published at the instance of the Admiralty, is a compendium of astronomical information originally intended (it first appeared in 1767) solely for the use of navigators. In course of time it came to contain not only all that they were ever likely to require, but also a great deal which it was morally certain they would never have any opportunity of using.
In recent years, therefore, it has appeared in two forms -
The Nautical Almanac contains an enormous amount of accurate information relating to the positions and astronomical elements of the heavenly bodies, calculated several years in advance. Originally a large staff of computers was employed on such work, but for some years past all computations required have been done mechanically.
Reward of £20,000
When dealing with the sextant, it was pointed out that a ship’s observations give only her latitude and her local time -
It is possible to find Greenwich time astronomically by the “method of lunar distances”, which involves measuring the angular distance between the moon and either the sun or a suitable star, and then performing a long and intricate process of calculation. But the errors involved multiply at an amazing rate -
In the following year however, a chronometer was entered by John Harrison -
The marine chronometer is essentially a large and accurate watch, mounted in “gimbals” so that it remains horizontal whatever the motion of the ship. Its mechanism is slightly more complicated than that of a watch and embodies various contrivances designed to remove, as far as possible all sources of error which might affect its rate of going. The accuracy of the modern chronometer is remarkable -
THE MARINE CHRONOMETER is essentially a large and accurate watch mounted in “gimbals” so that it remains horizontal whatever the motion of the ship. Its mechanism is slightly more complicated than that of a watch and embodies various contrivances designed to remove as far as possible all sources of error. A good modern chronometer will keep time at sea with an error of not more than one second per week, corresponding to an error in longitude not exceeding a quarter of a mile.
All that the best chronometer can do is to carry the Greenwich time -
Wireless telegraphy is, of course, immensely valuable to the navigator in other ways. For example, gale warnings, information as to changes in lights, buoys, and the like can always reach him instantaneously, wherever he is. Of more technical interest is the fact that by means of a direction finding set he can obtain, by day or night, and even in the thickest fog, bearings of distant wireless telegraphy stations. Plotting such bearings on his chart, he at once obtains his exact position.
In fact, of all the gifts which science -
Certainly the question seems, at first sight, difficult to answer. But the chances of the sea and the fact that all navigators are human -
The navigator chiefly uses his instruments to ascertain how far he really is from where he has computed that he ought to be. That he has perpetually to do this is due to the sea itself, which has a way of its own with a ship, and delights in deluding the navigator as to his true position.
Navigation is very easy in fine weather, in well-
A VIEW OF THE BRIDGE of the M.S. Pilsudski. From left to right in this picture can be seen the engine-
“Romance of the Chronometer” this website.