THE MAGNETIC COMPASS
An extract from the 1907 F. Barker & Son Catalogue
This useful instrument, made in many forms, is used for some purpose or other in all parts of the world on land and sea. At what time it was first invented is difficult to say. The earliest references to the use of the compass are to be found in Chinese history, 2634 B. C. Compasses of that period were nothing like the instruments of scientific construction made to-day. Even in the sixteenth century the mariner's compass must have been very crude, for we read in Barlowe's "Navigator's Supply", published in 1597, "that in steade of our compas, they use a magneticall needle of sixe ynches long, and longer, upon a pinne in a dish of white china earth filled with water, in the bottome whereof they have two crosse lines, for the foure principall windes; the rest of the divisions being reserved to the skill of their Pilots".
The improvement of the compass has been but a slow process. In a treatise entitled "Magnetical Advertisements", published in 1616, the compass needle is described as "being the most admirable and usefull instrument of the whole world, is both amongst ours and other nations for the most part, so bungerly and absurdly contrived, as nothing more". In 1820 Professor Barlow
reported to the Admiralty "that half the compasses in the Royal Navy were mere lumber, and ought to be destroyed". Since then the compass has been greatly perfected, not only for nautical purposes, but for using on land for surveying and other purposes.
The simpler kinds, such as those used by the tourist, cyclist and others, consist of seven parts: A. the box; B. the dial; C. the magnetized steel needle; D. the cap; E. the pivot; F. the glass cover; G. the spring-ring bezel.
THE BOX. - Usually of circular form and made of metal.
THE DIAL. - Divided on the circumference according to two methods.
1. The circumference divided into 32 equal parts, called, "points of the compass", or "cardinal" points; each point containing 11.25 degrees.
2. The circumference divided into 360 equal parts called degrees, and figured from 10 to 360, having 90 at the East, 180 at the South, and so on.
THE NEEDLE. - A piece of magnetized steel of special manufacture, which is made in various shapes, having in the centre a brass cap which in the best compasses contains a jewel stone fixed in its centre. The needle is balanced by the cap on a steel pivot and allowed to revolve freely in the horizontal plane. The North end is made conspicuous by stamping the letter N or other mark on its top surface.
A compass as described above is known as a "needle" compass, but many compasses have the dial fixed on top of the needle so that they both revolve upon the pivot. These are known as "floating dial" compasses.
In the best surveying compasses which have sights, and those having "floating" dials, a prism is usually fixed in such a manner so as to facilitate
reading the figures on the dial when using the sights. Sight compasses having "fixed" dials should have the East and West points and the figures of the degree scale lettered and figured in the reverse manner; in other words, the E. and W. should exchange places. This is to simplify reading off a bearing when using the sights. Theodolites and miners' dials have their dials made this way.
In using a compass, hold it perfectly level and allow the needle to revolve freely, and, when the dial has settled, turn the box steadily until the north point of the dial is immediately under the north end of the needle, the compass letters will then be pointing in their proper direction. This applies to the "needle" compass. In using a "floating dial" compass the dial is allowed to settle, the letters will then be pointing in their right direction.
It would also be as well for one to pay particular attention to the following hints. The needle is attracted in the presence of iron and steel. Iron ore in the ground will cause attraction to some considerable extent, and in certain parts of South Africa the compass is rendered almost useless, but in England ore will attract the needle to no greater variation than about 2° from its true magnetic direction. The metals of a railway cause similar attraction, but this can be obviated to some extent by using the compass in the centre of the permanent way. Troops armed with rifles should be avoided, the compass being used at about 50 yards away. If a high wind is blowing when using the compass it would be better to kneel, so as to get a steadier position and a more reliable result.
There are two conditions which affect the magnetic needle, known as Declination and Dip.
THE DECLINATION OF THE NEEDLE, better known as the magnetic variation, is due to there being two magnetic poles situated some distance away from the earth's true poles. The North Magnetic Pole was found by Sir James Ross to be situated in 90° W. longitude and 70° S. latitude. The South Magnetic Pole is situated in about 168° E. longitude and 76° S. latitude. The variation of the compass is not only different for different places on the earth, but is, moreover, liable to change slowly as time goes on. At Greenwich the variation of the magnetic declination is about 12' in summer and 7' in winter.
In 1580, at London, the needle had about 11.5° easterly variation; between 1657 and 1662 its direction coincided with the true or geographical meridian, which means that there was no variation. Since then it has travelled in a westerly direction, and reached the maximum western declination in 1815, and is at the present time (1907) moving slowly towards the east.
THE INCLINATION OF THE NEEDLE, OR DIP, as it it technically called, is caused through the terrestrial megnetism exerting a pull downwards of one of the ends of the compass needle. In the northern hemisphere the north end of the needle would be depressed, whilst in the southern hemisphere the south end would "dip". At the North Magnetic Pole the north end of the needle would point directly downwards, and at the South Pole the south end would point likewise. At the time of manufacture the needle of the compass is so balanced that it will revolve in a horizontal plane where it is used; for instance, a compass made in London for New Zealand would have the needle made heavy at the north end, so that on arriving at its destination the needle would be horizontal. The needles of the best surveying compasses have a sliding weight for the correction of dip. If a compass has no weight attached to the needle the simples plan would be to take the needle out of the box and drop a little hot sealing-wax on the under side. This done with care will be found to answer very well.
The "dip" is supposed to have been discovered by Robert Norman, a nautical instrument maker at Wapping, who, about 1590, introduced the employment of a sliding weight on the needle for the correction of the dip at different points of the Earth's surface. This method is still employed in the best make of compasses, as mentioned above.
THE DECLINATION is subject to secular and other variations, the duration of which is not accurately known. Accidental disturbances, due to magnetic storms, affect the needle. These variations occur with great suddenness, almost like ordinary telegraphic signalling, and are, generally speaking, coincident with the passage of great outbursts of sun-spots across the sun's central meridian.
THE DIP, like the declination, is subject to similar variations, the true laws of which are not yet understood, but for investigation magnetic observatories have been established all over the world to furnish a continuous record from hour to hour, and year to year, of the forces which act upon the needle.
The following table gives the mean magnetic elements which applies to Greenwich only for the last ten years.
A chart of the world below gives the lines of equal magnetic variation for the year 1907.
A. F. C. B.