Assorted Learning Activities
Learning Activity 2
Flinders' compass (teacher guided) - extra information
Magnetism in Ships
This extract dealing with magnetism in ships is taken from G. C. Ingleton's book Matthew Flinders: Navigator and Chartmaker. It begins with quotes from Flinders' pamphlet Magnetism in Ships.
The cause of all these changes in a compass on shipboard, and the modes by which they may be obviated, I have endeavoured to explain in the account of the experiments drawn by order of the Admiralty; and to obtain further proofs, it is desirable that the differences should be observed in as many ships, and as accurately as possible, in the following manner:
The azimuth compass to be used, should be a good one; the card traversing freely; and if possible, the needle should be retouched with magnetic bars, before the observations are made. A low stand or stool must be prepared; so that when the compass is placed upon it, it may be of the same height with that by which the ship is steered. The binnacle being then taken away, substitute the stand and the azimuth compass; or if the sun cannot be there seen all round, fix the stand as near to the situation as it can be seen; but clear of any iron, and exactly midships. Let azimuths be taken, using both sides of the vane; and this as often as convenient, and with the ship's head in various directions; but more particularly at, and near, East and West; noting the direction of the head to the nearest quarter of a point.
Flinders concluded his statement by advising that the observations should be entered in a table of ten columns, which he described; and that a good series of experiments could be made when the ship was lying steadily at anchor, and it would be more easily determined whether the differences at the binnacle were sufficiently in error to be of importance to ordinary navigation. Flinders finally ended his statement: With marine surveyors and all officers who wish to fix the position of places or of their ships, by compass bearings, the subject must necessarily be one of much interest.
The statement, Magnetism of Ships, was reprinted in The Naval Chronicle for 1812; with additional comments by Captain Horsburgh, Hydrographer to the East India Company, on the subject of the amount of variation in the English Channel, to which Flinders replied. It was also a matter of some considerable interest that Peter Heywood conducted further experiments in his ship, the polyphemus, moored at Spithead, with a series of observations on conspicuous shore features while the ship's head was East and again, when it was changed by the tide to West. Heywood found no aberration of the needles; and generally this was the opinion of most captains, commanders and masters who made such experiments. The consensus of opinion amongst naval officers was that since ships were steered to the nearest quarter point of the compass, and moreover, the variation of the compass by the earth's magnetism was so high around the British Isles, the additional small error caused by the effect of the ship's iron could be safely disregarded. It was not until some forty or fifty years later, when ironclads and iron steamers plied the seas, that the subject became an important issue in safe navigation.
Besides the statement issued to captains, commanders and masters, and reprinted in The Naval Chronicle for 1812, Flinders had two other outlets for his views on the subject of magnetism of ships. As already mentioned, he gave a paper to Dr Bumey to print in the new edition of Falconer's A New Universal Dictionary of the Marine. He also prepared a lengthy paper, which was printed as Appendix II in A Voyage to Terra Australis, under the title: On the Errors of the Compass arising from Attractions within the Ship, and others from Magnetism of Land; with the Precautions for obviating their Effects on Marine Surveying.'
In this paper, which extended over twenty pages of text. Flinders again referred to the corollaries of the magnetic needle and compass. He also gave many examples of magnetic observations in Australian waters, as well as referring to the printed examples from earlier navigators and observers especially to the remarks of William Wales, an astronomer with Cook. He stated also that Beautemps-Beaupre (D'Entrecasteaux's surveyor) had early abandoned the use of the magnetic compass as inaccurate for the fixing of ships by bearings, and had preferred to use offshore angles obtained by sextant.
It is noteworthy that Flinders made no recommendation in his statement to captains, commanders and masters of the fleet, on methods for counteracting the effects of iron in the ship; but in his Appendix II in his book, he concluded with this advice: These memoranda are mostly relative to the compass fixed on the binnacle; but the trouble of correction may be saved if a place can be found near the taffrel, where the attraction of the iron at the stern will counteract, by its greater vicinity, the more powerful attraction in the centre and fore parts of the ship; and should the after attraction be too weak, it may be increased by fixing one or more upright stanchions or bars of iron in the stern. If a neutral station can be found or made, exactly amidships, and of a convenient height for taking azimuths and bearings, let a stand be there set up for a compass."
It does seem unlikely that Flinders himself experimented with vertical soft iron bars - in the form of detached iron stanchions to counteract the attraction by the ship's iron on the compass. He probably devised the method while still detained at the Île de France; and his only opportunities to experiment, therefore, would have occurred in the otter or the olympia, when in port on his homeward passage to England in 1810.
While the question of ship's magnetism did not arouse much public or scientific interest during Flinders' lifetime, due to his early death; yet six years later, a scientific beginning was made by Professor Peter Barlow, who wrote a number of papers on the subject. Barlow, apparently, was unaware of Flinders' experiments. There was an increasing development of iron shipping, which presented a growing problem on the magnetism of ships -and the greater activity in polar exploration also contributed considerably to research on the earth's magnetism. The polar expeditions established very early in the nineteenth century the positions of both the north and south magnetic poles.
The Navy also began to take an interest in magnetism -both in the matter of variation and in the reports on ship's deviation - due both to the question of safe navigation and to the worldwide activity in hydrographic surveying and more accurate charting. The Admiralty Compass Committee, instituted in 1837, was active in this field for a number of years. It devised a standard compass binnacle, to which was attached a square iron plate, which Barlow had suggested in 1820 in his Essays on Magnetic Attraction, &'c.; but this plate only partially fulfilled the results of a satisfactory counteraction, which had been achieved by Flinders' proposal of a vertical soft iron bar. For that reason. Barlow's Plate never became popular. Professor Sir George Biddell Airy (later Astronomer Royal), in his Account of Experiments on Iron-built Ships, instituted for the purpose of discovering a correction for the Deviation of the Compass produced by the Iron of the Ship (published in the Philosophical Transactions of the Royal Society for 1839), suggested horizontal magnets and boxes of soft iron chain to correct the compass; but neither Barlow's Plate nor the magnets or the boxes of chain proved a satisfactory counterbalance to the ship's magnetism. There were other papers by Barlow and Airy, as well as a considerable number of other contributors to this magnetic problem, extending well past the nineteenth century.
Long before this, however, the Admiralty Compass Committee decided it would be better to choose a good position in the ship, where the ship's magnetism affected the compass the least; and on finding that position, not to correct the compass further but to rely on a deviation table, checked constantly. Flinders had made a similar proposal in his above Appendix II of his book.
The issue of correcting magnetic compasses for deviation was becoming chaotic and shipowners were greatly concerned. In 1855 they set up the Liverpool Compass Committee, which issued its First and Second Reports two years later and its final report in 1861. Although long vertical bars of soft iron were introduced into some ships (that in the aphrodite was twenty-one feet long), the term 'Flinders Bar' was not used in these reports.
A study of Captain H. Bencker's valuable Historical and Bibliographic Index concerning the development and improvement of the Compass (published in The Hydrographic Review, May 1941) shows, with numerous entries in the section on 'Improvements of the Magnetic Compass since 1800', that Flinders' pioneer work on deviation had largely been forgotten; and that it was not recognized again until about 1878, when Sir William Thomson (later Lord Kelvin) introduced the Flinders Bar as we know it today.
To substantiate this claim, I asked Commander W. Edward May, R.N., the author of A History of Marine Navigation (1973) and other works on the magnetic compass, whether he knew when the term 'Flinders Bar' was adopted. Fortunately, the problem interested him and a long correspondence between us ensued. As his professional career was closely associated with the Compass Department of the Admiralty, and as he was himself a well-known authority, I was particularly grateful for his generous willingness to assist me in this matter of some importance in keeping Flinders' name alive.
A large number of reports and papers on the magnetic compass were examined for a reference to the Flinders Bar, without success, until the works of Sir William Thomson were read. Even the celebrated Admiralty Manual for ascertaining and applying the Deviations of the Compass caused by the iron in a Ship, by Captain F. J. Evans, R.N. (later Hydrographer of the Navy) and Archibald Smith, contained no references to Flinders in any of its first five editions (1862, 1863, 1869, 1874 and 1882). In the sixth edition (1893), however, the Flinders Bar was definitely named and discussed credit being given to Sir William Thomson for having introduced it on his patent binnacle, 1878.
In 1874, Archibald Smith died and Thomson wrote an obituary, which led him to become interested in magnetic compasses. On preparing an article for Good Words for 1876, he realized how little he knew about the subject of magnetic compasses; but this lack of knowledge was soon rectified by his inventions during the next few years in his yacht salla rookh, when he perfected his compass and binnacle in 1878 and applied for a patent (No 679 of 1879). Flinders was not mentioned in the specification, the particular reference reading as follows:
For correcting the part of a fore-and-aft component of a ship's magnetism which is due to magnetism induced by the vertical component of the earth's magnetism I have a receptacle ... .fitted to the binnacle, in which a bar of soft iron can be placed.
Moreover, Thomson then described his newly patented and modem binnacle in an article, On Compass Adjustment in Iron Ships, in the Journal of the Royal United Services Institute for 1878; again, however, Flinders' name was not mentioned. In a further article entitled: Terrestrial Magnetism and the Mariner's Compass, in Good Words for 1879, Sir William Thomson at last mentioned the name Flinders Bar, and gave 1878 as the date when he first used the vertical bar of soft iron on his binnacle. In this last article, Thomson stated:
Lastly it has an appliance for fixing on the forward or after side of the binnacle a bar of soft iron to realize conveniently a most important but long strangely neglected correction, given so long ago as 1801 by Captain Flinders, by which the charge of inductive magnetism by the changing vertical component of the terrestrial magnetism force is annulled, when the requisite information for placing the bar properly has been obtained by observation on a voyage between places of sufficiently different vertical force. This last appliance has been very successful in ships of the Peninsular and Oriental and Cape Mail Services. In the Union Steamship Company's ship durban (Captain Warleigh) ,for instance, the first to which it was applied in connection with my compass, an error of 34° growing up in the voyage from England to Algoa Bay, has been corrected by a Flinders Bar attached to the front side of the binnacle, and the ship now goes and comes through that long voyage with no greater changes of compass error than might be experienced in the same time in a ship plying across the Irish Channel.
This was, as far as it can be ascertained, the first printed reference to the Flinders Bar, and possibly the first revival of Flinders' earlier experiments in the magnetism of ships. In the following year, however, Thomson printed a further article in the Journal of the R.U.S.I. for 1880, entitled: Recent Improvements in the Compass with correctors for Iron Ships, in which he again referred to the Flinders Bar, and its use for correcting ship's magnetism.
The Kelvin patented binnacle remained in use - with slight modifications for the standard magnetic compass, for more than 100 years. It was a glowing and useful feature on the bridge of numerous naval and merchant service ships; and with its shining brass container for the Flinders Bar, it was familiar to the seamen of the world. Thus Flinders' name became well known; and the credit for this wide promulgation of his services to safe navigation by the magnetic compass is due solely to that brilliant and innovative scientist. Lord Kelvin of Largs.
The highly polished brass cylinder contained the Flinders Bar of soft iron, varying in length from six to twenty-four inches, according to the amount needed to correct its component of the compass. Today, the Navy use a Flinders Bar greatly reduced in size, owing to the high percentage of non-ferrous metal used in the construction of the modem warships' superstructure.
[The author's wife, Mrs Nan Ingleton, and publisher, Genesis, kindly gave permission for the use of these pages, 411 - 413, from G.C. Ingleton's work, Matthew Flinders: Navigator and Chartmaker. Genesis Publications will soon be placing Geoffrey Ingleton's book on their website: http://www.genesis-publications.com]