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MAG N ETIC NE ED LE M AGN ETISM. 



salt. For a notice of Boratv of magnesia, see 

 Boracic Acid. 



MAGNETIC NEEDLE is a needle touched with 

 a loadstone, and sustained on a pivot or centre, on 

 which, playing at liberty, it directs itself to certain 

 points in or uiuler the horizon. See Magnetism, 

 aiul Compass. 



MAGNETISM. According to ancient tradition, 

 a shepherd named Magnes,'m walking up mount Ida, 

 found that certain stones were attracted by his iron 

 hook, and from the name of the discoverer these 

 stones were afterwards called magnets. Others assert 

 tiiat these stones derived the name magnet from hav- 

 ing been first discovered at Magnesia, a town in 

 Heracles. It is not our business to inquire whether 

 either of these traditions be true ; it is sufficient for 

 us to know that a peculiar species of iron ore, known 

 to us by the names natural magnet, or natural load- 

 stone, was known to the Greeks, and called by them 

 magnet. More recent research has shown that the 

 properties of the natural magnet (see Iron, ores of) 

 may be communicated to other substances, or that 

 artificial magnets may be formed, and the science 

 which explains the laws of the action of magnets, 

 whether natural or artificial, is called magnetism. 



History. The property of the natural magnet of 

 attracting iron seems to have been the amount of 

 the knowledge of the ancients on the subject. Yet 

 some suppose that they were also acquainted \vith the 

 formation of artificial loadstones. The most impor- 

 tant property of the magnet remained unobserved 

 until about the middle of the thirteenth century ; we 

 allude to the property of the magnet's taking a cer- 

 tain direction when allowed to move freely on an axis. 

 The discovery of the fact that a magnet when bal- 

 anced upon a centre will dispose itself in a direction 

 north and south, a discovery which has contributed 

 more than all the inventions yet known to the exten- 

 sion of knowledge and the diffusion of happiness over 

 the globe, is attributed to different individuals : the 

 invention of the mariners' compass has by some been 

 ascribed to Flavio, a Neapolitan ; but Dr Gilbert 

 supposes that the invention was brought to Europe 

 from China by Paulus Venetus, an Italian. It is 

 matter of deep regret that any uncertainty should 

 exist respecting the name of the man who has con- 

 ferred so great a benefit on the human race. From 

 a manuscript letter of R. Adsign, dated 1269, still 

 preserved in the collection of the University of 

 Leyden, it would appear that he was acquainted 

 with the principal properties of the magnet. In this 

 MS. we find the first notice of the declination or 

 variation of the needle, i. e. that the suspended mag- 

 net does not always point duly north and south : this 

 discovery has been erroneously attributed to Colum- 

 bus, whose voyage was made nearly 200 years 

 subsequent to the date of the Leyden manuscript. 

 The next important discovery was that the needle 

 though balanced with the utmost care before it was 

 magnetised, departed from the level after it had re- 

 ceived the magnetic influence. This phenomenon, 

 commonly called the dip of the needle, was first 

 observed and made known by Norman, a mathemat- 

 ical instrument-maker, near London, who wrote a 

 work on The New Attraction. The variation of 

 the needle was not at this time known to be change- 

 able, but Mr Bond in 1640, who wrote fifty-nine years 

 after Norman, clearly points it out as not only ob- 

 served by himself, but also by Mair, Gunter, and 

 Gellibrand. 



No attempt was made to form a theory of magne- 

 tism before the time of Dr Gilbert, queen Elizabeth's 

 physician, who in 1600 published his work de Mag- 

 ncte, a work that although it contains many fanciful 

 hypotheses, contains a statement of many important 



facts, and forms an important era in the history of 

 experimental science. Halley published a theory of 

 magnetism in 168:5, and in 1698, the command of 

 one of the ships of the royal navy was given him, in 

 order to discover the rule for the variation of the 

 compass. An account of this voyage was published 

 in 1701, together with a chart of the variation. The 

 continual inverting of the needle was now established, 

 although the law of the variation was not yet deter- 

 mined. More minute observations made by Mi- 

 Graham at London, in 1722, and subsequently, 

 showed that the variation hitherto observed was not 

 the only change in the direction of the needle, that 

 besides its tendency westward, it had a daily varia- 

 tion ; he observed that the needle made each day an 

 oscillation, the greatest variation taking place be- 

 tween ten and four o'clock in the day, and the least 

 between six and seven in the evening. About the 

 middle of the eighteenth century, experiments on the 

 same subject were made by Wargentin, a Swede, and 

 also by Canton, an English philosopher, which last 

 determined that the diurnal variation is less in winter 

 than in summer. Mr Canton makes the daily var- 

 iation for January 7' 8", and for June 13' 2''. Recent 

 observation establishes the fact, but the amount of 

 the variation suffers a change ; for we find the diurnal 

 variation for January 1819, to be 6' 3", and for June 

 11' 15". About 1750, professor Wargentin observed 

 that the aurora borealis had a peculiar effect upon 

 the magnetic needle, and more recently during one 

 of Captain Cook's voyages, the iron in the ship was 

 found to affect the direction of the magnet in a very 

 remarkable degree ; so much so, indeed, as to render 

 some remedy necessary, especially in high latitudes. 



The method of magnetising steel bars was not 

 properly known till about the year 1750 ; the discov- 

 ery being due to Dr Knight. Du Hamel, Canton, 

 and others improved upon his plan. Lambert of 

 Berlin was the first who discovered the law of mag- 

 netic attraction, viz. the magnetic influence decreases 

 from the magnetised object as the squares of the 

 distances increase, and Coulomb put the truth of 

 the law beyond doubt by his experiments with the 

 Torsion balance. Mr Peter Barlow, of the royal 

 military academy, from a series of well instituted 

 experiments, added much to the knowledge we pos- 

 sess of magnetic action. He established the impor- 

 tant law that the magnetic action increases with the 

 surface, not with the mass, and that if any body, as a 

 ball of iron, influence a compass needle, the squares 

 of the tangents of deviation will be proportional to 

 the cubes of the attractive forces. He also experi- 

 mented on the effect of the addition or abstraction of 

 heat on the magnetic power. Mr Bonnycastle fol- 

 lowed up these results by a beautiful analytical in- 

 vestigation which made its appearance in the ninth 

 volume of the Philosophical Magazine. The re- 

 searches of Biot, Sir H. Davy, Oersted, Mr Arago, 

 and more recently of Mr Faraday, have been directed 

 to the advancement of this highly interesting science, 

 but the plan of this work will only admit of our enter- 

 ing into minor details ; we shall therefore content 

 ourselves wiiii a brief notice of what has been done 

 by the. last mentioned philosopher. 



Mr Faraday, from recent experiments, considers 

 magnetism common to all metals. In a lecture 

 lately delivered at the royal institution, he showed 

 the distribution of the magnetic power at a heat of 

 250 ; and that there was a temporary loss of all 

 its .nagnetic power, when the metal was brought to a 

 red heat. In order to demonstrate this, he put a 

 magnet into an iron tube, by which means it lost all 

 magnetic power, excepting within the tube. He 

 then inserted it into a red hot tube, until the magnet 

 acquired sufficient heat to deprive it of its powers. 



