TERRESTRIAL MAGNETISM. 



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place on the stone, and draw another line in the same way. 

 This may be repeated with the needle in other positions. All 

 of the lines thus drawn will run together in two points, just as 

 all the meridian circles of the world run together in two opposite 

 poles of the world.'' The direction of the magnetic force, how- 

 ever, is not to be supposed to lie along the surface of the magnet. 

 Indeed, Peregrinus observed that a small needle stands perpen- 

 dicularly to the surface of the stone at the poles, where the 

 magnetic force also is strongest. 



A simple way of illustrating the distribution of magnetic force, 

 not only on the surface but also in the space surrounding a 

 magnet, is that of sprinkling iron filings over a sheet of paper 

 or glass laid over or in the neighbourhood of the body. The 

 little pieces of iron themselves become temporary magnets 

 owing to the magnetic force surrounding the stone, and if the 

 paper is lightly tapped they arrange themselves end to end so 

 as to make the directions of the lines of magnetic force clearly 

 apparent. 



By analogy with a spherical lodestone such as Peregrinus 

 used, our illustrious countryman, William Gilbert, Physician 

 to Queen Elizabeth, was led to the conclusion that the earth 

 itself is a great magnet. In his famous treatise De Magnete 

 (1600) he describes the earth as being a great round lodestone, 

 the magnetic poles of which were supposed to be coincident 

 with the poles of rotation. Some years earlier, in 1581, Robert 

 Norman, an instrument maker of London, and formerly a seaman, 

 had announced that, as in Peregrinus' lodestone, the magnetic 

 force of the earth does not lie along its surface, i.e., it is not 

 horizontal, but has also a vertical component. Ordinary 

 compass needles are pivoted and weighted so as to swing 

 horizontally, but if perfectly balanced before magnetization, 

 after they are magnetized they will exhibit a tendency to dip ; 

 in the northern hemisphere the north-seeking, and in the southern 

 hemisphere the south-seeking end is the one which dips. 

 The natural angle of the dip in London is in round figures 

 65° measured from the horizontal. This was discovered 

 by Norman in 1576, and is a salient feature in the analogy 

 between the earth and a spherical lodestone. Norman 

 himself undoubtedly had some inkling of the idea which 

 Gilbert afterwards clearly stated ; he perceived that the dipping 

 of the needle indicates that the point or source of force which 

 the needle " respects " is in the earth and not in the heavens. 



