MAGNETISM. 



57 



to the lofty plains of the Cordilleras, betwixt 

 the silver mines of Micuipampa and the old 

 seat of the Incas, Caxamarca, where I had an 

 opportunity of observing th'(? inclination, it trav- 

 erses the whole of South America, which, in 

 these southern latitudes, like the interior of 

 Africa, remains a magnetic terra incognita up 

 to the present time. 



Late observations collected by Colonel Sa- 

 bine("*), inform us that the node of the Island 

 of St. Thomas has travelled four degrees, from 

 east to west, between 1825 and 1837. It would 

 be of the highest importance to know whether 

 the opposite node of Gilbert's Island, in the 

 South Pacific, had not travelled as far west- 

 ward, towards the meridian of the Carolinas. 

 The general survey now given must suffice to 

 connect the different systems of not perfectly 

 parallel isoclinal lines with the great phenome- 

 non of equilibrium which manifests itself in the 

 magnetic equator. It is no small advantage 

 for the establishment of the laws of terrestrial 

 magnetism, that the magnetic equator, whose 

 fluctuating alterations of form, and whose nodal 

 motion in the midst of the various magnetic lat- 

 itudes, exert an influence("') upon the dip of 

 the needle in the remotest countries of the 

 world, is, with the exception of one-fifth, whol- 

 ly oceanic ; it is therefore, through the remark- 

 able relations betwixt the sea and the land, by 

 so much the more accessible, as we are now 

 in possession of a means of determining both 

 variation and dip, with great accuracy, on ship- 

 board, whilst the vessel is holding her course. 



We have now portrayed the distribution of 

 magnetism upon the surface of our planet, ac- 

 cording to the two forms of variation and dip. 

 The third form, that of intensity of the force, 

 still remains, and this is graphically expressed 

 by isodynamic curves (lines of equal intensity). 

 The investigation and measurement of this 

 force, in its terrestrial relations, by the oscilla- 

 tions of a vertical or horizontal needle, have 

 only excited general and lively interest since 

 the beginning of the nineteenth century. The 

 measurement of the horizontal force has been 

 made capable of a degree of accuracy, particu- 

 larly by the application of delicate optical and 

 chronometrical instruments, which far exceeds 

 that of all the other magnetical determinations. 

 If, with reference to the immediate application 

 to navigation and steering, the isogonal lines 

 be the more important, the isodynamic, espe- 

 cially those that indicate the horizontal force, 

 present themselves, according to the most re- 

 cent views, as those which promise the richest 

 harvest for the theory of terrestrial jnagnet- 

 ism(i28) One of the earliest facts discovered 

 by observation, was this : that the intensity of 

 the sum of the force increases from the equa- 

 tor towards the pole("'). 



For a knowledge of the measure of this in- 

 crease, and the establishment of all numerical 

 relations of the law of intensity, embracing the 

 whole earth, we are especially indebted to the 

 ceaseless activity of Colonel Sabine, who, ever 

 since the year 1819, after he had made obser- 

 vations on the same needle oscillating at the 

 American north pole, in Greenland, in Spitz- 

 bergen, on the coast of Guinea, and in the Bra- 

 zils, has been incessantly engaged m collecting 

 and arranging whatever may Borve to illustrate 

 H 



the direction of the isodynamic lines. I have 

 myself given the first plan of an isodynamical 

 system, divided into zones, for a small part of 

 South America. These isodynamic lines are 

 not parallel to the lines of equal dip ; the in- 

 tensity of the force is not, as was at first be- 

 lieved, weakest at the magnetic equator ; it is 

 not once equal at any part of the same. If 

 Erman's observations in the southern portion 

 of the Atlantic, where a zone of declining in- 

 tensity runs from Angola, over the island of 

 St. Helena, to the coast of Brazil (0-706), be 

 compared with the very latest observations of 

 that distinguished navigator Sir James Clark 

 Ross, it is found that the force upon the surface 

 of our planet increases nearly in the ratio of one 

 to three towards the magnetic south pole, and 

 where Victoria Land stretches away from Cape 

 Crozier towards Mount Erebus, that volcano 

 which rises from everlasting ice to the height 

 of 11,600 feet above the level of the sea("''). If 

 the intensity in the vicinity of the magnetic 

 south pole be expressed by 2052 ( — the inten- 

 sity which I found on the magnetic equator in 

 North Peru is still assumed as unity, or 1 000), 

 Sabine found it, in Melville Island, 24° 27' N. 

 lat., near the magnetic north pole, only 1-624 ; 

 whilst, in the United States, near New- York — 

 nearly under the same parallel of latitude as 

 Naples, consequently — it was 1-803. 



Through the brilliant discoveries of Oersted, 

 Arago, and Faraday, the electrical charge of 

 the atmosphere has been brought to approxi- 

 mate more closely to the magnetical charge of 

 the earth. If Oersted found that electricity in- 

 duced magnetism in the vicinity of the body 

 which was conducting it, so, on the other hand, 

 it was shown in Faraday's experiments that 

 free magnetism gave rise to electricity. Mag- 

 netism is one of the numerous forms in which 

 electricity manifests itself The ancient sus- 

 picion of the identity of electrical and magnet- 

 ical attraction has been demonstrated in the 

 present age. " If electrum" (amber), says 

 Pliny (^^^), in the sense of the Ionic natural 

 philosophy of Thales, " becomes inspired by 

 friction and warmth, it attracts bark and dried 

 leaves, exactly like the magnetic iron stone." 

 The same words occur in the literature of a 

 people inhabiting the easternmost parts of Asia, 

 in the discourse, laudatory of the magnet, of 

 the Chinese natural philosopher, Kuopho(^"). 

 It was not without surprise that I myself ob- 

 served, among the children at play on the woody 

 banks of the Orinoco, the offspring of native 

 tribes in the lowest grade of civilization, that 

 the excitement of electricity by friction was 

 known. The boys rubbed the dry, flat, and 

 shining seeds of a creeping leguminous plant 

 (probably a negretia), until they attracted fibres 

 of cotton wool and chips of the bamboo. 1 his 

 amusement of these coppery children is calcu- 

 lated to leave a deep and solemn impression be- 

 hind it. What a chasm lies between the elec- 

 trical play of these savages, and the discovery 

 of the lightning conductor, of the chemically 

 decompounding pile, of the light-evolving mag- 

 netical apparatus ! In such gulphs, millenni- 

 ums in the history of the intellectual progress 

 of mankind lie buried ! 



The ceaseless change, the fluctuating move- 

 ments which are observed in all magnetical 



