56 



MAGNETISM. 



those of Ancient Greece and Rome, to whom, 

 for example, the true course of the Apennines 

 and Pyrenees was never known. 



The magnetic force of our planet reveals it- 

 self on its surface in three classes of phenom- 

 ena, one of which shows the variable intensily 

 of the force, the two others indicate the varia- 

 ble direction in the inclination or dip, and in the 

 horizontal departure, or declination, from the ter- 

 restrial meridian of the place, the aggregate out- 

 ward effect of which may be graphically exhibit- 

 ed by means of three systems of lines, one isody- 

 namical, another isoclinial, a third isogonial ; 

 or lines of equal force, of equal dip, and of 

 equal variation. The distance and relative po- 

 sition of these ever-moved, oscillatingly-pro- 

 gressive curves, do not always remain the 

 same. The total variation or declination of 

 the magnetic needle has not, however, chan- 

 ged appreciably, or at all in certain parts of 

 the earthC"), in the Western Antilles and in 

 Spitzbergen, for example, in the course of a 

 whole century. Even so, the isogonial curves, 

 when, in the course of their secular movement, 

 they have passed from the surface of the sea 

 to a continent or island of considerable magni- 

 tude, are seen to linger long upon it, and then 

 they curve off again in their farther progress. 



These gradual transformations which accom- 

 pany the translation, and in the course of time 

 extend the empire of the Eastern and Western 

 variations so unequally, render it difficult, in 

 the graphic representations that belong to dif- 

 ferent centuries, to discover the transitions and 

 analogies of the forms. Every branch of a 

 curve has its own history ; but this history, 

 among the Western nations, nowhere mounts 

 higher than to.the remarkable epoch, the 13th of 

 September, 1493, when the rediscoverer of the 

 New World recognized a line of no variation, 

 three degrees west from the meridian of Flores, 

 one of the Azores('''^). The whole of Europe, 

 a small portion of Russia alone excepted, has, 

 at the present time, western variation ; whilst, 

 at the end of the 17th century, first in London 

 (1657), and then in Paris (1669), with a differ- 

 ence of twelve years, consequently, despite the 

 short distance between them, the needle point- 

 ed directly to the north pole. In East Russia, 

 to the east of the mouth of the Wolga, of Sar- 

 atow, Nijni-Novogorod and Archangel, the 

 Eastern variation presses in upon us from 

 Asia. Two excellent observers, Hansteen and 

 Ad. Erman, have given us intelligence of the 

 remarkable double curvature of the variation- 

 lines in the wide-spread realms of Northern 

 Asia ; convex towards the pole betwixt Ob- 

 dorsk and Obi and Turuchansk, concave be- 

 twixt lake Baikal and the bay of Ochotsk. In 

 this last part of the earth, in the north-east of 

 Asia, betwixt the Werchojansk mountains, 

 Jakutsk and Northern Corea, the isogonial lines 

 form a remarkable system enclosed within it- 

 self This ovoidal formation("') is more reg- 

 ularly repeated, and on a larger scale, in the 

 South Sea, nearly in the meridian of Pitcairn 

 island and the Marquesas group, betwixt the 

 parallels of 20° N. and 45° S. latitude. One 

 might feel disposed to regard so singular a 

 configuration of self-included, almost concen- 

 tric lines of variation, as the effect of a pecu- 

 liar local constitution of the body of the earth ; 



but should these apparently isolated systems 

 move on in the course of centuries, then, as 

 in all grand natural forces, must some more 

 general cause of the phenomenon be presumed. 



The hourly changes in the variation, depend- 

 ent on the true time, and apparently determin- 

 ed by the sun so long as it is above the horizon 

 of a place, decrease in their angular amount 

 with the magnetic latitude. Near the Equator, 

 in Rawak Island, for example, they are scarce- 

 ly more than from 3 to 4 minutes, whilst in the 

 middle of Europe they amount to from 13 to 14 

 minutes. Now, as the north end of the needle, 

 in the whole of the northern hemisphere, trav- 

 els, on an average, between half-past 8 a.m. and 

 half-past 1 P.M. from east to west; and in th8 

 southern hemisphere the same north end trav- 

 erses from west to east during the same period 

 of time, it has been recently, and with reason, 

 remarked("'), that there must be a region of 

 the earth situated, probably, between the ter 

 restrial and the magnetic equator, in which no 

 horary changes of the variation will be observ- 

 ed. But this fourth curve, that of no-move- 

 ment, or rather of no change in horary varia 

 tion, has not yet been discovered. 



As the points of the earth's surface where 

 the horizontal force disappears, are called mag 

 netic poles, and a greater degree of importance 

 has been attached to these points than belongs 

 to them of right(^'^*), in the same way is thai 

 curve called the magnetic equator upon which 

 the dip of the needle is nothing. The positioD 

 of this line, and its secular variations of form, 

 have been made objects of particular investiga 

 tion in recent times. From the admirable work 

 of Duperrey(^**), who, between the years 1822 

 and 1825, crossed the magnetic equator six 

 times, it appears that the two points in which 

 the line of no dip cuts the terrestrial equator, 

 and so passes from one hemisphere into anoth- 

 er, are so unequally divided, that, in the year 

 1825, the node by the island of St. Thomas, on 

 the west coast of Africa, lay in a direct line 

 188J° from the node in the South Sea by the 

 little Gilbert's Island (nearly in the meridian 

 of the Viti group), in the Southern Pacific. In ^^ 

 the beginning of the present century, at an el- 

 evation of 11,200 feet above the level of the 

 sea, in 70° V S. lat. and 48° 40' W. long., I 

 was enabled astronomically to determine the 

 point at which the Andes betwixt Quito and 

 Lima, in the interior of the New Continent, 

 are crossed by the magnetic equator. From 

 this point, proceeding westward, it lingers in 

 the southern hemisphere, through almost the 

 whole of the South Sea, slowly approaching the 

 terrestrial equator. It first crosses over into 

 the northern hemisphere shortly before it reach- 

 es the Indian Archipelago ; it then just touch- 

 es tlie south point of Asia, and enters the Afri- 

 can continent westward from Socotora, close 

 to the straits of Babelmandel, where it is at its 

 greatest elongation from the terrestrial equa- 

 tor. Traversing the unknown regions of cen- 

 tral Africa in a south-western direction, the 

 I magnetic equator returns, in the gulph of 

 I Guinea, into the southern tropic, and in its 

 j course across the Atlantic separates so far 

 j from the terrestrial equator, that it meets the 

 I coast of Brazil at Os Ilheos, to the north of 

 I Porto Seguro, in 15° S. latitude. Froii^enco 



