MAGNETISM. 



23 



declination; but it is more usually known 

 by the term Variation of the Compass. 

 The vertical plane which passes through 

 the direction of the horizontal needle at 

 any particular place is termed the mag- 

 netic mpri'iian of that place, in contra- 

 distinction to the geographical or true 

 meridian, which is a vertical plane pass- 

 ing through the poles of the earth. 



(93.) There are but few places on the 

 earth where the compass points directly 

 to the poles ; that is, where it exhibits 

 no variation. As far as observation 

 has extended, these places are situated 

 in a line which encompasses the globe, 

 and is called the line of no variation. 

 In many of its portions it appears to 

 form part of a great circle of the sphere, 

 but in others it deviates much from re- 

 gularity, presenting many flexures in its 

 course. It may be considered as com- 

 mencing from a point which may be 

 designated as the principal arctic mag- 

 netic pole of the earth, and the exact 

 situation of which is not yet perfectly 

 ascertained, although the late voyages 

 of discovery in these regions have en- 

 abled us to form a tolerable approxima- 

 tion to the precise spot, which appears 

 to be a point somewhere to the west- 

 ward of Baffin's Bay. After crossing 

 the United States of North America it 

 passes along a tract of the Atlantic, a 

 little to the eastward of the windward 

 West India Islands, till it touches the 

 north-eastern point of the South Ame- 

 rican continent. Thence it stretches 

 across the Southern Atlantic towards 

 the south pole, where navigators are 

 unable to follow it. It re-appears in the 

 eastern hemisphere to the south of Van 

 Dieman's Land, and passing across the 

 western part of the Australian continent, 

 is again found in the Indian Archipe- 

 lago. Here, according to Biot, it di- 

 vides into two branches, one of which 

 crosses the Indian Sea and enters Asia 

 at Cape Comorin ; it then traverses Hin- 

 dostan and Persia, and passing through 

 the western part of Siberia stretches over 

 to Lapland and the Northern Sea. The 

 second branch pursuing a more directly 

 northern course, traverses China and 

 Chinese Tartary, and makes its exit from 

 Asia in the eastern division of Siberia, 

 where we again lose it in the Arctic seas. 

 Between these there must exist an inter- 

 mediate line of no variation in some 

 part of the continent of Asia ; but the 

 observations we possess regarding it are, 

 as yet, too imperfect to admit of any 

 attempt to trace it correctly. 



(94.) If we consider these Asiatic 



lines of no variation as composing a 

 single band, we may then consider the 

 globe as divided by this and the corre- 

 sponding American line into two hemi- 

 spheres. In that hemisphere which 

 comprehends Europe, Africa, and the 

 western parts of Asia, together with the 

 greater portion of the Atlantic, the va- 

 riation is to the west. In the oppo- 

 site hemisphere, which comprises nearly 

 the whole of the American continents, 

 both North and South, and the entire 

 Pacific Ocean, together with a certain, 

 portion of Eastern Asia, the variation is 

 to the east. 



2. Dip of the Magnetic Needle. 



(95.) But in order to arrive at a 

 knowledge of the real influence which 

 the earth exerts on a magnetic needle, it 

 is not sufficient to ascertain the position 

 it assumes when its movements are con- 

 fined to a horizontal plane, as it is in the 

 mariner's compass of the ordinary con- 

 struction : we must place it in such cir- 

 cumstances as will allow it to move 

 freely in a vertical plane also. But to 

 effect this in an unexceptionable man- 

 ner is extremely difficult. The great ob- 

 stacle with which we have to contend is 

 the force of gravity, which by acting in 

 one direction, interferes with the opera- 

 tion of the force of terrestrial magnetism, 

 which acts in a different and in an ob- 

 lique direction. 



(96.) The readiest mode of removing the 

 influence of gravity, is to affix a steel nee- 

 dle to a cork, or other buoyant substance, 

 and to immerse it in water, adjusting the 

 specific gravity of the two bodies, so that 

 they may remain suspended in the middle 

 of the fluid without any tendency either 

 to float or to sink ; taking care at the 

 same time that the centre of gravity of 

 the whole coincides with the centre of 

 its figure, so that, when the needle is 

 unmagnetic, and united to the cork, the 

 two together, placed in any position in 

 the fluid, shall have no tendency to take 

 any other position. If the needle be 

 now rendered magnetic, and replaced as 

 before, it is found to assume a position 

 nearly vertical, that is, making an angle 

 with the plumb line of about 20 degrees, 

 the north pole of the needle being turned 

 about 25 degrees to the westward of the 

 true north. Its deviation from the plane 

 of the meridian is equal to the variation 

 of the horizontal needle. Its inclination 

 to the horizontal plane, or 70, is called 

 the dip. But this method, though well 

 fitted for illustrating the general fact, and 

 the principle on which it depends, is not 



