24 



MAGNETISM. 



adapted for accurate measurement. 

 For this purpose we must have recourse 

 to other contrivances. 



(97.) The magnetic force may, by the 

 ordinary dynamic method of the resolu- 

 tion of forces, be resolved into two 

 forces, the one acting vertically, the other 

 horizontally. The latter of these forces, 

 namely, the horizontal force, is the only 

 one with the action of which gravitation 

 does not interfere ; and accordingly, the 

 mariner's compass indicates by its mo- 

 tions, the effects of this part of the ter- 

 restrial magnetic force, and this only. 

 In order to ascertain the vertical force, 

 we must proceed in a different manner. 

 The needle must be furnished with an 

 axis, at right angles to its length, and 

 adjusted very carefully, so that it may 

 pass as exactly as possible through its 

 centre of gravity. This, of course, can 

 only be done when the needle is wholly 

 free from magnetism, and secured, in the 

 manner hereafter to be pointed out, from 

 all magnetic influence which the earth 

 might exert upon it. The axes should be 

 supported horizontally in such a man- 

 ner as to allow the needle complete free- 

 dom of motion in a vertical plane. The 

 needle being thus balanced, will have no 

 tendency to incline to one side rather 

 than to another, and will remain at rest 

 in any position in which it may happen 

 to be left, as long as no extraneous force 

 is applied to it. When this has been 

 accomplished, the needle is to be mag- 

 netized, by the methods hereafter to be 

 described, as strongly as possible, and it 

 is then to be replaced on its supports, 

 which are to be turned so that the plane 

 in which the needle is allowed to move, 

 may coincide with that of the magnetic 

 meridian. It will be found that, in this 

 situation, the end of the needle to which 

 a northern polarity has been imparted, 

 will preponderate, or dip, as it is called, 

 and after a certain number of oscilla- 

 tions, will settle at a determinate point. 

 The line which its axis assumes under 

 these circumstances, is termed the mag- 

 netical direction, or position. The dip of 

 the needle was first observed by Norman. 



(98.) The inclination of the needle, or 

 dip, like the variation, differs in differ- 

 ent parts of the globe. The latest accu- 

 rate observation of the dip in London, 

 of which we have any record, is that of 

 Captain Sabine, who ascertained it, in 

 August, 1828, to be 69 47'.* Asa 



* Philosophical Transactions for 1829. Since the 

 ove W as written, \ve are informed that the dip 



38' at e \v a ? c " r i ained b y Ca Pt- Segelcke to be 69* 

 PS at Woolwich, in Nov. 1830 



general rule, to which, however, there 

 are many exceptions, the dip diminishes 

 as we approach the equator, and in- 

 creases as we recede from it on either 

 side. Towards the polar regions it is 

 very great, and as we come near to the 

 poles, it approaches to a right angle. At 

 the magnetic poles themselves, the 

 dipping needle would, of course, be ex- 

 actly perpendicular to the horizon. 

 Those places on the earth where the 

 needle is perfectly horizontal, that is, 

 where there is no dip, are in a line that 

 encircles the globe, and is termed the 

 magnetic equator. 



(99.) As the magnetic poles are not 

 situated exactly at the poles of the earth's 

 relation, but at some little distance from 

 them; so, the magnetic equator does 

 not coincide with that of the earth ; 

 though it does not in any part deviate 

 widely from it. In a general way we 

 may consider it as a great circle of the 

 globe inclined to the terrestrial equator 

 at an angle of about 12 degrees ; its in- 

 tersections with it being situated at the 

 longitudes 113 14' west, and 66 46' 

 east from the meridian of Greenwich. 

 Such, at least, is the result given by all 

 the observations made for an extent of 

 more than one half of its circuit, in the 

 Atlantic and Indian Oceans, and that 

 part of the Pacific which is nearest to 

 the South American continent, as ap- 

 pears from a table of these observations 

 given by Biot*. But a remarkable ano- 

 maly is met with when we trace the 

 course of the magnetic equator across 

 the Pacific Ocean. This line is found 

 in the southern hemisphere in the 

 American continent, and joins the equa- 

 tor as before-mentioned, at a longitude 

 of about 113; but still further to the 

 westward, at longitude 156 30' it is 

 again met with at a distance from the 

 equator and to the south of it. In the 

 Sea of China at 116 east longitude, it is 

 found to the north of the equator, 

 which it must therefore have crossed at 

 some intermediate point ; and it is again 

 inflected towards the south, so as to tra- 

 verse the equator at the eastern node 

 already mentioned. 



It appears, therefore, from these ob- 

 'servations, that there are at least three 

 points in the terrestrial equator where the 

 magnetic equator coincides with it ; and 

 the probability is, that there are four : 

 because, if the latter curve passes to the 

 northern side of the equator at its western 

 coincidence, it must again cross it before 



* Traite de Physique, tome III. p. 130. 



