80 



MAGNETISM. 



priate formula, the joint operation of the 

 two forces is resolvable, and the posi- 

 tion which the needle should assume 

 from that of magnetism alone is dedu- 

 cible with great precision *. This in- 

 tentional separation of the centres of 

 gravity and suspension is effected by 

 tapping the needle with a fine interior 

 screw, in order that it may receive a 

 fine steel screw, projecting at some dis- 

 tance from the needle, and on which a 

 small brass ball traverses. By this con- 

 trivance, the needle may be deflected 

 from the true dip, in any degree that may 

 be desired, and the terrestrial action, 

 which varies as the sine of the angle of 

 deviation from the line of dip, may be 

 increased in almost any proportion. 



(305.) The following is the descrip- 

 tion of the needle constructed on this 

 principle, which was employed by Cap- 

 tain Sabine in the determinations of the 

 dip, as reported in the work already 

 quoted. This needle was a parallelopi- 

 pedon of eleven inches and a half in 

 length, four-tenths in breadth, and one- 

 twentieth in thickness ; the ends were 

 rounded ; and a line marked on the 

 face of the needle, passed through the 

 centre to the extremities, answering the 

 purpose of an index line. The cylin- 

 drical axis on which the needle revolved 

 was of bell metal, terminated, when it 

 rested on the agate planes, by cylinders 

 of less diameter ; the finer these termi- 

 nations can be made, as long as they 

 do not bend with the weight of the 

 needle, the more accurate will be the 

 oscillations. Small grooves in the 

 thicker part of the axis received the Y's, 

 which raised and lowered the needle on 

 its supports, and insured that the same 

 parts of the axis rested on the planes in 

 each observation. 



A small brass sphere traversed on a 

 steel screw, was inserted in the lower edge 

 of the needle, as nearly as possible in 

 the perpendicular to the index line pass- 

 ing through the axis of motion ; by this 

 mechanism, the centre of gravity of the 

 needle, screw, and sphere may be made 

 to fall more or less below the axis of 

 motion, according as the sphere is 

 screwed at a smaller or greater distance 

 from the needle, and according as 

 spheres of greater or less diameter are 

 employed. The object proposed in thus 

 separating the centres of motion and of 

 gravity, was to give to the needle a force, 

 arising from its own weight, to assist 



* See Captain Sabine's Account of Experiments 

 to determine the figure of the Earth, p. 46?. 



that of magnetism in overcoming the 

 inequalities of the axis ; and thus to 

 cause the needle to return, after oscil- 

 lation, with more certainty to the same 

 point of the divided limb than it would 

 do were the centres strictly coinci- 

 dent. 



(306.) The centres of motion and of 

 gravity not coinciding, the position 

 which the needle assumes, when placed 

 in the magnetic meridian, is not that of 

 the dip ; but the dip is deducible by an 

 easy calculation from observations made 

 with such a needle, according to the fol- 

 lowing directions : 



(307.) If the needle has been carefully 

 made, and the screw inserted truly as 

 described, the centres of motion and of 

 gravity will be disposed as in the lever 

 of a balance, where a right linejoining 

 them will be a perpendicular to the ho- 

 rizontal passing through its extremities, 

 that is, to the index line. This condi- 

 tion is not, indeed, a necessary one ; 

 but it is desirable to secure it, because 

 it shortens the observations, as well 

 as the calculation from whence the dip 

 is deduced. Its fulfilment may be as- 

 certained with great precision, by placing 

 the needle on the agate planes before 

 magnetism is imparted to it, and observ- 

 ing whether it returns to a horizontal 

 direction after oscillation, in each posi- 

 tion of the axis ; if it do not, it may be 

 made to do so at this time with no great 

 trouble. 



(308.) With a needle in which this 

 adjustment can be relied on, two obser- 

 vations made in the magnetic meridian 

 are sufficient for the determination of 

 the dip. The two faces of the needle 

 are in succession turned towards the 

 observer, by reversing the position of 

 the axis on its supports, in such a man- 

 ner that the edge of the needle which is 

 uppermost in the one observation, be- 

 comes lowermost in the other. The 

 angles which the needle makes with the 

 vertical in these two positions being 

 read, the mean of the tangent of those 

 angles is the co-tangent of the dip. 



(309.) But when needles are used in 

 which this previous adjustment has not 

 been made, or when its accuracy cannot 

 be relied on, four observations are re- 

 quired ; two being those that have been 

 already directed; and the other two 

 being similar to them, but made with 

 the poles of the needles reversed. 



Calling, then, the first arcs F and/; 

 and those with the poles reversed G and 

 g, calling the dip , and taking 



