MAGNETISM. 



tans:. F-j-tancr./= A 

 tans. F - tanir./ = R 

 tang. G -f- tan:?. g 1 = C 



r. G - tang.# = D 

 Then the dip may be calculated by the 

 following formula: 

 A . D , B . C 



B+3 + B + D =2cotangtS ' 



(310.) In reversing the poles, it is not 

 necessary that the magnetic force im- 

 parted to the needle should be the same 

 in degree as it possessed previously to 

 the operation. The coincidence of the 

 poles with the extremities of the longi- 

 tudinal axis may always be insured by 

 adopting the precaution of placing the 

 needle in a groove, to prevent its lateral 

 motion, and by confining the sides of 

 the magnet by parallel strips of wood, 

 so that in moving along the needle they 

 may preserve its direction. 



(311.) If the distance between the 

 centres of motion and of gravity be con- 

 siderable, the arcs in the alternate ob- 

 servations will be on different sides of 

 the vertical, especially when the dip is 

 great; in such cases the arcs to the 

 south of the vertical are read nega- 

 tively. The arcs in each of the four 

 positions, forming the data from which 

 the dip is deduced, are the arithmetical 

 means of several observations, usually 

 six, half of which should be made with 

 the face towards the east, and half with 

 the face towards the west; the needle 

 being lifted by the Y's and lowered gently 

 on its supports between each observa- 

 tion. The arcs indicated by both ends of 

 the needle should also be read, in order to 

 correct the errors arising from inequa- 

 lity in the divisions, or from the axis of 

 the needle not passing correctly through 

 the centre of the circle. 



(312.) In order to insure the perfect 

 horizontality of the agate planes which 

 supported the axis of the dipping-needle 

 on Mayer's construction, employed by 

 Captain Sabine, a spirit level was at- 

 tached to a circular brass plate, of the 

 proper diameter to be placed upon the 

 planes themselves, with adjustments to 

 bring it parallel to the plate. The 

 errors of the level were shown by placing 

 the plate in various positions horizon- 

 tally ; and the errors of the planes by 

 turning the whole instrument upon its 

 horizontal centre. When these errors 

 were adjusted, and the planes and plate 

 perfectly horizontal, the apices of two 

 cones, which proceeded perfectly at 

 right angles from the plate uniting them 

 at their base, and were equal to the dia- 



meter of the divided circle of the instru- 

 ment, ought to have coincided with the 

 divisions 90 and 90 of the circle; 

 when they did not, the cones afforded, 

 in this case also, the means of correct- 

 ing the adjustment. 



(313.) The dipping-needle affords a 

 method of determining the position of 

 the magnetic meridian, independently of 

 the horizontal needle; for if we turn round 

 the whole instrument horizontally (so as 

 to place it successively in different azi- 

 muths), till we find that in which the 

 needle assumes an exactly vertical po- 

 sition, the plane of its motion is then ex- 

 actly at right angles to the magnetic 

 meridian ; and the latter may therefore 

 be determined from the former. 



(314.) By comparing the inclination 

 of the dipping-needle to the horizon, in 

 two different positions, such that the 

 planes of its rotation are perpendicular 

 to each other, we may, by the following 

 trigonometrical formula, deduce the dip. 

 If the inclinations, observed in the two 

 azimuths, be represented respectively by 

 $' and $", and the dip itself (or the incli- 

 nation in the magnetic meridian) by , 

 then, 



Cot. 2 $=cot.*S' + cot.*S" 

 By multiplying observations of this kind 

 in different azimuths, and taking the 

 mean of all, we may arrive at a very 

 accurate determination of the dip. 



(315.) Mr. Scoresby has proposed an 

 ingenious method of finding the dip, by 

 observing the situation in which bar-iron, 

 void of permanent magnetism, loses all 

 power of affecting the compass placed 

 at a certain distance from it ; for, as 

 Mr. Barlow has ascertained, its position 

 must then be in the plane of the mag- 

 netic equator. The inclination of the 

 plane to the horizon is, of course, equal 

 to the complement of the dip. Mr. 

 Scoresby has described an instrument 

 calculated for making this species of 

 observation, in the Transactions of the 

 Royal Society of Edinburgh*. 



(316.) Other methods, of a nature 

 somewhat more refined, exist for dis- 

 covering the dip, which depend on the 

 admeasurement of the intensities of the 

 magnetic forces by which the needle is 

 urged in different positions of the axis 

 and plane of rotation. The magnetic 

 force derived from the influence of the 

 earth, and acting in the direction of the 

 dip, may be resolved into other forces, 

 which will bear to one another the same 

 ratios as the sides of the triangles 

 * Vol. ix, p, 247. 



