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s 



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o 



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f. 



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t* "^ »' 



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hk 



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Appendix.] 



TERRESTRIAL MAONETIS3r. 



45 



6, Two such detenniaatioDS will generally be found sufficient, bat it 

 the results differ from each other more than 3- or 4' it is desirable to 

 repeat the observations, 



7. On arriving at a new station it is alwavs desirable to 



- .'^ <> r^' 



e the 



needk^ afresh before the observations are comi-enc.-:!.- It is iuaitferent 

 whetla. . an observation is commenced with the end marked A as a North 

 or as a South pole ; bat it is convenient to call that state of the needle in 

 which ^^e end A is a South pole, and the end B a North pole, "poles 

 direct/' and vice versa. 



4- 



Dr. Lloyd has recently suggested a niode of employing this iustrumen 

 for measuring the absolute total force in localities where the inclination 

 approach* ? 90"^, and where the usual method of ascertaining the total 

 force from its horizontal component ceases to be satisfactory. It consists 

 in deflecting the dipping-needle from its natural direction in the line of 

 the dip by a second magnet whose magnetic moment is correctly known, 

 placed at one or more accuratcdy measured distances from ihe centre of 

 the dipping-needle, in a line at right angles to tlie magnetic axis of the 

 needle, and in observing ^.^refully the amount of the deflections thus 

 produced. For this purpose the vernier-plate of the inclinometer is fur- 

 nished with two additional arms perpendicular to those which carry 

 the microscopes and verniers ; one of these arms is prolonged by an 

 additional piece (removable when not in use) carrying a gn -::iac-.d tube 

 in which the magnet is placed, and in which it can be fixed by binding 

 screws, with its centre at different distances from the centre of the 

 dipping-needle and in the same vertical plane with it. The deflectin 

 magnet being placed in the tube, and fixed at a suitable distance from 

 the dipping-needle, with its North pole towards the needle, and the face 

 of the circle towards the East, the microscopes are brought to coincide 

 ^ith the ends of the needle in the new or deflected direction. The 

 deflecting magnet is then also in its required position, namely, at right 

 angles to the line joining the two extremities of the needle. The arcs 

 tre read with the precautions already described in the directions for 

 observing the inclination. The deflecting magnet is then taken out of 

 the tube and replaced at the same distance as before^ but with its South 

 pole towards the dipping-needle, and the arcs are again read. This 

 double process is repeated with the vernier-p- :te turned nearly through 

 a seuiicircle, so that the deflecting magnet is brought on the other side 

 of the dipping-needle to that which it occupied before: the face of the 

 circle is now turned towards the West, and the observations already 

 described are repeated. The arcs thus obtained i^ive four values for the 

 angle of deflection, the arithm.^'tie:^' mean of which is taken as the 

 deflection («), corresponding to the distance (r) in decimals of a foot. 



