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TERKESTUIAL MAGNETISM 





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tvro other magnets are supplied to be used as deflectors. In replacing 

 the needles and deflectors in tlie travelling box, cnre should always 

 bo taken that the poles of each occupy the places marked for them in 

 the box. 



It is desirable to use always the same needle at sea, and to keep it 

 always mountedj clamping it before it is put away for the day 5 but in 

 case of its undergoing any considerable deteriorntion from use or accident, 

 one of the other intensity needles may be substituted for it. 



When changing the needles at a land station, be very careful not to 

 injure the jewels, or the terminations of the axles of the needles ; when a 

 needie is changed it is desirable to hold it chiefly by the grooved wheel ; 

 the piTOt should iirst be put into the outer jewelled hole, and the opposite 

 pivot should be carefully guided into the hole at the back whilst the 

 bi*acket is screwed up. 



With respect to the constant weights, it is dc^'rable that the smallest 

 angle of defitction produced hy any of the weights employed should not 

 be less than 30°. On account of possible instrumental irregularities 

 is usual to employ more constant weights than one, with difiierences be- 

 tween each of half a grain, (as for exaniplp 2 grains, 2^ grains, 3 grains, 



Great care is taken that all the weights which have the same 

 nominal value sh -"rid be equiponderant, but it is desirable if p.\ : Ible to 

 preserve the same identical weights throughout the whole observations 

 of the same relative series. 



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II, — Observations at Sea. 



A. — IncV ation, 



1. Direct Observation, — The instrun: ^t having been placed on the 

 gimball stand and levelled, and the plane of the circle made parallel to 

 the magnetic meridian, with the face of the circle towards the East, 

 release the needle, which will immediately take approximately die direc- 

 tion of the inclination ; rub gently the centre pin at the back wdth th 



ivory disk, and read otf successively the divisions of the limb indicated 

 by the two ends of the needle t note the readings, which will be +, or 

 positive, when the North pole of the needle dips, and — , or negative, 

 when the South pole of the needle dips : repeat the observation three 

 times, turning the bracket which supports the needle a small quantity be- 

 tween each observation, and being careful to rub the centre pin at the 

 back with the ivory disk whilst reading off. The bracket is turned by 

 means of the screw heads at the back of the circle, and the object of turn- 

 ing it is to cause the ends of the axle of the needle to hare different points 

 of bearing on the jewels in each obsen/: 



In reading the divisions on the limb, be careful always to bring the 



^on. 



