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Appendix.] 



TERRESTRIAL MAG XETIS3I. 



AP, 



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APPENDIX No. 3 



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OBSrBTATIONS OF THE InCT.IM^TION, ^^VITH BaIIKO%V^S CiECLE FURNISHEB 



WITH Mtchoscopks ANB VEaNISIlS. 



1. Place the instrument on a tripod stand, and level it by mcun^ of the 



.ridian bv 



f jot-screws : then bring the ycrtical circle into the masrnetic n 



the following process :~PIace the needle designed for the observation 

 of the dip on the agate supports, -with the side of the needle on which 

 the letters are inscribed facing the microscopes. Turn the vernier plate 

 so that the microscopes mdj be nearly in a yertical line; clamp the 

 plate, and set the lower vernier to 90'^ by the tangent screw. Turn the 



d 



3 



vertical circle in aximuth^ so that its face may be to^rards the Souths an 

 until the North pole of the needle is bisected by the wire of the microsc . .e 

 raise the Ys and lower gently ; if the bisection of the needle has been 

 altered, correct by turning the circle in azimuth. Clamp the horizontal 

 circle, and read off its yr-rnier, calling the reading A. Now set the 

 upper vernier to 90^, imclamp the horizontal circle, and roovc in azi- 

 muth (if required) until the South pole of the needle is bisected bj C.^ 



i. Lve 



of the upper microscope. Raise the Ys and lower gently; correct the 

 bisection (if necessary) by moving the circle in azimuth : clamp the 

 liorizontal circle and read its veraierj calling? the reading B. Now 



0^^ 



unclanip the horizontal circle, and turn the vertical circle IS 

 azimuth, so that its face (by which is meant the side on which the mi- 

 croscopes are) which was before to the South may now be to the North. 

 Repeat the process described above, which will give two other readings 

 of the vernier of the horizontal circle, which call C and D. Then 



A + B + C -i- D 



4 



E. 



will coincide with the magnetic me 



the division of the hor' -nital circle to which the vernier should be 

 set, in order that the plane of the vertical circle may be at 7-ii/ht angles 

 to the magnetic m— idian ; theretore, when the vernier is set to 90 ±E. 

 the plane of the vertical circle 

 ridian. 



The vertical circle being now placed in the magnetic meridian, 

 with its face to the East, the needle will direct itself approximately to 

 the inclinati<m ; raise it bj the Ys and lower it gently on itb supports - 

 bring the lower microscope to bisect the North end of the nei-dle, clamp 

 and adjust exactly by the tangent screw, read off the vernier, which will 

 be Face East^ North EncL By means of the tangent screw of the vernier- 

 plate bring the upper microscope to bisect the South end of the needle, 



rirl 



