Introduction. 



Xlll 



Table 2. — ^Value of the Divisions of the Long Scale in the Declinometer during 1842. 



Scale 



Theodolite 



Scale 



Theodolite 



Value of 100 



Division. 



Reading. 



Division. 



Reading. 



Scale Divisions. 



140 



4 51 300 



240 



3 44 22-5 



i 7 7-5 



150 



4 44 47.5 



250 



3 37 400 



1 7 7-5 



160 



4 38 50 



260 



3 30 57-5 



1 7 7-5 



170 



4 31 20-0 



270 



3 24 150 



1 7 50 



180 



4 24 37-5 



280 



3 17 32-5 



1 7 50 



190 



4 17 550 



290 



3 10 500 



1 7 5-0 



200 



4 11 12-5 



300 



3 4 7-5 



1 7 50 



210 



4 4 30-0 



310 



2 57 250 



1 7 50 



220 



3 57 47-5 



320 



2 50 400 



1 7 7-5 



230 



3 51 5-0 



330 



2 44 00 



1 7 50 



240 



3 44 22-5 



340 



2 37 17-5 



1 7 50 



250 



3 37 40-0 



350 



2 30 35-0 



1 7 50 



260 



3 30 57-5 



360 



2 23 50-0 



1 7 7-5 



The mean value of 1 Scale division is 0''6710. 



11. Determination of the point of the scale at the magnetic axis of the bar, or 

 the zero of the scale. 



June 28. 1841. Magnet with short scale. 



The torsion having been removed from the suspension thread, and the Reading 

 Telescope fixed temporarily, readings were made, the magnet being direct and also 

 inverted (the scale and lens below the magnet). 



Table 3. — ^Reading of the Short Scale for the Magnetic Axis of the Declination Magnet. 



Position of 

 Magnet. 



Scale Reading. 



Mean of each 

 alternate two. 



Reading for 

 Magnetic Axis. 



Direct 



140-3 







Inverted 



159-1 



139-9 



149-50 



Direct 



139-6 



159-3 



149-45 



Inverted 



159-5 



139-1 



149-30 



Direct 



138-5 



159-9 



149-20 



Inverted 



160-3 



137-8 



149-05 



Direct 



1371 







The mean reading for magnetic axis = 149*3 Sc. div. 



July 12 till 26. 1841. Effect of Bifilar and Balance Magnets (See Table 5), 

 + 0-3 Sc. div., corrected zero 149*0 Sc. div. 



July 27 till Dec. 28. 1841. Effect of Bifilar and Balance Magnets (See Table 5), 

 —0*3 Sc. div., corrected zero 149.6 Sc. div. 



MAG. AND MET. OBS., VOL. I. d 



