Unifilae Magnetometer. 



XXlll 



Table 6. — Determinations of the Value of Angle A. 



Date. 



Readings of Horizontal Circle 



Angle A. 







For Declination 



Telescope. 



For North Mark. 



Verniers. 



Mean. 



Verniers. 



Mean. 



A. 



B. 



C. 



A. 



B. 



C. 



1843. 

 Jan. 18 



/ 



52 



n 



50 



/ ti 

 52 100 



52 



It 

 55-0 



O 1 II 



233 52 23-3 



/ // 



42 52-5 



42 



17-5 



42 



n 



32-5 







77 



42 34-2 



o 



23 



50 



10-9 



Mar. 11 



52 



100 



52 12-5 



53 



0-0 



233 52 27-5 



42 42-5 



42 



300 



42 



100 



77 



42 27-5 



23 



50 



00 



June 3 



52 



00 



51 57-5 



52 



45-0 



233 52 14-2 



42 37-5 



42 



3-5 



42 



200 



77 



42 20-3 



23 



50 



61 



June 3 



52 



1-2 



51 57-5 



52 



450 



233 52 14-6 



42 32-5 



42 



00 



42 



150 



77 



42 15-8 



23 



50 



1-2 



June 29 



52 



2-5 



52 2-5 



52 



500 



233 52 18-3 



42 450 



42 



12-5 



42 



350 



77 



42 30-8 



23 



50 



12-5 



July 27 



51 



52-5 



51 57-5 



52 



45-0 



233 52 11-7 



42 300 



42 



2-5 



42 



15-0 



77 



42 15-8 



23 



50 



41 



Sept. 23 



52 



10-0 



52 100 



52 



56-0 



233 52 25-3 



42 57-5 



42 



20-0 



42 



400 



77 



42 39-2 



23 



50 



13-9 



Sept. 29 



52 



400 



52 450 



53 



33-5 



233 52 59-5 



43 15-0 



42 



41-5 



43 



500 



77 



43 15-5 



23 



50 



160 



Nov. 6 



53 



17-5 



52 57-5 



53 



160 



53 53 10-3 



42 55-0 



43 



25.0 



43 



42-5 



257 



43 20-8 



23 



1 



50 



10-5 



The mean value of angle A=23° 50' 8"-4. 



Tlie value of angle Z, by the observations, Table 8, Introduction 1841-2, 

 = 1° 37' 38"-8. 



Whence A + Z = 25° 27' 47"-2 = 25° 27'-79 (25° 27'-75 was used), which is the 

 absolute westerly declination corresponding to the long scale reading (as corrected. 

 No. 19.), 256*30, and to the short scale reading (as corrected, No. 19.) 146-35. For 

 other scale readings, differing from these zeros by the angular quantity +_ D, the 

 declination is obtained as indicated above. Tables have been formed from the above, 

 and the known angular values of the scale divisions (8.), by means of which the 

 scale readings observed are at once converted into the absolute westerly declination. 



§ 4. Unifilae Magnetometee and Observations op the Absolute Horizontal 

 Intensity of the Eaeth's Magnetism. 



24. In the beginning of April 1843 two small wooden houses were erected about 

 19 yards to the north of the Magnetic Observatory ; the larger of the houses contains 

 the unifilar magnetometer and the dip circle, and the smaller, which is 10 feet to 

 SSE. of the larger, contains a reading telescope for the magnetometer. The instru- 

 ments were in their positions in June, and the first observation of the absolute hori- 

 zontal intensity was made in August. 



25. The unifilar magnetometer rests on a strongly braced wooden stand, which 

 is fixed by copper battens and plaster of paris to a stone slab, resting on a stone 



