Unifilar Magnetometee. xxiii 
Table 6. — Determinations of the Value of Angle A. 
Readings of Horizontal Circle 
Date. 
For Declination Telescope. 
For North M ark. 
Verniers. 
Verniers. 
Angle 
A. 
A. 
B. 
C. 
i>iean. 
A. 
B. I C. 
Mean. 
1843. 
Jail, is 
52 
50 
' 
52 
II 
100 
52 
550 
o 
233 
52 
23-3 
42 52-5 
42 
17-5 
/ 
42 
32-5 
o 
77 
42 
If 
34-2 
o 
, 23 
50 
a 
Id 
' 1 
iviar. 1 1 
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 
0 
() 
June 3 
52 
00 
51 
57-5 
52 
450 
233 
52 
14-2 
42 37-5 
42 
3-5 
42 
200 
77 
42 
20-3 
23 
50 
6 
1 
June 3 
52 
1-2 
51 
57-5 
52 
45-0 
233 
52 
14-6 
42 32-5 
42 
0-0 
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 45 0 
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 30 0 
42 
2-5 
42 
150 
77 
42 
15-8 
23 
50 
4 
i 
Sept. 23 
52 
10-0 
52 
100 
52 
56-0 
233 
52 
25-3 
42 57-5 
42 
20-0 
42 
40- 0 
77 
42 
39-2 
23 
50 
13 
9 
Sept. 29 
52 
40-0 
52 
45-0 
53 
33-5 
233 
52 
59-5 
43 15-0 
42 
41-5 
43 
500 
77 
43 
15-5 
! 23 
50 
16 
0 
Nov. 6 
53 
17-5 
52 
57-5 
53 
160 
53 
53 
10-3 
42 55 0 
43 
250 
43 
42-5 
257 
43 
20-8 
23 
50 
10 
5 
The mean value of angle A = 23° 50' 8 "-4. 
The 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 Obseevations of the Absolute Hoeizontal 
Intensity of the Earth'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 
