CHANGE IN DIRECTION OF EARTH’S MAGNETIC FIELD. 277 
Lat. 7° 
55.5' S. 
Magnetic 
Year. 
declina¬ 
tion. 
o 
1700. 
—2.00 
1754.3. 
8.10 
1768.25... 
9.88 
1775.4. 
10.87 
1806.3. 
15.67 
1816. 
15.50 
1825. 
16.87 
1830*. 
20.16 
1836. 
17.60 
1839. 
18.52 
1842. 
19.27 
1846. 
19.27 
1861. 
21.75 
1863. 
21.63 
f 23.10 
1876.25 ... 
22.77 \ 22.53 
L22.68 
f 22.6 
1890.3. 
23 - 00 |23.4 
ASCENSION ISLAND. 
Long. 14° 24 / W. 
Authority. 
Observer. 
De la Caille.. 
Wallis. 
Cook. 
Bonsoe. 
Brine. 
Duperrey. 
Foster. 
Fitz Roy. 
Du Petit Thouars. 
Belcher.. 
B6rard. 
Denham. 
H. M. S. Hecate. 
Nares and Thomson,R.N. 
E. D. Preston, assistant, 
- United States Coast 
and Geodetic Survey. 
Where recorded. 
Halley, in his Tabula Nautica, etc. 
Hansteen’s Magnetismus der Erde, Christiana, 
1819. 
Do. 
Do. 
Do. 
Becquerel’s Trait6 du Magnetisme. 
Do. 
Phil. Trans. Royal Society, Part II, 1877. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 
Report of Voyage of H. M. S. Challenger. 
} Bulletin No. 23, United States Coast and Geodetic 
Survey, March, 1891. 
Year. 
Observed. 
Computed. 
O-C. 
o-c 2 . 
0 
0 
1700. 
—2.00 
—2.47 
+0.47 
0.2209 
1754.3. 
8.10 
7.68 
+0.42 
0.1764 
1768.25... 
9.88 
9.66 
+0.22 
0.0484 
1775.4. 
10.87 
10.66 
+0.21 
0.0441 
1806.3. 
15.67 
14.93 
+0.74 
0.5476 
1816. 
15.50 
16.15 
+0.35 
0.1175 
1825. 
16.87 
17.23 
—0.36 
0.1296 
1836. 
17.60 
18.62 
—1.02 
1.0404 
1839. 
18.52 
18.95 
—0.43 
0.1849 
1842. 
19.27 
19.27 
0.00 
0.0000 
1846. 
19.27 
19.70 
—0.43 
0.1849 
1861. 
21.75 
21.13 
+0.62 
0.3844 
1863. 
21.63 
21.31 
+0.32 
0.1024 
1876.25 ... 
22.77 
22.32 
+0.45 
0.2025 
1890.3. 
23.00 
23.15 
—0.15 
0.0225 
3.4065 
Probable error of a single observation = + 21.6' 
Period = 600 years. 
(♦Omitted in investigation.) 
Empirical equation for determining the declination for any year: v = 10°.55 + 
9.47 sin 0.6 {t — 1850) -f* 9.56 cos 0.6 (t — 1850). 
41-Bull. Phil. Soc., Wash., Vol. 13. 
