CHANGE IN DIRECTION OF EARTH’S MAGNETIC FIELD. 303 
Lat. 17° 31' S. 
TAHITI, SOCIETY ISLANDS. 
Long. 149° 34' W. 
Year. 
Magnetic 
declina¬ 
tion. 
Authority. 
Observer. 
Where recorded. 
1768., 
—5.75, Wallis, 1767.. 
—4.75, Cook, 1769 .. 
1774.6 
1794.. . 
1824.3 
1833.. .. 
1838.5 
1859.. .. 
1871.. .. 
1875.. .. 
1878.. .. 
—5.68 
— 6.20 
—7.02 
—7.53 
—6.80 
—7.18 
—7.77 
—7.78 
—7.55 
—5.67, Wales, 1773. 
—5.82, Bayley, 1774. 
[—5.57, Cook, 1777. 
Vancouver. 
f —6.67, Diiperrey, 1823... 
j —6.83, Kotzebue, 1824... 
[—7.55, Beechey, 1826.... 
—7.50, Irland, 1831. 
—7.57, Fitz Roy, 1835.... 
—7.17, Bruce, 1837. 
—6.50, Belcher, 1840. 
—6.90, Friesach, 1859 .... 
—7.45, Novara Ex., 1859. 
Hansteen’s Magnetismus der Erde, Christiania, 
1819. 
Voyage of the Adventure and Beagle, London, 
1839. 
Do. 
Do. 
Voyage autour du Monde, Paris, 1829. 
Phil. Trans. Royal Society, 1877. 
Do. 
Do. 
Do. 
Do. 
Hydrographic Office archives document. 
—8.04, Nares & Thomson, 
R. N., 1875. 
—7.58, Nares & Thomson, 
R. N., 1875. 
—7.85, Nares & Thomson, 
R. N., 1875. 
Report of Challenger Expedition. 
Letter from French Government hydrographer. 
Year. 
Observed. 
Computed. 
O-C. 
o-c 2 . 
o 
o 
1768. 
—5.25. 
—5.07 
—0.18 
0.0324 
1774.6 .... 
—5.68 
—5.28 
—0.40 
0.1600 
1794. 
—6.20 
—5.93 
—0.27 
0.0729 
1824.3 .... 
—7.02 
—6.87 
—0.15 
0.0225 
1833. 
—7.53 
—7.08 
—0.45 
0.2025 
1838.5 .... 
—6.80 
—7.22 
+0.42 
0.1764 
1859. 
—7.18 
—7.52 
+0.34 
0.1156 
1871... 
—7.77 
—7.58 
—0.19 
0.0361 
1875. 
—7.78 
—7.58 
—0.20 
0.0400 
1878. 
—7.55 
—7.58 
+0.03 
0.0009 
0.8593 
Probable error of a single observation 
Period = 360 years. 
: ± 14' 
Empirical equation for determining the declination for any year 
— 0.8288 sin {t — 1850) — 1.7697 cos (t —1850). 
— 5°. 64 
