CHANGE IN DIRECTION OP EARTH’S MAGNETIC FIELD. 321 
Lat. 21° 18' N. HONOLULU. Long. 157° 52' W. 
Year. 
Magnetic 
dip or in¬ 
clination. 
Authority. 
Observer. 
Where recorded. 
1830. 
o 
41.70 
Tlnnirlas .. 
Phil. Trans. Rova.l Sneietv. Part T. 1875. 
18 3fi. . 
42.10 
<s Bonite ” 
Do. 
1837.. 
41.60 
Rfifip.besv .. 
Do. 
1838. 
41.30 
Belcher... 
Do. 
1852.5 .... 
41.18 
Swedish Magnetic Survey... 
Annalen der Hydrographie, 1877. 
1875,5 .... 
39.78 
H. M. S. Challenger. 
Voyage of H. M. S. Challenger. 
1890.6 .... 
40.53 
Lieutenant Courmes, Fr. N.. 
Annales Hydrographiques, 2d vol., 1892. 
1892.5 .... 
40.70 
E. D. Preston, assistant, 
Report of United States Coast and Geodetic Sur¬ 
United States Coast and 
vey, Part II, 1893. 
Geodetic Survey. 
Year. 
Observed. 
Computed. 
O-C. 
-2 
O-C . 
1830. 
o 
+41.70 
o 
+42.10 
—0.40 
0.1600 
1836.. 
+42.10 
+41.65 
+0.45 
0.2025 
1837... 
+41.60 
+41.59 
+0.01 
0.0001 
1838.. 
+41.30 
+41.51 
—0.21 
0.0441 
1852.5 .... 
+41.18 
+40.74 
+0.44 
0.1936 
■ Probable error of a single observation == ± 17'. 
1875.5 .... 
+39.78 
+40.28 
—0.50 
0.2500 
1890.6 .... 
+40.53 
+40.50 
+0.03 
0.0009 
1892.5 .... 
+40.7Q 
+40.55 
+0.15 
0.0225 
0.8737 
Empirical equation for determining the inclination for any year: I = 40°.31 — 
0.0093 (t — 1870) + 0.00089 (< — 1870) 2 . 
