CHANGE IN DIRECTION OF EARTH’S MAGNETIC FIELD. 283 
Lat. 19° 26' N. CITY OF MEXICO. Long. 99° 05' W. 
Year. 
Magnetic 
declina¬ 
tion. 
Authority. 
Observer. 
Where recorded. 
1769.. 
o 
—5.47 
—6.70 
—8.13 
—8.50 
—8.59 
—8.77 
—8.37 
—8.50 
—8.45 
—8.14 
—8.17 
—8.58 
—8.31 
Don Alzate 
Velasques 
Hansteen’s Magnetismus der Erde, 1819. 
Mem. del Observatorio Met. Central de Mexico, 
1880. 
Hansteen’s Magnetismus der Erde, 1819. 
Mem. del Observatorio Met. Central de Mexico, 
1880. 
Tratado de Topografia y Geodesia, Mexico, 1869. 
Phil. Trans. Royal Society, 1875. 
Tratado de Topografia y Geodesia, Mexico, 1869. 
Do. 
Mem. del Observatorio Met. Central de Mexico, 
1880. 
Tratado de Topografia y Geodesia, Mexico, 1869. 
Do. 
Mem. del Observatorio Met. Central de Mexico, 
1880. 
1775. 
de Leon 
1803. 
1849. 
Gomez de 
Velasques 
Muller and 
Salazar Ill? 
Diaz Covar 
Iglesias 
a Cortin 
and Ter 
Sontag 
iregui... 
1850. 
1856.. 
1858. 
I860. 
ubias 
1862.. 
1866.5 .... 
1868. 
Ponce deI 
Fernandez 
}. 
jeon. 
1879. 
1885.3 .... 
Year. 
Observed. 
Computed. 
O-C. 
-2 
O-C . 
1769. 
o 
—5.47 
o 
—6.03 
-{-0.56 
0.314 
1775. 
—6.70 
—6.35 
—0.35 
0.123 
1803. 
—8.13 
—7.65 
—0.48 
0.230 
1849. 
—8.50 
—8.60 
+0.10 
0.010 
1850. 
—8.59 
—8.61 
—0.02 
0.000 
1856. 
—8.77 
—8.59 
—0.18 
0.032 
Probable error of a single observation = ± 14'. 
1858. 
—8.37 
—8.58 
+0.21 
0.044 
Period = 360 years. 
1860. 
—8.50 
—8.56 
+0.06 
0.004 
1862. 
—8.45 
-Is. 53 
+0.08 
0.006 
1866.5 .... 
—8.14 
—8.47 
+0.33 
0.109 
1868. 
—8.17 
—8.45 
+0.28 
0.078 
1879. 
—8.58 
—8.21 
—0.37 
0.137 
1885.3 .... 
—8.31 
—8.03 
—0.28 
0.078 
1.165 
Empirical equation for the determination of the declination for any year : v = 
— 5°.52 + 0.027 sin (t — 1850) — 3.09 cos (t — 1850). 
