L. A. Bauer— Variation of Terrestrial Magnetism. 113 
was obtained. A column of mean results is then given which 
perhaps exhibit some periodical inequality. 
The inclination has been similarly treated in the subsequent 
columns. 
There was a suspicion that perhaps columns headed “ Mean” 
exhibited the sun-spot period (11% year). Upon plotting the 
quantities, however, nothing decisive could be discerned. 
From the bottom row of mean results the following laws 
are derived : 
I. The mean declination along a parallel of latitude is 
always westerly, the minimum occurring near the equator; 
the quantities, in general, imerease upon leaving the 
equator. 
Il. Lhe mean inclination along a parallel of latitude follows 
quite closely the law: 
tan I = 2tan g, 
I being the inclination and ¢ the geographical latitude. 
Ill. The minomum range in declination along a parallel of 
latitude occurs near the equator, generally increasing 
- upon leaving the equator. 
IV. Lhe maximum range mm melination along a parallel of 
latitude occurs near the equator, generally diminishing 
upon leaving the equator. 
The asymmetry of the mean quantities with respect to the 
geographical equator is, doubtless, to be referred to the eccen- 
tricity of the magnetic axis of the earth. How closely II 
follows the law given can be seen from the following figures : 
Latitude. I obs’d. I comp’d. 0.-C. 
O° [e) (eo) le) 
60 N. + 75°2 +73°9 +13 
40 +59°6 +59°2 +0°4 
20 +34°4 + 36°1 —1°7 
0 — 2°3 0°0 —2°3 
20 —36°'0 —36°l +0°1 
40 —57°6 —59°2 +1°6 
60 8. —70°6 —73°9. +3°'3 
The theoretical significance of this empirical law will be 
pointed out in the next number. | 
Huplanation of Table I[.—Here we have given the largest 
secular change in declination and inclination during the epoch 
1780 to 1885 for the 126 hypothetical stations. To illustrate: 
In latitude 60° N. and longitude 0°, the largest declination 
we have recorded occurred in 1842, the amount being 26-0 W.; 
the lowest recorded one we find in 1885, viz: 20°0 W., and 
Am. Jour. So1.—THIRD SERIES, Vou. L, No. 296.—AugGust, 1895. 
8 
