L. A. Bauer— Variation of Terrestrial Magnetism. 323 
Both deductions are amply borne out by observations and are 
prettily illustrated by a chart, as yet unpublished, to which I 
have made allusion several times, viz., the chart showing 
graphically the secular variation in declination for points on 
the earth distant 20° in latitude and in longitude. 
As a second illustration of the importance of study of the 
secondary magnetic poles, I shall reproduce a few of the secu- 
lar motion curves given in previous papers, and endeavor to 
explain a phenomenon which was a source of perplexity to me 
when I first observed it. 
On the diagram* opposite we have given the curves that 
have been described in the course of time by a magnetic 
needle freely suspended. It will be noticed that while the 
direction of motion is clockwise throughout there is a marked 
difference between some of the curves, as for example, between 
those of Ascension and St. Helena Islands and that of Rio de 
Janeiro. It would appear as though the latter curve has 
swung itself around. The same thing is exhibited by the 
curve for station 40° N. and 60° W. and by the curves west 
thereof. West of this station, the curves for the interval 
drawn follow the American type; east, the European. It is 
evident that some important change in magnetic distribution 
must have occurred in the interval 1750-1890 in the region 
between the stations exhibiting the contrasted motions. 
Upon looking into this matter I find that the secondary north 
end attracting pole during the stated interval moved between 
Ascension Island and Rio de Janeiro—away from Ascension 
Island and towards the latter station. Hence, it follows that 
the dip of the north end of the needle at Ascension Island 
decreased, or that of the south end increased, while on the 
other hand, the northerly dip at Rio de Janeiro increased or 
that of the southerly decreased. At Ascension Island and 
stations east (up to a certain line), the southerly dip increased ; 
at Rio de Janeiro and west (up to a certain line), it decreased. 
This is precisely the state of affairs exhibited by the curves, 
as will be seen by inspection. 
For the better understanding of the foregoing the following 
geographical positions are given: : 
Latitude. Longitude. 
Breeteiena ie... ....-. 15° 5a &. 5°44’ W. 
macemmionw is _.....-:.. 7 56 SB. 14 23 W. 
mide Janeiro. .....-- 99° bas. 43 10 W. 
From the table on p. 318 we find that the position of the 
secondary north pole was : 
in 1780 10°. 30° W. 
PEIBBS: 20 8. 45 .W. 
* Taken from the Physical Review, vol. iii, No. 13, Plate I. 
