ON THE PHENOMENA OF TERRESTRIAL MAGNETISM. 69 
again two points of convergence : one, of the observations in the 
northern parts of America comprised between the meridians of 
230° E. and 290° E., which, computed by spherical trigonometry, 
is in 259° 58' E. and 70° 17 ; N., corresponding to the year 1769, 
and to a mean of all the intersections; the other, for the obser¬ 
vations in the North of Asia,between the meridians of 70° E. and 
130° E., in 101° 29' E. and 85° 43'N., corresponding also to a 
mean of all the intersections of observations of the year 1769. 
Here also the arrows in the intermediate spaces, viz. between 
130° E. and 230° E., and between 290° E. and 70° E. (passing 
through 360°), have no point of common convergency; but their 
direction appears to be determined by one or other of the influ¬ 
ential points, according to proximity. 
We trace, then, by means of the polar maps four principal 
points of convergence in the direction of the magnetic needle; 
two in the southern hemisphere, designated by M. Hansteen 
A and «, and two in the northern hemisphere, B and b. Those 
in the southern hemisphere correspond to the year 1774, when 
A was situated in 136° 15' E. and 69° 27' S.; and a in 236° 45'E. 
and 77° 17' S. Those in the northern hemisphere to the year 
1769, when B was in 259°58'E. and 70°17 , N., and b in 101 ° 
29' E., and 85°43'N. 
These points are called by M. Hansteen simply points of 
convergence”. Each is considered by him to mark the vicinity 
of a magnetic pole; the mode of deducing the positions of the 
poles from those of the points of convergence is a subject of 
discussion in a subsequent chapter. 
M. Hansteen next proceeds to inquire whether these points 
of convergence appear to be stationary, or otherwise; whether 
they are equally the points of intersection of the directions of 
the compass-needle observed in the same localities, but at other 
dates than those above stated. The observations recorded in the 
appendix furnish him with the place of A corresponding to the 
year 1642 ; of a to 1586 and 1670; of B to 1725 ; and of 1) to 
1594 and 1805. The result in every case shows that the points 
are not stationary; the earlier observations concur uniformly 
in making the intersections, in the northern hemisphere to the 
westward, and in the southern hemisphere to the eastward, of 
the later observations. 
Neither the degree of exactness of the earlier observations, nor 
the intervals elapsed between them and the later ones, are suffi¬ 
cient to warrant any very decided inference of the periods in 
which each or any of the points would complete the circle of all 
the meridians. The periods indicated are, however, as follows : 
A 4609 years ; a 1304 years; B 1740 years ; and b 860 years* 
