84 
FIFTH REPORT- 1835. 
fectly known values, each of which influences the direction of the 
needle ; and in the present still imperfect state of our knowledge 
of the phenomena themselves, it does not appear possible to 
determi we precisely how great is the influence due to each. It is 
possible, however, to assign approximate values ; and by com¬ 
paring the results computed with these with the observed phse- 
nomena continually to approximate within narrower limits. It 
is true that a final determination must await more exact and mul¬ 
tiplied observations ; but those which have been already obtained 
are sufficient to show that the three phenomena of variation, 
dip, and intensity, observed indiscriminately over the whole sur¬ 
face of the globe, do admit of a very approximate representation, 
on the supposition of two magnetic axes, with such values as 
may at present be assigned. 
To obtain such approximate values, the points of convergence 
found for the year 1775 were in the first instance assumed as 
the ends of the magnetic chords. Combining these with the 
dip observed in Hudson’s Bay by Hutchins, and in Petersburg 
by Kraft, it appeared, after repeated trials, in which the length 
of the magnetic axis was taken successively as &c., toyy 
of the earth’s axis, that were the positions of the points of con¬ 
vergence those of the ends of the magnetic chords, the propor¬ 
tion of t X q of the earth’s axis for that of the magnet would best 
accord with the dips observed in high magnetic latitudes : and 
as the result of calculations made with an axis so small in pro¬ 
portion as yq scarcely differs in the majority of instances from 
those made with an infinitely small axis, and as the latter sup¬ 
position is the more convenient in calculation, it was thought 
preferable to assume it, until more correct places could be de¬ 
duced for the ends of the magnetic chords. The values of a, 
s, §, &c. were then deduced for both axes, as well as the peri¬ 
centric point and poles of the magnetic equator for each axis. 
The geographical situation was then calculated of the two points 
where the magnetic equators would intersect each other; these 
points would necessarily be opposite points on the globe ; and 
at them it is obvious that the dip, whether influenced by one or 
other axis, should be 0: consequently, without reference to the 
dimensions, the relative forces, or the eccentricity of the axes, 
and provided -only that the mathematical and magnetical centres 
of each axis respectively are the same, there should be on the 
globe tw r o points precisely opposite to each other where the dip 
should be 0 ; and two such points should accordingly be found 
on the observed line of no dip. Now if we refer- to the map 
of the dip for 1780, w r e find that there are two points in the line 
of no dip opposite to each other on the earth’s surface, viz. in 
