296 Theory of the Magnetical Variation. 
necessarily be interrupted and imperfect. Its general history, 
however, enables us to conclude that its attractive power for iron 
was its first known property; that in subsequent times its po- 
larity was discovered, when it was found to possess a line of at- 
traction situate in its own proper axis, and conforming its ener- 
gies agreeably to that direction. In aftertimes its absolute po- 
larity was discovered in the verticity of the needle, when it was 
found to conform with the poles of the world, a discovery of all 
others the most important to navigation and commerce : this was 
the origin of the mariner’s compass. Lastly, the vertical or dip- 
ping needle was discovered, whose use and theory remain for 
the cultivation of the moderns. The variation of the horizontal 
needle, and the variation of that variation, have been discoveries 
of later times. 
If, as some philosophers have asserted, the earth was originally 
a right sphere, having its polar and equatorial diameters equal, 
and its whole surface a perfect globe; it may be argued with 
equal probability, that at that period the magnetic power strictly 
conformed itself with the true poles of the world universally, and 
of consequeuce there could be no variation of the magnetic needle. 
But since extensive surveys have been made by eminent mathe- 
maticians, and it has been determined that the present figure of 
the earth is a spheroid, having its polar axis less than its equa- 
torial; and that it has been gradually increasing from the power 
of gravity acting upon its polar surfaces; it is equally possible 
that the variation of the magnetic poles hath originated from a 
similar cause; and that so long as the earth shall obtain the form 
of a spheroid, and increase in its obliquity, so !ong will the va- 
riation of the magnetic poles continue to. increase in different 
parts of the earth, as now we find it. In the former case, which 
is an hypothesis not altogether improbable, the lines of the mag- 
netic sphere coincided with those of the true sphere; and by 
consequence all the other imaginary lines constituting either 
sphere were common, such as the magnetic meridians, equator, 
and parallels: but in the latter case an absolute difference is found 
in the existing variation, known to all persons experienced in 
Navigation, 
A projection of the lines of the magnetic sphere on a globe is 
not altogether impracticable, as may appear from what has al- 
ready been effected in some maps and charts of the Magnetical 
Variations already published. ‘The lines of quantity being trans- 
ferred from the chart to the globe, the magnetic meridians, 
equator, and parallels, may also be laid down with equal pre- 
cision ; and by the help of these the whole complement may be 
supplied, and thereby the theory of the magnetic sphere will be 
most interestingly displayed and understood. 
I shall 
