Theory of the Magnetical Variation. 297 
I shall for the sake of illustration give a figure of the Magnetic 
equator, and meridians, © ~ 
‘ A B 
Let A B represent the 
two hemispheres of a 
“globe: Aad Bb a mag- 
netic meridian: CD a © D 
magnetic parallel, which 
being the greatest of all 
the parallels in the mag- 
netic sphere, is here, for b 
distinction sake, called the magnetic equator. It is evident from 
inspection, that the magnetic lines are all curvilinear, and differ 
from the circles on a globe. ‘That the various inclination of a 
magnetic meridian with the true meridian, produces the varia- 
tion in quantity more or less, according to the parts of the earth 
so affected: and so likewise in any line drawn at right angles 
with a magnetic meridian; as for instance, the magnetic equator, 
which in its traverse round the globe, makes different quantities 
of variation according to its different inclination with the terres- 
trial equator. his variation increases towards the poles, as repre- 
sented in the figure: and the same magnetic meridian which in 
the northern hemisphere produces a west variation, may produce 
an east variation in the southern hemisphere, as in the meridian 
marked A a; and that meridian which in the southern hemi- 
sphere may produce an east variation, may produce a west va- 
riation in the northern hemisphere :—in like manner, and by the 
same rule, the magnetic equator produces the various quantities 
of east and west variation. 
To delineate, either on a chart or on a globe, a series of lines 
representing the magnetic meridians and parallels, is equivalent 
to projecting the north and south and east and west magnetic 
rhumbs. It is owing to the variation of the magnetic rhumb 
from the true rhumb of the course, that the mariner is frequently 
necessitated to make observation of the variation at sea in order 
to correct his course: this is most convincingly taught in the de- 
lineation of any magnetic meridian. 
According to the present state of the variation between the 
tropics, the magnetic equator, traversing the whole compass of the 
glohe, completes its range within 23 degrees of latitude or nearly 
12 degrees north, and 12 ditto south of the equator. Its maximum 
quantity of variation any where known is about 23 degrees on 
the coast of Africa, near the line; and hence the variation in- 
creases gradually until it is lost in the lines of no variation east 
and west. The west variation comprehends about 110 degrees 
of longitude, reckoned from the western coast of South America 
about 
