206 
LINES OF xMAGNETIC DECLINATION IN THE ATLANTIC. 
branches of which form the limits of the four great systems of isogonic lines, viz. of 
the four great systems which prevail respectively in the N.W., N.E., S.W. and S.E. 
directions, and have one isogonic line common to them all, the branches of which 
meet at a common point of junction or intersection near the west coast of Africa, 
and not far from the terrestrial equator. The theoretical value of this line is 22° 13' W.; 
whereas its true value is certainly not more than between 19° and 20°, and it appears 
to have been in the year 1840 nearly midway between those values. Had the theo- 
retical value been in defect instead of in excess, the difference might have been 
ascribed to an effect of secular change ; in which case M. Gauss’s map might be 
supposed to represent an earlier state of the phenomena than that of the year 1840. 
But the fact is otherwise ; the theoretical value of the line in question is in excess, 
although its actual value has been progressively increasing since the earliest record 
of the phenomena, and was greater in 1848 than in any antecedent year since the 
phenomena have been observed. The theoretical error is the more remarkable 
because Mr. Barlow’s map for 1833, which was the one employed by M. Gauss for 
the declination, gives the value of that element correctly at 20° or thereabouts, at 
the spot where M. Gauss’s theoretical line of 22° 13' cuts the coast of Africa. 
The form of the lines in Mr. Barlow’s map in that quarter of the globe is indeed 
not correct, since on no part of the surface of the earth do the magnetic lines forh in 
the manner there represented, and which is very different from the intersection of 
lines of equal value forming four branches of one and the same isogonic line, which 
is the character of the remarkable line now referred to; but the value of the declina- 
tion on that part of the African coast is more correct in Mr. Barlow'’s map than in 
M. Gauss’s theory, which is partly based upon it. The geographical position of the 
intersection is also given by M. Gauss’s theory about 10° of longitude too much to 
the east. The secular movement of the intersection is to the west, and may be 
estimated, very roughly, at about 10° in the last half-century. 
It is obvious from this comparison that the General Theory will require to have 
its numerical coefficients reconstructed before it can become available for practical 
purposes ; and that those who desire to take a correct view of the magnetic 
phenomena, must for the present at least, have recourse to the maps constructed 
directly from the observations themselves. 
