ON THE MAGNETIC INTENSITY OF THE EARTH. 75 
characteristics of the system most strongly marked, I have 
chiefly employed them, where observations would permit, in 
describing its general features. The characteristics soften 
gradually as the distance increases from the governing cen- 
tres; but even in the intertropical regions the distribution of 
the intensity and the arrangement of the lines contribute their 
testimony to the same system. I have nowhere attempted to 
assign the precise geographical positions of the centres ; and in 
regard to those of the southern hemisphere especially, have 
expressly stated, that the facts yet acquired would not enable 
this to be done within fair limits of approximation. Thus 
much, however, may be safely said in regard to them, that the 
primary in the southern, and the secondary in the northern, 
are at the present time not far from the same meridian; and 
that the primary in the northern, and the secondary in the 
southern, are similarly situated, except that their difference of 
longitude is somewhat greater. If we respectively connect 
the centres, which thus approximate in longitude, by lines 
on the globe crossing the equator, the lines will mark those 
localities within the tropics where the influence of the cen- 
tres should produce a higher intensity than elsewhere in 
the same latitudes. Thus we should have two maxima in the 
_ intertropical regions; and these should not be in opposite me- 
aera SA ake and 
~ 
ridians, because the centres are unsymmetrical. Such is ac- 
tually the distribution of the intensity in these regions. The 
isodynamic lines which represent unity are the weakest which 
run unbroken round the globe, and appear twice in every me- 
ridian; these approach each other in the meridians of 110° 
and 260°, whilst, intermediately, they recede from each other, 
and inclose spaces occupied by a still weaker intensity; the 
largest of these spaces, corresponding to the widest interval 
between the centres, is of 210 degrees of longitude, and the 
smallest of 150 degrees. In the middle of the largest, as the 
point most distant from all the four centres, we should expect 
to find the weakest intensity existing anywhere at the surface of 
the globe; and accordingly at St. Helena, which is nearly in 
that situation, the intensity observed by Captain Fitz Roy, 0°84, 
is the lowest determination recorded in this report, and is the 
locality of the weakest intensity yet observed on the globe. 
Between St. Helena and the lines of unity on either side, we 
should have a line representing the value of 0°9, a part of 
which has been extremely well determined by concurrent ob- 
_ servations. This line, being comprehended by the lines of 
unity, is necessarily a closed one. Observations are yet want- 
ing to show whether the intensity descends as low as 0°8 in the 
