198 LZ. A. Bauer— Variation of Terrestrial Magnetism. 
What we actually do, however, is simply to draw lines through 
all the places on the earth’s surface having the same departure 
from the normal or computed dip. Such lines, I call lines of 
equal-departing inclination or briefly, “ isapoclinies.”* On the 
diagram opposite they have been drawn for the year 1885. 
The sign of the differences, ¢, is determined in accordance 
with the formula: : 
z=I obs’d —Icomp’d 
regarding dip of north end of needle below the horizon as posi- 
tive, above as negative. A plus means then that the dip of 
the north end of the needle has been encreased by the disturb- 
ing force, or the north end of the needle has been attracted ; a 
minus ¢ signifying, that dip of north end has been deminished, 
or the south end attracted. 
The two heavy lines marked zero, which are but branches of 
the same closed curve, mark out all those places where ¢ =0, or 
where the dip has the value assuming the earth to be uniformly 
magnetized about the rotation axis. The region between them 
on the left has everywhere plus departures, i. e. over this part 
of the earth the deflecting force attracts the north end of the 
needle. It is seen that these plus isapoclinics encircle a 
focus, approximately in latitude 20°S. and in longitude 40° W. 
Here the inclination attains a value of —7°:5 instead of —36°°1 
prescribed by formula, hence 
t= —7°5 —(—36°'1) = + 28° 6 
or the north end of the dipping needle is drawn downwards at 
this point about 29°—this being the maximum plus deflection 
produced by the disturbing force. We mzght regard then this 
point as the SEAT of the force which deflects the north end of 
the dipping needle from its normal amount, or briefly a sec- 
ondary north magnetic pole. 
Similarly, the curves of negative departures on the right 
lead up to a focus in about latitude 5° S. and longitude 40° E. 
Here the sowth end of the dipping needle is drawn downwards 
by about 24°. Hence this focus marks the seat of the south 
end disturbing force and corresponds thus to a secondary south 
magnetic pole. 
* The analogous terms for the other elements would be, ‘‘isapogonics,” “ isapo- 
dynamics” and ‘ equiabspotentials.”’ These lines are not quite, though nearly, 
the same as the “ isanomalous lines ” successfully introduced in terrestrial magne- 
tism by von Bezold and von Tillo which are the equal departing lines, taking the 
mean elements for a parallel of latitude as the normal ones. I have been obliged 
to adopt the above method for my purposes because: First, it enables me to draw 
‘‘ equal departing lines” without the aid of magnetic charts, since I can compute the 
normal elements for any latitude. This is important when considering the secu- 
lar variation. Second, it enables me to give precision to the interpretation of the 
residual or secondary field obtained with the aid of these lines. 
