TERRESTRIAL MAGNETISM.—MURPHY. 293 
the figure which I place before you we find the parailels of lati- 
tude and meridians of longitude appear at regular curves. But 
from a focus at N. radiate a series of curves, which take sinuous 
forms and finally converge towards another focus at the anti- 
podes. These foci are the magnetic poles of the earth toward 
which the compass needle ever points, not directly but parallel 
to the lines of force. It will be seen that these magnetic poles 
are far removed from the geographical poles. 
These three features of the earth’s magnetic action which are 
chiefly attended to by observers are then—the inclination, the 
declination and the intensity. 
The inclination is the angle at which the dipping needle is 
inclined to the horizon. The declination is the angle at which 
the horizontal needle is inclined to the north and south line. 
The intensity is the magnitude of the force with which the needle 
seeks the position of rest. 
Now if we travelled over the whole surface of our earth and 
carefully determined the declination, the inclination and the 
intensity of the magnetic action at every point, we should be 
able to map down ona chart of the earth the relations thus 
presented to our notice, we should—speaking generally—have 
the following peculiarities to deal with : 
First, as to the declination, we should find that in certain 
regions the magnet’s northern end was to the west of north, 
whilst in certain other regions the reverse was the case. If we 
marked in the boundary line between these regions, it is obvious 
that we should have traced a line along which the needle 
would lie due north and south. This is what is termed the line 
of no declination. On charts of the earth’s magnetic relations 
the position of this line for the middle of the present century is 
usually indicated. In some maps a set of lines used to be added 
along each of which the magnetic needle had a definite declina- 
tion. These lines are now omitted on account of their complexity. 
Secondly, as to the inclination, we should find as we travelled 
over the earth’s surface that the dipping needle tends to verti- 
eal at two nearly opposite points—one close to the Arctic, and 
he other to the Antarctic circle. These are called the northern 
