TERRESTRIAL MAGNETISM.—MURPHY. 299 
To show the variability of this attraction in direction and 
amount in various parts of the globe, the mariner’s compass is 
everywhere subject to the influence of these magnetic lines of 
force, and it is their influence that gives steadiness and direction 
to the needle. At e, e’, e” and e”’ is a magnetic needle repre- 
Sented as suspended by a thread from the zenith, and assuming 
as it always will, a direction parallel to the line of force. At 
the magnetic equator, m, m, this line is parallel to the horizon, 
and so is the needle, e” ; we go north and the line becomes bent, 
so the needle inclines as at e”; proceeding further, the line bends 
more, and the needle inclines accordingly ; finally, at e, it is bent 
vertically in the vicinity of the pole. In all these cases the force 
or intensity of the magnetic field steadily increases from the 
first towards the last position of the needle, so that it will vibrate 
slowly at e”’, whilst at e” it will be quicker, the are smaller, and 
the time less, and so on, until it comes to e, when it will have a 
few quick, jerky movements, and then stop short. 
Now, a needle dipping thus at every remove from equatorial 
regions is of no value to guide a ship. It must always be hori- 
zontal, and this is practically obtained by placing a small sliding 
counterpoise on the needle, to overcome the downward pull of 
the magnetism. It is easily adjusted to change. In this con- 
stantly horizontal direction of the needle, however, the portion 
of the magnetic intensity that gives it steadiness is materially 
changed, lessened, and more diminished as we proceed from e”’ 
toe’. As we proceed from the magnetic equator towards the 
pole, the compass becomes less steady and reliable, while at the 
same time the total intensity of the magnetic field increases. 
Humboldt, in the first volume of the Cosmos, after discussing 
translatory motion, terrestrial phenomena, geographical distribu- 
tion, and the figure, density and internal heat of the earth, 
refers to terrestrial magnetism. He says: “If we present to 
ourselves the interior of the earth as fused and undergoing an 
enormous pressure, and at a degree of temperature the amount 
of which we are unable to assign, we must renounce all idea of a 
magnetic nucleus of the earth. All magnetism is certainly not 
lost until we arrive at a white heat, and it is manifested when 
