Diagram of declinations of the magnetick needle, when
_excited, in the various portions of the sphere, and horizons_
of the earth, in which there is no variation
_of the declination_.
[Illustration]
{190} As æquator let A B be taken, C the north pole, D the south, E G
dipping-needles in the northern, H F in the southern part of the earth or
of a terrella. In the diagram before us all the cusps have been touched by
the true Arctick pole of the terrella.
Here we have the level position of the magnetick needle on the æquator of
the earth and the stone, at A and B, and its perpendicular position at C,
D, the poles; whilst at the places midway between, at a distance of 45
degrees, the crosses of the needle dip toward the south, but the cusps just
as much toward the north. Of which thing the reason will become clear from
the demonstrations that follow.
_* Diagram of the rotation and declination of a terrella_
conforming to the globe of the earth, for a
_latitude of 50 degrees north._
[Illustration]
A is the boreal pole of the earth or of a rather large terrella, B the
southern, C a smaller terrella, E the southern pole of the smaller
terrella, dipping in the northern regions[233]. The centre C is placed on
the surface of the larger terrella, because the smaller terrella shows some
variation on account of the length of the axis; inappreciable, however, on
the earth. Just as a magnetick needle dips in a regional latitude of 50
degrees, so also the axis of a stone (of a spherical stone, of course) is
depressed below the horizon, and its natural austral pole falls, and its
boreal pole is raised on the {191} south toward the Zenith. In the same way
also a circular disc of iron behaves, which has been carefully touched at
opposite parts on its circumference; but the magnetical experiments are
less clear on account of the feebler forces in round pieces of iron.
_Variety in the declinations of iron spikes at various latitudes of a
terrella._
[Illustration]
The declination of a magnetick needle above a terrella is shown by means of
several equal iron wires, of the length of a barleycorn, arranged along a
meridian. The wires on the æquator are directed by the virtue of the stone
toward the poles, and lie down upon its body along the plane of its
horizon. The nearer they are brought to the poles, the more they are raised
up by their versatory nature. At the poles themselves they point
perpendicularly toward the very centre. But iron spikes, if they are of
more than a due length, are not raised straight up except on a vigorous
stone.
* * * * *