s 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.
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. 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 thesouth
