rth) and heated
for eight or ten hours, then cooled away from the fire, in the same
position towards the poles, acquires a verticity in accordance with the
position of its heating and cooling. Let a rod of cast iron be heated
red-hot in a strong fire, in which it lies * meridionally (that is, along
the path of a meridian circle), and let be removed from the fire and
cooled, and let it return to its former temperature, remaining in the same
position as before; then from this it will turn out that, if the same ends
have been turned to the same poles of the earth, it will acquire verticity,
and the end which looked toward the North on water with a cork before the
heating, if it have been placed during the heating and cooling toward the
fourth, now turns round to the south. But if perchance sometimes the
rotation have been doubtful and somewhat feeble, let it be placed again in
the fire, and when it is taken out at a red heat, let it be perfectly
cooled toward the pole from which we desire the verticity, and the
verticity will be acquired. Let the same rod be heated * in the contrary
position, and let it be placed so at a red heat it is cool; for it is from
its position in cooling (by the operation of the verticity of the earth)
that verticity is put into the iron, and it turns round to parts contrary
to its former verticity. So {141} the end which formerly looked toward the
north now turns to the south. In accordance with these reasonings and in
these ways the boreal pole of the earth gives to the end of a piece of iron
turned toward it a southern verticity, and that end is attracted by that
pole. * And here it must be observed that this happens to iron not only
when it is cooled in the plane of the horizon, but also at any angle to it
almost up to the perpendicular toward the centre of the earth. So the
heated iron conceives vigour and verticity from the earth more quickly in
the course of its return to its normal state, and in its recovery, as it
were (in the course of which it is transformed), than by its mere position
alone. This is effected better and more * perfectly in winter and in colder
air, when the metal returns more certainly to its natural temperature, than
in summer and in warm regions. Let us see also what position alone and a
direction toward the poles of the earth can effect by itself without fire
and heat. Iron rods which have been placed and fixed for a long time,
twenty * or more years, from south to nort