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 north (as they not infrequently are fixed in buildings and across windows), those rods, I say, by that long lapse of time acquire verticity and turn round, whether hanging in the air, or floating (being placed on cork), to the pole toward which they were pointing, and magnetically attract and repel a balanced iron magnetick; for the long continued position of the body toward the poles is of much avail. This fact (although conspicuous by manifest experiments) is confirmed by an incident related in an Italian letter[213] at the end of a book of Maestro Filippo Costa, of Mantua, _Sopra le Compositioni degli Antidoti_ written in Italian, which translated runs thus: "A druggist of Mantua showed me a piece of iron entirely changed into a magnet, drawing another piece of iron in such a way that it could be compared with a loadstone.