How a more robust Loadstone may be recognized.

Very powerful loadstones sometimes lift into the air a weight of iron equal to their own; a weak one barely attracts a slender wire.

Those therefore are more robust which appeal to and retain larger bodies, if there is no defect in their form, or the pole of the stone is not suitably moved up. Moreover, when placed in a boat a keener influence turns its own poles round more quickly to the poles of the earth or the limits of variation on the horizon. One which performs its function more feebly indicates a defect and an effete nature. There must always be a similar preparation, a similar figure, and a like size; for in such as are very dissimilar and unlike, the experiment is doubtful.

The method of testing the strength is the same also with a versorium in a place somewhat remote from a loadstone; for the one which is able to turn the versorium round at the greater distance, that one conquers and is held the more potent.

Rightly also is the force of a loadstone weighed in a balance by B. Porta; a piece of loadstone is placed in one scale-pan, in the other just as much weight of something else, so that the scale-pans hang level. Soon a piece of iron lying on the table is adjusted so that it sticks to the loadstone placed in the scale, and they cling together most perfectly, according to their friendly points; into the other scale-pan sand is gradually thrown, and that until the scale in which the loadstone is placed is separated from the iron. Thus by weighing the weight of sand, the magnetick force becomes known.

Similarly also it will be pleasing to try with another stone, in equilibrium, the weight of the sand being observed, and to find out the stronger by means of the weights of sand. Such is the experiment of Cardinal Cusan in his De Staticis[185], from whom it would seem that B. Porta learnt the experiment.

The better loadstones turn themselves round more quickly toward the poles or points of variation; then they also lead along and turn round more quickly, according to the greater quantity and mass of wood, a boat and other stuff. In a declination instrument, the more powerful force of a loadstone is looked for and required. Those therefore are more lively when they get through their work readily, and pass through and come back again with speed, and swiftly at length settle at their own point. Languid and effete ones move more sluggishly[186], settle more tardily, adhære more uncertainly, and are easily disturbed from their possession.

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CHAP. XXXVII. Use of a Loadstone as it affects iron.

By magnetick coition we test iron ore in a blacksmith’s forge. It is burnt, broken in pieces, washed and dried, in which way it lays down its alien humours; in the bits collected from the washing is placed a loadstone, which attracts the iron dust to itself; this, being brushed off with feathers, is received in a crucible, and the loadstone is again placed in the bits collected from the washing, and the dust wiped off, as long as any remains which it will attract to itself.

This is then heated in the crucible along with sal nitri[187] until it is liquid, and from this a small mass of iron is cast.

But if the loadstone draws the dust to itself quickly and readily, we conjecture that the iron ore is rich; if slowly, poor; if it seems altogether to reject it, there is very little iron in it or none at all. In like manner iron dust can be separated from another metal. Many tricks there are also, when iron is secretly applied to lighter bodies, and, being attracted by the motion of a loadstone which is kept out of sight, causes movements which are amazing to those who do not know the cause. Very many such indeed every ingenious mechanician will perform by sleight of hand, as if by incantations and jugglery[188].