CHAP. XXXIII.
On the Varying Ratio of Strength, and of the Motion
of coition, within the orbe of virtue.
hould a very large weight, which at a very small distance is drawn towards a loadstone, be divided into ever so many equal parts, and should the radius of the orbe of magnetick attraction be divided into the same number of parts, the like named parts of the weight will correspond to the intermediate parts of the radius.
The orbe of virtue extends more widely than the orbe of motion of any magnetick; for the magnetick is affected at its extremity, even if it is not moved with local motion, which effect is produced by the loadstone being brought nearer. A small versorium also is turned when a good distance off, even if at the same distance it would not flow towards the loadstone, though free and disengaged from impediment.
The swiftness of the motion of a magnetick body to a loadstone is dependent on either the power of the loadstone, on its mass, on its shape, on the medium, or on its distance within the magnetick orbe.
A magnetick moves more quickly towards a more powerful * stone than towards a sluggish one in proportion to the strength, and [as appears] by a comparison of the loadstones together. A lesser mass of iron also is carried more quickly towards a loadstone, just as also one that is a little longer in shape. The swiftness of magnetick motion towards a loadstone is changed by reason of the medium; for bodies are moved more quickly in air than in water, and in clear air than in air that is thick and cloudy.
By reason of the distance, the motion is quicker in the case of bodies near together than when they are far off. At the limits of the orbe of virtue of a terrella a magnetick is moved feebly and slowly. At very short distances close to the terrella the moving impetus is greatest.
A loadstone which in the outmost part of its orbe of virtue * hardly moves a versorium when one foot removed from it, doth, if a long piece of iron is joined to it, attract and repel the versorium more strongly with its opposite poles when even three feet distant. The result is the same whether the loadstone is armed or unarmed. Let the iron be a suitable piece of the thickness of the little finger.
For the vigour of the loadstone excites verticity in the iron and proceeds in the iron and through the iron much further than it extends through the air.
The vigour proceeds even through several pieces of iron (joined * to one another end to end), not so regularly, however, as through one continuous solid.
Dust of steel placed upon paper rises up when a loadstone is moved near above it in a sort of steely hairiness; but if the loadstone is placed below, such a hairiness is likewise raised.
Steel dust (when the pole of a loadstone is placed near) is cemented * into one body; but when it desires coition with the loadstone, the mass is split and it rises in conglomerated parts.
But if there is a loadstone beneath the paper, the mass is split in the same way and many portions result, each of which consists of very many parts, and remains cemented together, as individual bodies. Whilst the lower parts of these pursue greedily the pole of the loadstone placed directly beneath, even they also are raised up as magnetick wholes, just as a small iron wire of the length of a grain or two grains of barley is raised up, both when the loadstone is moved near both beneath and above.