Chap. vii. On the Potency of the Magnetick Virtue, and on its nature capable of spreading out into an orbe.
rom about a magnetical body the virtue magnetical is poured out on every side around in an orbe; around a terrella; in the case of other shapes of stones, more confusedly and unevenly. But yet there exists in nature no orbe or permanent or essential virtue spread through the air, but a magnet only excites magneticks at a convenient distance from it. And as light comes in an instant (as the opticians teach), so much more165 quickly is the magnetick vigour present within the limits of its strength; and because its activity is much more subtile than light, and does not consent with a non-magnetick substance, it has no intercourse with air, water, or any non-magnetick; nor does it move a magnetick with any motion by forces rushing upon it, but being present in an instant, it invites friendly bodies. And as light strikes an object, so a loadstone strikes a magnetick body and excites it. And just as light does not remain in the air above vapours and effluvia, and is not reflected from those spaces, so neither is the magnetick ray held in air or water. The appearances of things are apprehended in an instant in mirrors and in the eye by means of light; so the magnetick virtue seizes upon magneticks. Without the more intangible and shining bodies, the appearances of things are not seized or reflected; so without magnetical objects the magnetick power is not perceived, nor are the forces thus conceived sent back again to the magnetick substance. In this, however, the magnetick power excels light, in that it is not hindered by any opaque or solid substance, but proceeds freely, and extends its forces on every side. In a terrella and globe-shaped loadstone the magnetick power is extended outside the body in an orbe; in a longer one, however, not in an orbe, but it is extended in an ambit conformably to the shape of the stone. As in the somewhat long stone A, the vigour is extended to the ambient limit F C D, equidistant on every side from the stone A.
165 Page 77, line 2. Page 77, line 2. multo magis.— This is an à fortiori argument. It is interesting to find Gilbert comparing the velocity of propagation of magnetic forces in space with the velocity of light. The parallel is completed in line 13 by the consideration that as the rays of light require to fall upon an object in order that they may become visible, so the magnetic forces require a magnetic object in order to render their presence sensible.
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