Demonstration of direction, or of variation from the true direction, at the same time with declination, by means of only a single motion in water, due to the disposing and rotating virtue.
ix a slender iron wire of three digits length through * a round cork, so that the cork may support the iron in water. Let this water be in a good-sized glass vase or bowl. Pare the round cork little by little with a very sharp knife (so that it may remain round), until it will stay motionless one or two digits below the face of the water; and let the wire be evenly balanced. [239]Rub one end of the wire thus prepared on the boreal end of a loadstone and the other on the southern part of the stone (very skilfully, so that the cork may not be moved ever so little from its place) and again place it in the water; then the wire will dip with a circular motion on its own centre below the plane of the horizon, in proportion to the latitude of the region; and, even while dipping, will also show the point of variation (the true direction being perturbed). Let the loadstone (that with which the iron is rubbed) be a strong one, such as is needed in all experiments on magnetick declination. When the iron, thus put into the water and prepared by means of the loadstone, has settled in the dip, the lower end remains at the point of variation on the arc of a great circle or magnetick meridian passing through the Zenith or vertex, and the point of variation on the horizon, and the lowest point of the heavens, which they call the Nadir. This fact is shown by placing a rather long magnetick versorium on one side a little way from the vase. This is a demonstration of a more absolute conformity of a magnetick body with the earth's body as regards unity; in it is made apparent, in a natural manner, the direction, with its variation, and the declination. But it must be understood that as it is a curious and difficult experiment, so it does not remain long in the middle of the water, but sinks at length to the bottom, when the cork has imbibed too much moisture.
The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.
239 ↑Page 203. This figure of the experiment with the simple dipping needle suspended in water in a goblet is due to Robert Norman. In his Newe Attractiue (London, 1581, chap. vi.) he thus describes it:
"Then you shall take a deepe Glasse, Bowle, Cuppe, or other vessell, and fill it with fayre water, setting it in some place where it may rest quiet, and out of the winde. This done, cut the Corke circumspectly, by little and little, untill the wyre with the Corke be so fitted, that it may remain under the superficies of the water two or three inches, both ends of the wyer lying levell with the superficies of the water, without ascending or descending, like to the beame of a payre of ballance beeing equalie poysed at both ends.
"Then take out of the same the wyer without mooving the Corke, and touch it with the Stone, the one end with the South of the Stone, and the other end with the North, and then set it againe in the water, and you shall see it presentlie turne it selfe upon his owne Center, shewing the aforesay'd Declining propertie, without descending to the bottome, as by reason it should, if there were any Attraction downewards, the lower part of the water being neerer that point, then the superficies thereof."
