On the lodestone and magnetic bodies and on the great magnet the earth : a new physiology demonstrated with many arguments and experiments / William Gilbert of Colchester, physician of London ... a translation by P. Fleury Mottelay.

  • Gilbert, William, 1544-1603.
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: On the lodestone and magnetic bodies and on the great magnet the earth : a new physiology demonstrated with many arguments and experiments / William Gilbert of Colchester, physician of London ... a translation by P. Fleury Mottelay. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    the terrella ; for there is error when the needle is long or placed at a distance, as it has a truly proportionate movement only at the superficies of the terrella. But were the quadrant —being remote from the terrella—to be moved into its sphere of influence toward the pole on a circle concentric with the terrella, then the needle would indicate on the quadrant the degrees of dip in ratio and symmetry with that circle, not with the terrella. CHAPTER IV. OF A SUITABLE LENGTH OF NEEDLE ON THE TERRELLA FOR SHOWING THE DIP. WHEN it is sought to define the dip by means of a dip- indicating instrument on the earth itself, we may use either a short versorium or one ever so long, provided only the mag- netic property of the loadstone with which it has been stroked is able to pervade its whole substance and length. For the greatest length of a versorium, as compared with the earth's diameter, is insignificant and has no ratio perceptible by sense. But on a terrella, or on a plane nigh a meridian of a terrella, a short needle is required, one barley-corn's length ; for longer versoria (because they reach farther), in the first degrees of dip, descend suddenly and irregularly, and turn to the body of the terrella. For example, as soon as the long versorium in the figure is moved onward from the equator A to C, it lays hold of the stone with its point C as though with a long outspread wing, when the point reaches the parts around B, which give it a greater revolution than those at C. And the ends of