Page:On the Magnet - Gilbert (1900 translation of 1600 work).djvu/208

 Now how much it dips at every horizon may be ascertained in the first place by a contrivance, which, however, is not so easily made as is that in dials for measuring time, in which the needle turns to the points of the horizon, or in the mariners' compass. From a plank of wood let a smooth and circular instrument be prepared, at least six digits in diameter, and affix this to the side of a square pillar, which stands upright on a wooden base. Divide the periphery of this instrument into 4 quadrants: then each quadrant into 90 degrees. At the centre of the instrument let there be placed a brass peg, at the centre of the end of which let there be a small hollow, well polished. To this wooden instrument let a brass circle or ring be fixed, about two digits in width, with a thin plate or flat rod of the same metal, representing the horizon, fixed across it, through the middle of the circle. In the middle of the horizontal rod let there be another hollow, which shall be exactly opposite the centre of the instrument, where the former hollow was made. Afterward let a needle be fashioned out of steel, as versoria are accustomed to be made. Divide this at right angles by a thin iron axis (like a cross) through the very middle and centre of the wire and the cross-piece. Let this dipping-needle be hung (with the ends of the cross resting in the aforesaid holes) so that it can move freely and evenly on its axis in the most perfect æquilibrium, so accurately that it turns away from no one point or degree marked on the circumference more than from another, but that it can rest quite easily at any. Let it be fixed upright to the front part of the pillar, whilst at the edge of the base is a small versorium to show direction. Afterward touch the iron, suspended by this ingenious method, on both ends with the opposite ends of a loadstone, according to the scientifick method, but rather carefully, lest the needle be twisted in any way; for unless you prepare everything very skilfully and cleverly, you will secure no result. Then let another brass ring be prepared, a little larger, so as to contain the former one; and let a glass or a very thin plate of mica be fitted to one side of it. When this is put over the former ring, the whole space within remains inclosed, and the versorium is not interfered with by dust or winds. Dispose the instrument, thus completed, perpendicularly on its base, and with the small versorium horizontal, in such a way that, while standing perpendicularly, it may be directed toward the exact magnetical point respective. Then the end of the needle which looks toward the north dips below the horizon in northern regions, whilst in southern regions the end of the needle which looks toward the south tends toward the centre of the earth, in a certain proportion (to be explained afterward) to the latitude of the district in question, from the æquator on either side. The needle, however, must be rubbed on