NIZDIA, =poetical, in navigation, a needle touched with a loadstone, and sustained on a pivot or centre ; on which, playing at liberty, it directs itself to cer tain points in or under the horizon ; whence the magneticsl needle is of kinds, viz. horizontal and inclinatory.
Horizontal needles are those equally balanced on each side the pivot that sus tains them ; and which, playing horizon tally with their two extremes, point out the north and south points of the horizon. In the construction of the horizontal needle a piece of pure steel is provided, of a length not exceeding six inches, lest its weight impede its volubility, very thin, to take its verticity the better, and not pierced with any holes, or the like, for ornament sake, which prevent the equable diffusion of the magnetic virtue. A perforation is then made in the middle of its length, and a brass cap or head sol dered on, wbose inner cavity is conical, so as to play freely on a style or pivot headed with a fine steel point. The north point of the needle in our hemi sphere is made a little lighter than the southern ; the touch always destroying the balance, if well adjusted before, and rendering the north end heavier than the south, and thus occasioning the needle to dip.
The needle is not found to point pre cisely to the north, except in very few places, but deviates from it more or less in different places, and that too at dif ferent times, which deviation is called its declination or variation from the meri dian.
Inclinatory or dipping-needle, a mag netical needle, so hung, as that, instead of playing horizontally and pointing out north and south, one end dips or inclines to the horizon, and the other points to a certain degree of elevation above it. Or
a dipping-needle may be defined to be a long• straight piece of steel, every way poised on its centre, and afterwards touched with a loadstone, but so con trived as nut to play on the point of a pin, as does the common horizontal nee dle, but to swing in a vertical plane, about an axis parallel to the horizon ; and this to discover the exact tendency of the power of magnetism. See MAGNETISX.
TO find the longitude or latitude by the dipping-needle. If the lines of equal dip below the horizon be drawn on maps or sea charts from good observations, It will be easy, from the longitude known, to find the latitude, and from the lati tude known, to find the Ibngitude, either at sea or land. Suppose, for example, you were travelling or sailing along the meridian of London, and found the angle of dip with aneedle of one foot to be 75°, the chart will show that this meridian and the line of dip meet in the latitude 53° 11', which' is. therefore, the latitude sought. See LATITUDE. Or suppose you were travelling or sailing along the paral lel of London, that is, in 51° 32' north latitude, and you find the angle of dip to be 74°. The parallel and the line of this dip will meet in the map in 1° 46', of east longitude from London, which is there fore the longitude sought.