Magnetical NEEDLE, in Navigation, a needle touched with a loadstone, and sustained on a pivot or centre; on which playing at liberty, it directs itself to certain points in or under the horizon; whence the magnetical needle is of two kinds, viz. horizontal or inclinary. See the article MAGNET.
Horizontal needles are those equally balanced on each side of the pivot that sustains them, and which, playing horizontally with their two extremities, point out the north and south points of the horizon. For their application and use, see the article COMPASS.
In the construction of the horizontal needle, a piece of pure steel is provided; of a length not exceeding six inches,
Needle. inches, lest its weight should 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 folded on, whose 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 hemisphere 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 method of giving the needle its verticity or directive faculty has been shown already under the article MAGNET; but if, after touching, the needle be out of its equilibrium, something must be filed off from the heavier side, till it balance evenly.
Needles in sea compasses are usually made of a rhomboidal or oblong form; we have given their structure already under the article COMPASS.
The needle is not found to point precisely to the north, except in very few places; but deviates from it more or less in different places, and that too at different times; which deviation is called its declination or variation from the meridian. See the article VARIATION.