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 inclinatory. 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 extremes, 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 soldered 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.
Surgeons Needles are generally made crooked, and their points triangular; however, they are of different forms and sizes, and bear different names, according to the purposes they are used for.
The largest are needles for amputation; the next, needles for wounds; the finest, needles for sutures. They have others, very short and flat, for tendons; others, still shorter, and the eye placed in the middle, for tying together of vessels, &c. Needles for couching cataracts are of various kinds; all of which have a small, broad, and sharp point or tongue, and some with a sulcus at the point. Surgeons have sometimes used two needles in this operation; one with a sharp point for perforating the coats of the eye, and another with a more obtuse point for depressing or couching the opaque crystalline lens; but care should be taken in the use of any of these, that they be first well polished with cloth or leather, before they are applied to the eye.
Mr Warner observes, that the blade of the couching needle should be at least a third part larger than those generally used upon this occasion, as great advantages will be found in the depressing of the cataract, by the increased breadth of the blade of that instrument. The handle, also, if made somewhat shorter than usual, will enable the operator to perform with greater steadiness than he can do with a larger handled instrument.
It is to be observed, that needles of silver pierce more easily in stitching arteries after an amputation, than those made of steel.