DIPPING NEEDLE. An instrument for showing the direction of the magnetic force of the earth. It is a magnetic nee dle, furnished with an axis at right angles to its length, and passing as exactly as possible through its centre of gravity, about which it moves in a vertical plane. When a needle thus mounted is placed any where not on the magnetic equator, it dips, or points downwards : and if the vertical plane in which it moves coincides with the magnetic meridian (which is al ways known by means of a variation com pass), the position which it assumes shows at once the direction of the mag netic force ; and the intersection of two or more directions, found by making the experiment at different places, indicates the place of the Magnetic pole. Though the principles on which the dipping needle acts are abundantly simple, its practical construction is found to be exceedingly difficult. It must. be accurately balanced on its axis ; the axis must be placed ex actly horizontal ; the friction mast be diminished to the utmost extent possible; and the adjustments can only be made when the needle is perfectly free from magnetism, and also secured from the effects of the magnetic influence of the earth. It must be subsequently magna tized, and during this process much care is required to guard against derangement. The simple construction is represented in the annexed figure. Tee needle D d consists of a flat oblong piece of steel, tapering to a point at both ends, and hav ing a slender cylindrical axis passed through its centre of gravity. The axis moves freely in circu lar holes made in the lateral horizontal bars lI h, which support a vertical 'circular C C, graduated for the pur pose of showing the inclination of the nee dle to the horizon.
The stand S. T, to which the circle is fixed, is provided with levels, and ad justed to horizontality by means of screws. But in the most improved form of construction of the dipping needle, the axis, instead of being a cylinder, is a knife edge, resting perpendicularly, like the supports of a pendulum, on two agate planes. A needle thus supported, how ever, must necessarily make small oscil lations ; consequently it must be so ad justed that when it points in the direc tion of the magnetic force, the knife edges may be perpendicular to the agate planes. The mean value of the angle of the dip must therefore be known previously to its construction ; but it is the best adapt ed, on account of its delicacy, for ascer taining the minute variations of the dip at the same place. The angle of the dip, like that of the variation, changes its value even at the same Iflace, following of course the motion of the magnetic poles, which, from the observations made by Scoresby, Parry, Ross, and others, in high latitudes, appear to have a motion westward, the annual amount of which is about 11' 4". lu the summer of 1831, Commander Ross, in an excursion from the vessel in which his party were so long detained in the polar seas, reached a spot on time continent of North Ameri ca, which had been calculated to be the position of the magnetic pole. There he found the dip of the needle to be 89° 59', within one minute of the vertical ; and compass-needles suspended in the most delicate manner possible exhibit no pol arity whatever. The latitude of this spot is 70° 171' north, and its longitude 46' 45" west.