DECLINATION NEEDLE. When a magnetic needle is suspended or made to rest on a point so as to be free to move in a horizontal plane, it finds its position of rest in a line joining two fixed points on the horizon; and when made to leave that position, after several oscillations, it returns to it again. At certain places on the earth's surface, these two points are the n. and s. points of the horizon; but generally, though near, they do not coincide with these. A vertical plane passing through the points on the horizon indicated by the needle, is called the magnetic meridian, in the same way that a similar plane, the n. and s. points, is known as the astronomical meridian of the place. The angle between the magnetic and astronomical meridian is termed the declination or variation of the needle. The declination is e. or w., according as the magnetic n. lies e. or w. of the true north. Instruments for determining magnetic declination are called declination needles, or declinometers. In this instrument there arc two things essential—the means of ascertaining the astronomical meridian, and a needle for showing the magnetic meridian. The common form of declinometer -con sists of a tripod provided with leveling screws, and supporting a pillar, to which is fixed a graduated azimuthal circle. A compass-box, with vernier attached, moves on the azimuthal circle by means of a pivot at the top of the pillar. Two uprights are fixed to the 5j.de of the compass-box, on which rests the axis of a telescope. A graduated arc is irxed to the got om of one of the uprights, and the angle of elevation of the teles cope is marked by a vernier on the arm attached to the axis of the telescope. A level is also hung on the axis of the telescope, for adjusting the instrument. Inside the
compass-box is another graduated circle, the line joining the zero-points of which is par allel to the axis of the telescope. All the fittings are in brass or copper, iron, of course, being unsuitable. The compass-box and telescope move round as one piece on an axis passing through the center of the azimuthal circle. When an observation is made, the telescope is pointed to a star whose position with regard to the astronomical merid ian is known at the time of observation. The telescope"with the compass-box is then brought the proper number of degrees on the azimuthal circle, until its axis is in the meridian of the place. If, when the telescope is in this position, the n. end of the needle stand at the zero-point of the inner circle, the declination would be 0*; but if it lie c. or w. of this point, the declination is shown by the degree at•which the needle stands. It is difficult to construct a needle so that the line joining its poles exactly coincides with the line joining its visible extremities. If this coincidence be not per fect, the geometrical axis of the needle according to which the reading is made lies to the right or left of • the magnetic the true reading. To rem edy this, the needle is so made that it can rest either on its lower or upper surface. In Sliding the true reading, the position of the needle is marked, and then it is turned upside down, and again marked, the mean of the two readings giving the true one. The declination of the needle may be also ascertained by the " dipping needle "(q.v.). The ordinary compass which must be used by making allowance for declination, is a decli nation compass. See MAGNETISM.