DECLINATION NEEDLE. An instrument for determining magnetic declination. In this instrument, which is also known as a deelino meter, there are two things essential—the means of ascertaining the astronomical meridian, and a needle for showing the magnetic meridian. The common form of declinometer consists of a tri pod. provided with leveling screws, and support ing a pillar, on which is fixed in a horizontal plane a graduated circle. A compass-box with vernier attached is placed on this divided circle and is free to move about a pivot at the top of the pillar. Two uprights arc fixed to the side of the compass-box, on which rests the axis of a telescope. A graduated are is fixed to the bottom of the uprights, and the angle of eleva tion of the telescope is read with a vernier on the arm attached to the axis of the telescope. A level is also hung ou the axis of the telescope. for adjusting the instrument. Inside the com pass-box is another graduated circle, the line joining the zero-points of which is parallel to the axis of the telescope. All the fittings are of brass or copper. iron, of course, being unsuitable. The compass-box and telescope move round as one piece on an axis passing through the centre of the divided circle. When an is made, the telescope is pointed at a star position with regard to the astronomical merid ian is known at the time of observation. The telescope with the compass-box is then revolved the proper mini her of degrees until its axis is in the meridian of the place. If. when the tele
scope is in this position, the north end of the needle stands at the zero-point of the inner circle. the declination would be 0 = zero; but if it lie east or west, the declination is shown by the degree at whicrli 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 perfect, the geometrical axis of the needle, pecording, to which the reading is made, lies to the right or left of the magnetic axis, and consequently of the One reading. To remedy this, the needle is so made that it can rest either on its lower or upper surface. Tn finding the true reading. the position of the marked, and then it is turned upside down, and again marked, the mean of the two readings giving the true one. The most accurate method of determining declination is with the magnet ometer q.v. , and theodolite. This instrument is employed by the Division of Terrestrial Mag netism of the United States Coast. and Geodetic Survey, and the apparatus and results are de scribed in the annual reports. The declination of the needle may be also ascertained by the 'dipping needle' (q.v.). The ordinary surveyor's compass, which must be used by making allow ance for declination, is a declination compass. See COM PASS; ENGINEERING I NSTRU ENTS _MAGNETISM, TERRESTRIAL : and MAGNETOMETER.