DIPPING-NEEDLE. If a magnetic needle be supported so as to be free to MO NA! in a verti cal plane, at most places on the earth's surface, it does not rest in a horizontal position, but has a certain amount of inclination. If the vertical thine in which the needle moves is the meridian of the place, the angle between the needle and the horizontal line is called the dip or inclination of the needle. l'he dip of the mag netic needle at any place can be ascertained with exactness by means of the dipping-needlc. It consists of a graduated virile fixed vertically in a frame, Cthuht.h can be revolved on a horizontal graduated circle. This last is supported on it tripod furnished with leveling screws. At the (intre of the virtual circle. there are two knife edges of agate, supported loy the frame, and parallel to the plane of the circle. The needle rests on these knife-edges by means of two line I (dished evlimh•rs of steel, which are placed act uratilY of the needle. and project at right angles from it. The needle is adjusted -oo as to net\ it with little or no friction; it is so balanced, IlliteenVer, that before it is magne tized it will remain indifferently in any position: after neignetization, therefore, the dip which it shows is wholly due to the magnetic intluenee of the earth.
In order to understand how an observation is made with the dippingmeedle, we must regard the diret-ting fort, of the earth's magnetism ex upon the poles of the needle in any vertical plant. in which it may happen to be, as resolved into two forces. (one acting at right angles to the plane, and the other acting in 1 he id a Ile. There being a corresponding Ibiti Opposite force at pole, \\ it ha Vit I 1111S. twit statical couples siting an the needh•mie tending to turn it at right angles to the plane in Whi•ll it niece., and the tending to bring it round to a position in the plane such that the needle and the forces of the couple may be in a line. In the dipping-needle, the mode of support completely neutralizes the tied of the couples: and the position that the needle takes in any plane is due wholly to second. NI hen the planc of the needle is at right angles to the magnetic meridian, 1.11(• forces of 'inns latter couple act vertically, and luring the needle to t he Sallie position. t hen, gives us t he mean, of determining the magnetic. meridian, for We hay(' Only to circle round till the needle stands at ' to put it in it plane at right angles to that meridian; and then by moving the vernier im the horizontal circle over Mt', we place the tipper circle and needle in (Inc plane of the magnetic meridian. The dip
ping needle thus serves t he purpose of a declina tion needle (q.v.). In bringing the needle round from the plane at right angles to the magnetic meridian. the (lip is less and less, till it become:, least in the plane of that meridian. \Ve might thus also find the magnetic meridian, for it is that plane in which, the dip of the needle is least. 1\•ien the needle is in the plane of the magnetie meridian, the couple which acts in other vertical planes at right angles to them disappears, and the whole force of the terrestrial magnetism :lets at each pole of the needle, forming a volatile which swings the needle round till it stands in a line with itself. The degree on the circle then pointed to by the needle is the (lip at the place of oloser%at ion. Two readings are necessary, for the reason stated tinder the l)t:(.1.1NATioN Niamf thie reading is taken, the needle is then reversed so as to change its supports, rind then a seeonol reading is noted, and the mean of the two gives the correct reading. The position of the needle when the dip is read off is manifestly the same that a needle suspended in air, if that were pos sible, and free to move in any way, would finally assume. In resolving. therefore, the total direc tive force of the earth as we have done above, we must keep in mind that it always acts parallel to the direction of the dipping-needle.
The direction and intensity of the earth's mag netism may he roughly represented by supposing that there is a magnet at the centre of the earth whose length is very small in comparison with the earth's diameter, and which makes an angle of about 20 degrees with the earth's axis of rota tion. A line passing through the earth coincid with the length of the magnet would mark the north and south magnetic poles on the sur face of the earth. The magnetic equator would therefore be a circle around the earth perpen dicular to the magnetic axis, making an angle of about •0 degrees with the geographical equa tor. As We proceed the magnetic equator towards the magnetic poles, the amount of dip increases from zero to 00 degrees, and there is also a considerable change of dip from time to time at any one place. The dip in Paris was 75' when it was first observed in 1671, and from this it had fallen to 65' IS' in 1885, and at the present time is about 65'. See MAGNETRilI, TERRESTRIAL: INCLINATION.