Page:The New International Encyclopædia 1st ed. v. 06.djvu/324

* DIPPER. 276 DIPPING-NEEDLE. loni of the water. They carry their rather short t;iils elevated alter the inaiiiier of wrens, wliieh ihev al>^o resemble in their "fivqueiit beeks,' or dipping of the head accompanied with an up- ward jerkinj; oi the tail. The common European dipper, water-ousel, or water-crow (Ciiwlus 071/0- iicun), is a bird smaller than any of the British thrushes, of a j;enerally dark-brown color, with throat and upper ])art of breast pure white. It is found eliielly in hilly or mountainous districts, and is not gregarious. The dijjper never fails to r.ltraet iu)tice, as it sits u])on some stone in the midst of or beside the stream, its white breast rendering it conspicuous as it repeats the move- ments from which it derives its name. It builds a very curious nest of interwoven moss, domed and with the entrance in the side, usually in some mossy bank close >y a stream, and often near or under a cascade. The eggs are pure white. The assertion that the dipper walks with- out apparent nuiscular elTort at the bottom of the water is incorrect: its feet are not well formed for walking, and it scrambles about under water by help of its short wings. The st.itement, also often made, that it oats the spawn of salmon and other fishes, in the belief of which it is nmch persecuted in Scotland, has proved erroneous. A!)out twelve species of dipper are known, one of which {Ciiiclii.i Mcxicaiius) is found in the moun- tains of westt>rn Xorlh .Vmerica from Alaska to Mexico; this lacks the pure white under parts of the European species, but otherwise resembles it closely. Its habits have been eloquently de- scribed by Muir, in "The IIumming-Hird of the Californian Waterfalls," an illustrated article in Scribner's Monthly (New York, 1878). Con- sult, also, Keyser, Birds of the Rockies (Chi- cago. I!t02). ( 2 ) The butHehead, or spirit-duck. See Buffle- lIKAll. DIPPING-NEEDLE. If a magnetic needle be svipi)orted so as to be free to move 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 inclinaticm. If the vertical plane in which the needle moves is the magnetic meridian of the ])lace, the angle between the needle and the horizontal line is called the dip or inclination of the needle. The dip of^the mag- netic needle at any place can be ascertain<'d with exactness by means of the dipping-needle. It consists of a graduated circle fixed vertically in a frame, which can be revolved on a horizontal graduated circle. This last is supported on a tripod furnished with leveling screws. At the centre of the vertical circle, there are two knife- edges of agate, supported by the frame, and l>arallcl to the plane of the circle. The needle rests on these knife-edges by means of two fine polished cylinders of steel, which are placed accurately at the centre of the needle, and project at right angles from it. The needle is adjusted so as to move with little or no friction; it is so balam'cd, moreover, that before it is magne- tized it will remain indilTcrently in any position; after magnetization, therefore, the dip which it shows is wholly due to the magnetic influence of the earth. In order to understand how an observation is made with the dipping-needle, we inust regard the din^ting force of the earth's magnetism ex- erted u])on the poles of the needle in any vertical plane in which it may happen to l>e. as resolved into two forces, one acting at right angles to the plane, and the other acting in the plane. There being a corresponding but opposite force at each pole, wc have thus two statical couples acting on the needle — one tending to turn it at right angles to the plane in which it moves, and the other tending to bring it round to a position in the plane such that the needle and the forces of the cinijjlc may be in a line. In the dipping-needle, the mode of support completely neutralizes the first of the couples; and the |)Osilinn that the needle takes in any i)lane is due wholly to the second. When the plane of the needle is at right angles to the magnetic meridian, the forces of this latter couple act vertically, and bring the needle to the same position. This, then, gives ua the means of determining the magnetic meriilian, for we have only to bring the vertical circle round till the needle stands at 'M" to ])ut it in a ]danc at right angles to that meridian: and then by moving the vernier on the horizontal circle over 90", we place the up|)er circle and needle in the ])lane of the magnetic meridian. The dip- ping-needle thus serves the purpose of a declina- tion needle (q.v.). In bringing the needle round from the ])lane at right angles to the magnetic meridian, the dip is less and less, till it becomes least in the plane of that meridian. We night thus also find the magnetic meridian, for it is that plane in which the dip of the needle is least. When the needle is in the ])lane of the magnetic meridian, the couple which acts in other vertical planes at right angles to them disappears, and the whole force of the terrestrial nuignetism acts at each jiole of the needle, forming a couple IMPCINO-.NEEDLE — KKW l'.TTK«N. 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 dip at the place of obserxalion. Two readings arc necessary, for the reason stated under the Ofxiination NKF.ni.K. One reading is taken, the needle is then reversed so as to changi' its supports, and then a secopd