Dioscu´ri. See Castor and Pollux.
Dios´pyros, a large genus of trees or shrubs, natives of the warmer regions of the world, nat. ord. Ebenaceæ. The trees of this genus supply ebony wood. That from Ceylon is the wood of D. Ebĕnum; from India, of D. melanoxylon and other species; and that from Mauritius, D. tesselaria. The Chinese date-plum (D. kaki) is an apple-like tree which produces large red fruits resembling tomatoes. In China and Japan this tree is as important as the apple is in Northern Europe.
Dip, of the horizon, the angle of depression of the visible horizon at sea below the true horizontal direction, due to the height of the eye above the level of the sea. The dip in minutes of arc is approximately equal to the square root of the height in feet.—Dip, magnetic, or Inclination, is the angle which a magnetic needle free to move in a vertical circle in the magnetic meridian makes with the horizon. See Dipping Needle.
Dip, in geology, the inclination or angle at which strata slope or dip downwards into the earth. The degree of inclination or amount of the dip, which is easily measured by a clinometer, is the steepest angle made with a horizontal plane by a line drawn in the surface of the stratum. The line of dip is hence perpendicular to the intersection of the stratum with the horizontal, which is called the strike.
Dip Circle. See Dipping Needle.
Diphthe´ria is an acute infectious disease characterized by the formation of membrane in the throat and air-passages, and associated with severe disturbances affecting especially the heart and nervous system. It is due to a bacillus described by Koch in 1883. It is essentially a disease of the early years of life, and the period between two and twelve years covers the vast majority of cases. The commonest modes of infection are direct and indirect contact, infected milk, and defective drains. Of late years, the domestic cat has been held to be a source of infection. The disease runs a rapid course, beginning with fever, headache, chilliness, lassitude, and occasionally vomiting, while usually there is early complaint of sore throat. The membrane, which appears on the side of the throat, is usually of a dirty yellowish-white colour. It may be limited to a small area, but usually, if untreated, it would spread extensively over the throat, involving the palate and uvula. Diphtheria beginning in the larynx (windpipe) is what is popularly called croup (q.v.). There is danger of death in severe cases during the first few days from early heart failure, and almost any time during convalescence late heart failure may occur. Late heart failure is one of the forms of post-diphtheritic paralysis which arise from disturbances of the nervous system. The other common varieties are paralysis of the palate, of the pharynx, of the eye muscles, and of the respiratory muscles. An effective treatment is found in diphtheric antitoxin, which should be administered as early as possible in the disease. It is given under the skin, and the dose is regulated by the severity of the attack.—Bibliography: W. F. Litchfield, Diphtheria in Practice; W. R. Smith, Harben Lectures.
Diphthong (Gr. di-, double, and phthongos, sound), a coalition or union of two vowels pronounced in one syllable. In uttering a proper diphthong both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in void, bough. The term improper diphthong is applied to the union in one syllable of two or more vowels of which only one is sounded, as in bean.
Di´phyodont, a term applied to those animals which develop two sets of teeth, a deciduous or milk set, and a permanent set—as distinct from the monophyodonts, which develop only one set. The majority of mammals are diphyodont, though the number of teeth replaced may vary: thus in man twenty teeth of the adult are preceded by a milk set.
Diplacan´thus, a genus of ganoid fishes, found only in the Old Red Sandstone. They have small scales, a heterocercal tail, and two dorsal fins with a strong spine in front.
Diplei´doscope, an instrument for indicating the passage of the sun or a star over the meridian, by the coincidence of two images of the object, the one formed by single and the other by double reflection. It consists of an equilateral hollow prism, two of whose sides are silvered on the inside so as to be mirrors, while the third is formed of glass. The prism is adjusted so that one of the silvered sides shall be exactly in the plane of the meridian, and the transparent side towards the object.