DIPPING-NEEDLE. 2 reading is noted, and the mean of the two gives the correct reading. The position of the needle when the dip is read otT 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 oartli 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 tiie earth's mag- netism may be roughly rc])resented 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 eartli's a.xis of rota- tion. A line passing through the earth coincid- ing 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 niag:ietic axis, making an angle of about 20 degrees with the geographical equa- tor. As we proceed from_ the magnetic equator towards the magnetic poles, the amount of dip increases from zero to 90 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" 18' in 1885, and at the present time is about 65". See Magnetism, Terrestrial: I>clix.tiox. DIPROTODON (Xeo-Lat., from Gk. Si-,di-, doulile + zpCiror prOtos, first + 6duv, Ionic for dSois, odous, tooth ) . An extinct herbivorous marsupial, of the size of a rhinoceros, related lo the kangaroos and phalangers, and found fos- sil in the Pleistocene deposits of Australia. The skull is heavy, about three feet in length, and has a peculiar dentition. There are no canine teeth; the inner pair of upper incisors are enlarged, elongated, and chisel-shaped, like those of rodents. The lower incisors, two in number, are likewise elongated to meet those of the upper jaw, and, like these, they grew from persistent pulps : i. e., they continued to grow from their roots as they were worn away at their cro^vns, after the fashion of the incisors of rodents. Another related but somewhat smaller genus. Nototherium, the re- mains of which are found with those of Diproto- don. has a very similar dentition. These two fossil genera are the giants among the marsu- pials, .s-cc Maksupialia. DIP'SACUS. See Teasel. DIP'SAS (Ok. SiVof, venomous snake, whose bite causcrl intense thirst, from Shpa, dipsa, thirst). A genus, or family, of tropical serpents — the nocturnal tree-snakes. See Tree-snake. DIPSOMANIA (Keo-Lat., from Gk. Sifa, di/w/. thirst -f- /jav'ia, ?»ania, madness) . A va- riety of periodic insanity. The patient mani- fests, during certain periods, an xmcontroUable desire for alcoholic liquors, and drinks to great excess, while during the inter'als he abstains entirely from the use of liquor and seems men- tally sound. The dipsomaniac is not a drunk- ard, nor a 'moderate drinker.' He often experi- ences a positive aversion for the smell and taste of liipiors, but when the imperative appetite seizes him. after a period of abstinence and of orderly, refined, and irreproachable conduct, he abandons himself to unrestrained indulgence. Dipsomania is a degenerative insanity; that is. 7 DIPTYCH. an insanity dependent upon an hereditary or acquired constitutional condition. The outlook for recovery is bad. DIP'SOSAU'IITTS (Xeo-Lat., from Gk. «M,n, dipsa, thirst -+- aavfjoc, saiiros, lizard). A small, remarkable iguanid lizard (rJipsosaiiriis dor- salis) of the Colorado and ilohave deserts and Lower California. It has a very small head, a ni^AD op DIPSOSAITRUS DOUSALIS. conspicuous ridge of keeled scales along the back, and a long, compressed tail. It is sand-colored, w ith obscure blotches on the sides, and the sides and legs streaked with broH-nish red. It is a strict vegetarian, feeding wholly on buds and flowers. Its nearest ally is the collared lizard (q.v.). DIP'TERA (Neo-Lat. nom pi., from Gk. Si-r- Tepoi dipterox, two-winged, from dc-, di-, double -+- ■!v-ep6v ptcron, wing). An order of insects, the flies, characterized prominently by the de- velopment of only two wings, which are the front pair and membranous, while the posterior pair is represented by two small central rods known as halteres or 'poisers;' and by having mouth-parts fitted only for sucking and piercing, but not for gnawing. Their metamorphosis is complete. See Fly. DIP'TERTJS (Neo-Lat., from Gk. St-repoc, diplcros, two-winged). A genus of fossil dipnoan fishes of the family Ctenodontidae, found in the Old Red Sandstone of the Scottish Devonian. The body is small, rather slender, tapering gijuiually toward the heterocercal tail, and is covered by circular enameled scales. The head is low and blunt anteriorly, and the skull-bones are small and numerous. There are a pair of pectoral fins and two dorsal fins, situated posteriorly, opposite the pelvic and anal fins, and all of these have their bases covered for a considerable distance with the enameled scales of the bodv. See LrxG- DIPTYCH, dip'tik (Lat. diptycha, nom. pi. from Gk. oi-rvxoi;. dipiychos. folded double, from Si-,di-, double -{-Trrvxi, pt yche, iold, from Trrvaaew, ptyssehi, to fold). A double writing-tablet, or two writing-tablets which could be folded to- gether. Herodotus speaks of such a t'ablet. made of wood and covered with wax. It was in the later Roman time, however, that they were most used, and those which have been preserved belong chiefly to the period when classical was merging into mediieval life. The beautiful carving with which they are often covered on the outside, con- sequently, represents not unfrequently a combina- tion of classical and of Christian subjects. Ivory and metal were sometimes employed fn place of wood ; but the construction was always the same, the wax with the writing being in the inside. T'nder the emperors, diptychs were distinguished into consular and ecclesiastical. The former, which were presented by the consuls and other
