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Example, Tetragona hispidum (Quoy and Gaimard).

[Tetragona hispidum.]
1. Tetragona hispidum : 2, 3, 4, details of the same.

M. de Blainville is of opinion that this is only the pos

terior or inferior natatory organ of a true Diphyes. Sulculeolaria.

Body ? subcartilaginous, transparent, elongated, cylindróid, traversed throughout its length by a very large furrow, bordered with two membranes, truncated at the two extremities, with a posterior aperture, with appendicular lobes on its circumference and leading into a very long and blind cavity.

Example, Sulceolaria quadrivalvis (Lesueur). Locality, Mediterranean (Nice).

(Sulculcolaria quadrivalvis.]

A genus characterized by De Blainville, who found it established in the figures of Lesueur, from those figures; but the former is strongly inclined to believe that the genus is founded on the part of an animal, and not on an entire one. If these bodies should turn out to be merely organs or parts, M. de Blainville thinks they ought to belong to the genus Calpe of Quoy and Gaimard.

Galeolaria.

Body gelatinous, rather firm, perfectly regular, symme: trical, subpolygonal or oval, compressed on the sides and furnished with two lateral rows of extremely fine cirrhi. A large posterior aperture pierced in a sort of diaphragm with appendicular lobes, binary above, leading into a large cavity with muscular walls. An ovary at the anterior superior surface, coming out by a mesial and bilabiated orifice.

Example, Galeolaria australis, Beroides australis (Quoy and Gaimard).

[Galeolaria australis.]

Known to M. de Blainville from the drawings of Lesueur, who gave the form the name of Galeolaria, which the former adopts in preference to Beroides, the term employed by Quoy and Gaimard. Galeolaria however is the name given by Lamarck to a genus of his Serpulidae. From the manuscript memoir of Quoy and Gaimard lent by them to De Blainville, he ascertained the peculiarity of the two rows of cilia on each side. Botta sent him also in spirit

many individuals obtained in the course of his circumnavigation. It seemed to De Blainville that these animals differed really from the Diphydae, and approached the Beroes. To confirm this approximation it would have been necessary to find the posterior aperture of the internal canal, of which, he remarks, no observer has spoken; but it appears to him that the existence of the two series of cirrhi, their relation with a canal which follows their root, the distinct and muscular walls of the cavity, and the position of the ovary, are sufficient to show in these animals a passage at least towards the Beroes.

Rosacea. Body free, gelatinous, very soft, transparent, suborbicular, with a single terminal aperture at one of the poles leading into an oval cavity which communicates with a depression, whence proceeds a cirrhigerous and oviferous production. Example, Rosacea Ceutensis (Quoy and Gaimard).

[Rosacea Ceutensis.]

Eschscholtz unites this genus with those of Calpe and Abyla under the first appellation. , De Blainville, who states that he only knows the form from the figures and description given by Quoy and Gaimard, is at a loss to determine positively what it is, but he supposes it to be a Physsophora rather than a Diphyes.

Noctiluca. Body free, gelatinous, transparent, spheróidal, reniform, with a sort of infundibuliform cavity, whence proceeds a proboscidiform, contractile production. Example, Noctiluca miliaris, Lamarck.

[Noctiluca miliarie J

M. † a doctor of medicine, while investigating the cause of the phosphorescence of the sea-water at Hávre, appears to have been the first who observed and called attention to the genus Noctiluca, which he described and figured in the memoir that he communicated to the class of sciences of the French Institute. Its size hardly equals that of a small pin's head, and it is as transparent as crystal; he found it very common in the basins at Hávre, sometimes in such abundance as to form a considerably thick crust (crosite assez epaisse) on the surface of the water. Lamarck adopted the genus, placing it between Beroe and Lucernaria, which last, in his system, immediately precedes Physsophora. To these minute animals #. Surriray attributes the phosphorescence of the sea at aWre. M. de Blainville states that he has often had occasion to observe these minute beings with Dr. Surriray, aided by the microscope. ... “It appeared to me,’ says De Blainville, speaking of Noctiluca miliaris, “nearly regularly spherical, but somewhat notched (fendu), or excavated on its anterior part so as a little to resemble a cherry. From the middle of the excavation proceeds a sort of long cylindrical ten; tacle diminishing little in size throughout its extent, and terminating in an obtuse extremity. During life this organ moves in all directions somewhat after the manner of an elephant's trunk (ense repliant, un peu a la manière de la trompe de l'elephant). It seemed to me, in fact, to be composed of annular fibres and traversed by a canal throughout its length, so that it may be supposed to be terminated by a sucker. The body is enveloped in a transW. membrane, forming sometimes irregular plaits. ithin may be perceived a kind of funnel-like opsophagus

(espèce d'oesophage en entonnoir) commencing anteriorly

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towards the proboscis and terminating posteriorly by a sort of spherical stomach. I was unable to determine whether there was an intestinal canal with an anal opening. In some individuals, but, as it would appear, at a certain period of the year only, may be seen in the interior many groups, or small masses irregularly placed, and composed of a transparent envelope, containing small globules of blackish brown, which M. Surriray considers to be eggs. At a more advanced period, which M. Surriray supposes to be that of spawning, the water becomes of a red colour (d'un rouge lie de vin), and then there are found a certain number of individuals which have the proboscidiform production twice its usual length (du double plus long), and which he regards as newly-born animals. The general movements of these small animals appear to be very slow, and are essentially executed by means of the species of trunk which is continually moving from right to left. M. Surriray, who had frequently occasion to .. them, has seen them sometimes disencumber themselves entirely of their membranous envelope even to the tentacula. During life the Noctiluca are excessively phosphorescent, and I have verified with M. Surriray the fact that at Hávre the phosphorescence of the sea is owing to these animals; also, that in |. it through a strainer (à travers une étamine), it oses this property, which is much the strongest in warm and stormy weather, much weaker in the winter, and null under a west wind.” De Blainville remarks that though he arranges this animal provisionally in this section, he is far from considering that it is its true place, and that it seems to him, in fact, to have much relation with that form of which MM. de Chamisso and Eisenhardt have made their genus Fagellum, and which MM. Quoy and Gaimard have also designated under a particular denomination: he asks, in conclusion, whether Noctiluca may not be an animal near the Cucubali and Cuculli, whose natatory organs have been reduced to the membranous envelope?

Doliolum.

Body? gelatinous, hyaline, cylindrical, truncated, and equally attenuated at the two extremities, which are largely opened and without apparent organs.

Example, Doliolum Mediterraneum (Otto).

[Doliolum Mediterraneum.]

M. Otto describes the organism on which he has established this genus as swimming by ejecting and absorbing the water by means of the alternate dilatation and contraction of its two orifices. M. Delle Chiaje (Mem., tom. iii.) seems inclined to believe that the }}. of Otto is merely a fragment of a species of Holothuria, which he names Holothuria inhaerens. De Blainville observes that if Otto's description of the motion, &c., above stated, be correct, it is probable that the animal is a true Biphore; but if, by any chance, there should be but one opening, then it would be the organ of some Physsophora, which would agree better with the total absence of internal organs. M. de Blainville's ‘Manual' was published in 1834, and in his “ Nouvelles Additions et Corrections,’ dated at the Paris Museum, December, 1836, he declares his persistence in the belief that the Physograda, Dyphidae, and Ciliograda, ought not to be comprised in the type of the Actino30airia, but that they ought to form an ‘entretype, under the denomination of Malactinozoaria, indicating that they are, so to speak, intermediate between the Mollusca and the Radiata. With regard to the Diphydae, in particular, he remarks that since the appearance of his “Manual,” MM. Quoy and Gaimard have published their observations on this group of animals, and that they have abandoned the different generic distinctions (coupes géneriques) which they had established in their first memoirs; j in fact, which scarcely rested on more than the difference \f form and the proportion of the natatory organs. They have, moreover, observed that their polymorphous Biphore (Uranie, Zoolog., pl. 73, fig. 4) is certainly nothing more than a part of their Diphyes Abyla. M. de Blainville then continues thus:–“The structure of

the Physsophora, which I have named Diphysu, by reason of the existence of two natatory organs only, which are median and placed one before the other, and of rows of cartilaginous squamellae upon the root of the cirrhigerous productions, does not permit a doubt of the great relationship which exists between the Diphyes, properly so called, and the Physograda ; and that these two great genera ought to be united under the same family, as has been previously stated. M. Brandt has proposed to establish two subgenera only among the Diphyes, the first consisting of those in which the cartilaginous scales of the cirrhigerous production are scattered or distant, as in Diphyes dispar, and the second, which he names Diphyomorpha, in which the scales are so close-set as to be imbricated, as is seen in the new species observed by Mertens, and named by him Diphyes Stephanomia. Among the genera incerta sedis, which, wrong or right, have been connected with Physsophora or Diphyes, without even being very certain that they are animals, we shall cite the two following genera intentionally omitted in our work.' De Blainville then mentions the following: CUPULITEs (Quoy and Gaimard), placed among the Physsophorae, whose capsules are .# on each side of a very long axis, established on an organized body, figured pl. 87, fig. 4–16 in the zoological part of the Voyage of the Uranie. Not having met with this animal in their second voyage, MM. Quoy and Gaimard doubt (Astrolabe, Zoolog., t. iv. p. 53 n.) whether it is an incomplete Physsophora or a Stéphomonise (Stephanomia 2) with hollow natatory organs. Cuvier places the genus between Hippopus * and Racemis. PolytomA (Quoy and Gaimard, 1. of the Uranie, pl. 87, fig. 12, 13), which may be defined to be an oval mass of globular trivalvular corpuscles (corpuscules globuleux comme trivalves), and which MM. Quoy and Gaimard conceive to be rather a Biphore than a Physograde. TETRAGoNA (p. 10), Quoy and Gaimard, Zool. of the Uranie, pl. 86, fig. 11). This the authors themselves (Astrolabe iv. p. 103) have recognized as being nothing more than the posterior point of Diphyes hispida. RAcEMIs (Delle Chiaje, Cuvier), figured by Delle Chiaje, Mem. tab. 50, f. 11, 12, and described as a globose vesicle endowed with a very quick motion, and disposed towards an ovate shape; but, observes De Blainville, the figures and description are too incomplete to afford a supposition of what it is; in fact, Delle Chiaje confines himself to stating that his Racemis ovata executes all the rotatory and rapid motions at the surface of the water, and that those of each vesicle are so lively that it has been absolutely impossible to perceive the aperture with which, according to Delle Chiaje, they are provided. Cuvier only adds to the description of Delle Chiaje, who also places Racemis near the Physsophorac, a . membrane with which each vesicle is furnished. M. De Blainville concludes by observing that he had seen a drawing, by M. Laurillard, which had been taken at Nice from one of these organized bodies while alive, and that he supposed that it might well be a mass of eggs of Mollusca. From the difficulties with which the distinguished zoologists above quoted have found this subject surrounded, and the differences of opinion expressed by them, the reader will perceive that the natural history of these extraordinary organized bodies is anything but complete; and we have laid before him the information above given in order that he may see what has been done and how much remains to be elucidated. DI'PHYES. [DipHypes.] DIPHYLLI'DIA. [INFERoBrANchiATA.] DIPHY'SA. [PHysogFADA.] DIPLECTRON. [PAvonid E.] DIPLOCTE'NIUM. [MADRE phyllice A.] DIPLODA'CTYLUS, a genus of Lizards established by Mr. Gray, and regarded by him as forming a new genus in the family of Geckos. Generic character. Scales subconformable, minute, smooth; the abdominal scales rather large; the caudal scales annulate and larger; the labial scales moderate, distinct, the three anterior ones on each side much the largest; no gular scales. Tail cylindrical, ventricose. Tots, 5, 3, simple, subequal, subcylindrical, the points subdilated, i. beneath, with two oval, oblique, smooth, fleshy disks;

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claws 5, 5, small, very retractile. No femoral pores. (Gray.) This genus differs from Phyllodactylus of the same zoologist in having the under sides of the tips of the toes furnished with two rather large oblong tubercles truncated at the tip and forming two oval disks placed obliquely, one on each side of the claw, instead of having, as in Phyllodactylus, two membranaceous scales. The scales of Diplodactylus are, moreover, uniform, whilst in Phyllodactylus there is a row of larger scales extending along the back. Example, Diplodactylus vittatus. Description. Brown, with a broad longitudinal dorsal fillet; limbs and tail margined with rows of yellow spots. There are two rows of rather distant small spots on each side of the body. the spots become larger on the upper surface of the tail, and are scattered on the limbs. *i. of head and body 2 inches, that of the tail 14 inch. cality, New Holland, whence it was brought to England by Mr. Cunningham. (Zool. Proc. 1832.)

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conchifers, Naïades of Lea. [NAi sp.:s J DIPLOMACY is a term used either to express the art of conducting negotiations and arranging treaties between nations, or the branch of knowledge which regards the principles of that art and the relations of independent states to one another. The word comes from the Greek diploma, which properly signifies anything doubled or folded, and is more particularly used for a document or writing issued on any more solemn occasion, either by a state or other public body, because such writings, whether on waxen tablets or on any other material, used antiently to be made up in a folded form. The principles of diplomacy of course are to be found partly in that body of recognized customs and regulations called public or international law, partly in the treaties or special compacts which one state has made with another The superintendence of the diplomatic relations of a country has been commonly entrusted in modern times to a minister of state, called the Minister for Foreign Affairs, or, as in England, the Secretary for the Home Department. The different persons permanently stationed or occasionally employed abroad, to arrange particular points, to negotiate treaties commercial and general, or to watch over their execution and maintenance, may all be considered as the agents of this superintending authority, and as immediately accountable to it, as well as thence deriving their appointments and instructions. For the rights and duties of the several descriptions of functionaries employed in diplomacy, see the articles AMBAss ADoR, CHARGE D’AFFAIREs, CoNsui, JENvoy. DIPLOMATICS, from the same root, is the science of the knowledge of antient documents of a public or political character, and especially of the determination of their authenticity and their age. But the adjective, diplomatic, is usually applied to things or persons connected, not with diplomatics, but with diplomacy. Thus by diplomatic proceedings we mean proceedings of diplomacy; and the corps diplomatique, or diplomatic body, at any court or seat of government, means the body of foreign agents engaged in diplomacy that are resident there. Some of the most important works upon the science of diplomatics are the following:— Ioannis Mabillon de Re Diplomatica, lib. vii., fol., Paris, 1681-1709, with the ‘Supplementum,' fol., Paris, 1704; to which should be added the three treatises of the jesuit, Barthol. Germon, addressed to Mabillon, “De Veteribus Regum Francorum Diplomatibus,’ 12mo., Paris, 1703, 1706, and 1707:—Dan. Eber. Baringii ‘Clavis Diplomatica,’ 2 vols. 4to., Hanov., 1754; Ioan. Waltheri “Lexicon Diplomaticum,’ 2 vols. fol., Götting, 1745-7: “Nouveau Traité de Diplomatique,' par les Bénédictins Tassin, &c., 6 vols. 4to., Paris, 1750-65; ‘His* We are indebted to Mr. Gray for the figure of this animal.

toria Diplomatica,’ da Scipione Maffei, 4to., Mant., 1727, Io. Heumann von Teutschenbrunn “Commentarii de Re Diplomatica Imperiali,’ 4to., Nurem., 1745; Dom de Vaines, • Dictionnaire Raisonné de Diplomatique,’ 2 vols., 8vo., Paris, 1774; J. C. Gatterer “Abriss der Diplomatik,' 8vo., Götting., 1798; and C. T. G. Schoenemann “Versuch eines vollständigen Systems der allgemeinen besonders ältern Diplomatik,' 8vo., Götting., 1802.

#. MERULID.E.] DIPPING-NEEDLE, an instrument, the essential part of which is the xmagnetised needle employed to ascertain the DIP or inclination. [INCLINATIon.] DIPROSIA. [Poecilopoda.] DIPSA'CEAE, a small natural order of exogenous plants, with monopetalous flowers, nearly allied to Compositae (otherwise called Asteraceae), from which it differs in the ovule being pendulous instead of erect, in the embryo being inverted, in the anthers being distinct, not syngenesious, and in the corolla having an imbricated, not valvate aestivation. In habit the species are similar to Compositae, having their flowers constantly arranged in heads. None of the species are of any importance except the common teazle, Dipsacus Fullonum, whose prickly flower-heads are extensively employed in carding wool. Many of the species have handsome flowers, especially the Scabiouses, and are cultivated in the gardens of the curious. Purple and starry Scabions are common hardy annuals.

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The subjoined cut, from Guerin (Iconog) will illustrate

the form.

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The term Dipsas is also used by Dr. Leach to distinguish a genus of fresh-water conchifers; and he states that its systematic situation is between Unio and Anodonta (Anodon); Unio of Sowerby; Naiades of Lea. [NAIADEs.] DIPSASTRAEA.. [MADREPhylliceA.] DI'PTERA, one of the orders into which insects are divided. This name was first applied by Aristotle, and has subsequently been adopted by almost all entomologists to designate those insects the most striking characteristic of which is the possession of two wings only. The common house-fly and blue-bottle fly afford familiar examples of this order. Some dipterous insects, however, are destitute of wings (such as the species of the genera 3/elophagus, Nycterobia, &c.); hence it is necessary that we should here notice other peculiarities observable in these insects. The Diptera have six legs, furnished with five-jointed tarsi, a proboscis, two palpi, two antennae, three ocelli, and two halteres or poisers. The wings are generally horizontal in their position and transparent; their nervures are not very numerous, and are for the most part longitudinally disposed, a character in which the wings of dipterous insects differ from those of the orders Neuroptera and Hymenoptera. The proboscis, situated on the under part of the head, is generally short and membranous, and consists of a sheath (or part analogous to the under lip or labium in mandibulate insects), which serves to keep in situ other parts of the mouth, which, when they are all present, represent the mandibles, maxillae, tongue, and labium. There are however considerable modifications in the structure of the proboscis: in some it is long, slender, and corneous, and the number of enclosed pieces, which are generally very slender and sharp, varies from two to six. It is evident that this structure of mouth is adapted only to the extraction and transmission of fluids; and when these fluids are contained within any moderately tough substance, the parts enclosed by the sheath of the proboscis are used as lancets in wounding and penetrating so as to allow the escape of the fluid, which by their pressure is forced to ascend and enter the desophagus.

The palpi are situated at the base of the proboscis. The antennae are placed on the fore part of the head, and approximate at their base; they are generally small and three-jointed; the last joint, however, is often furnished With an appendage, called the stylet, which is considerably diversified, not only in form but in its position. In some of the insects of this order, the Tipulidae for instance, the antennae are long, and composed of numerous joints; and in the Culicidae they resemble little plumes. The eyes in dipterous insects are generally large, especially in the male sex, where they often occupy nearly the whole of the head. The halteres or poisers are two small organs of a slender form, and furnished with a knob at their apex, situated at the base of the thorax on each side, and immediately be. hind the attachment of the wings. These organs have been considered by many as analogous to the under wings of four-winged insects. Latreille and others, however, have come to a different opinion, from the circumstance of their not being attached to the same part of the thorax. The use of these organs is not yet ascertained; it is however supposed by some that the little knob which we mentioned is capable of being inflated with air, and that they serve to balance the insect during flight, at which time these organs are observed to be in rapid motion. As regards the thorax, it is only necessary here to observe that the chief part of that which is visible from above consists of the mesothorax; the prothorax and metathorax being comparatively small. The scutellum varies considerably in form, and is sometimes armed with spines; we find it developed in an extra9rdinary manner in the genus Celyphus" (Dalman), where it is very convex and covers the whole abdomen. The abdomen seldom presents more than seven distinct segments; its form is very variable. ipterous insects, undergo what is termed a complete transformation: their larvae are devoid of feet, and have a head of the same soft substance as the body and without determinate form. The parts of the mouth exhibit two. scaly pointed plates. The stigmata are nearly all placed on the terminal segment of the body. When about to assume the pupa state, they do not cast their skin (as is the case with the larvae of most insects), but this becomes gradually hardened, and after a time the animal assumes the pupa state within, so that the skin of the larva forms as it Were a cocoon. There are however exceptions to this rule, for many change their skin before they assume the pupa state, and some spin cocoons. We may here observe, that in some of the species of the genus Sarcophaga the eggs are hatched within the body of the mother, whence the insect first makes its appearance in the larva state; and in the Pupipara, not only are the eggs hatched within the body of the parent but the larvae continue to reside there until their transformation into pupae. As regards the habits of dipterous insects, they will be found under the heads of the several families and genera: we shall therefore conclude by noticing the two great seetions into which this order is divided by Macquart. These are the Nemocera and the Brachocera. The species of these two sections are distinguished chiefly by the number of joints of the antennae and palpi. Their characters are as follows:– Section 1. Nemocera. Antennae filiform or setaceous, often as long as the head and thorax together, and composed. of at least six joints. Palpi composed of four or five joints: body generally slender and elongated; head small; proboscis sometimes long and slender, and inclosing six lancets; sometimes short and thick, and having but two lancets: thorax large and very convex; legs long; wings long, and with elongated basal cells. Section 2. Brachocera. Antennae short, composed of three joints; the third joint generally furnished with a stylet; palpi composed of one or two joints; head usually hemispherical, and as broad as the thorax; proboscis either long, slender, coriaceous, and protruded, or short, thick. and retracted, and containing either six, four, or two laneels; thorax moderately convex; legs usually of moderate length. wings with the basal cells rather short. -The principal works on dipterous insects are, Wiedemann, • The great development of the scutellum in the insists of this genus has its parallel in the order Hemiptera, for in the genus Tw'yra the scuollum also covers the abdomeu.

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Diptera Erotica, 1 vol. 8vo. 1821, Meigen, Systematische Beschreibung der bekannten Europäischen zweiflügelingen Insekten, 6 vols. 8vo. with figures; Macquart, in the Suites a Buffon, Histoire des Insectes, ‘Diptéres,’ 2 vols. 8vo. DIPTERA'CEAE or DIPTEROCARPEAE, an important order of East Indian exogenous polypetalous trees, allied to Malvaceae. They have a tubular \nequal permanent calyx, with five lobes, which after flowering become leafy and very much enlarged, surmounting the fruit without adhering to it. There are five petals, with a contorted abstivation, an indefinite number of awl-pointed narrow anthers, and a few-celled superior ovary, with two pendulous ovules in each cell; of these all are eventually abortive, except one, which forms the interior of a hard dry leathery pericarp. The seed is solitary, contains no albumen, and has an embryo with two large twisted and crumpled cotyledons, and a superior radicle. The leaves are long, broad, alternate, rolled inwards before they unfold, with strong straight veins running obliquely from the midrib to the margin, and oblong deciduous stipules rolled up like those of a Magnolia. The different species produce a number of resinous, oily, and other substances; one a sort of camphor (Dryobalamops); another a fragrant resin used in temples; a third Gum Animi; while some of the commonest pitches and warnishes of India are procured from others.

(Dipterocarpus gracilis:

1, two of the stamens; 2, a ripe fruit surrounded by the calyx whose segments have become large and leafy, and very unequal.

DIPTERAL. [Civil ARCHITECTURE.]

DIPTERIX. [Coum ARounA.]

DIPTEROCARPUS, a genus of East Indian, and chiefly insular, trees, of which Blume gives the following as the essential character: “Calyx irregularly five-lobed at the mouth: the two opposite segments very long and ligulate. Petals five, convolute when unexpanded. Stamens numerous; anthers long, linear, terminating in an awl-shaped point. Nut rather woody, and one-celled and one-seeded by abortion, inclosed in the enlarged calyx. The species are described as enormous trees, abounding in resinous juice, with erect trunks, an ash-coloured bark, strong spreading limbs, and oval leathery entire leaves, with pinnated veins. The flowers are large, white or pink, and de

liciously fragrant. The pubescence is always stellate when present. The resinous juice of D. trinervis, a tree from 150 to 200 feet high, inhabiting the forests of Java, is made into plaisters for ulcers and foul sores; and when dissolved in spirit of wine, or formed into an emulsion with white of egg, acts upon the mucous membranes in the same way as balsam of copaiva. Dryobalanops Camphora, the Camphor tree of Sumatra, is usually referred to this genus; but, according to Blume, is really a distinct genus. [DRYoBALANOPs. DIPUS. [JERBoA.] DIPYRE or leucolite, a silicate of alumina and lime, which occurs in small slender prisms, the primary form or which has not been determined; their colour is greyish or reddish white, and fasciculated into masses. Internally the lustre is shining; vitreous; opaque; hardness sufficient to scratch glass; specific gravity about 2' 6. It is found in the Western Pyrenees. By analysis it yielded—silica 60, alumina 24, lime 10, and water 2. When heated by the blow-pipe it becomes milk white, and then fuses into a blebby colourless glass. DIRECT and RETROGRADE, two astronomical terms, the former of which is applied to a body which moves in the same direction as all the heavenly bodies except comets; the second to one which moves in a contrary direction. The motions of the planets round the sun, of the satellites round their primaries, and of the bodies themselves round their axes, all take place in one direction, with the exception only of the comets, of which about one-half the whole number move in the contrary direction. The course of these celestial motions is always from west to east, which is the direct course. The retrograde is therefore from east to west. The real diurnal motion of the earth being direct, the apparent motion of the heavens is retrograde, so that the orbital motion of the sun and moon has, so far as it goes, the effect of lessening the whole apparent motion: or these bodies appear to move more slowly than the fixed stars. With regard to the planets, the effect of the earth's orbital motion combined with their own, makes them sometimes appear to retrograde more in the day than they would do from the earth's diurnal motion only. [PLANETARY Motions.] In the Latin of the seventeenth century, the direct motion is said to be in consequentia, and the retrograde in antecedentia. The most simple way of remembering direct motion, is by recalling to mind the order of the signs of the zodiac. From Aries into Taurus, from Taurus into Gemini, &c., up to from Pisces into Aries, is direct motion; while from Taurus into Aries, from Aries into Pisces, &c., is retrograde motion. DIRECTION, a relative term, not otherwise definable than by pointing out what constitutes sameness and difference of direction. Any two lines which make an angle point in different directions; a point moving along a straight line moves always in the same direction. Permanency of direction and straightness are equivalent notions. A body in motion not only changes its direction with respect to other bodies, but also the direction of other bodies with respect to it. The most common measure of direction, for terrestrial purposes, refers to the north as a fixed direction, and uses the points of the compass. But any line whatever being drawn from the point of view, the directions of all other points may be estimated by measuring the angles which lines drawn from them to the point of view make with the standard line. When a point describes a curve, it cannot at any one moment be said to be moving in any direction at all ; for upon examining the basis of our notion of curvature, we find that it consists in supposing a line to be drawn, no three contiguous points of which, however near, are all in the same straightline. But this is a mathematical notion, which is contradicted in practice by any attempt at a curve which we can make on paper. For it is found that, as must be the case from the proposition mentioned in the article ARC (vol. ii., p. 256), when two points of a curve are taken very near to each other, and joined by a chord, the widest interval between the chord and the arc disappears or becomes imperceptible long before the chord and arc disappear. Hence arises the notion that a curve may in fact be composed of very small straight lines, each of which has of course a definite direction. But though such motion must be abandoned in geometry, yet it leads to the stricter notion of a TANGENT (see also Costact], or of a straight line of which, as soon as the term is explained, we unhesitatingly

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