quarto; "A preface to the Sciographia of John Wells, of Brembridge, Esq." 1635, 8vo; "An Institution, trigonometrical, explaining the doctrine of plane and spherical Triangles, after the most exact and compendious way, by Tables of Sines, Tangents, &c. with the application thereof to questions of Astronomy and Navigation," 1634, octavo; and afterwards republished with enlargements by William Leybourn, 1652, octavo; “An Epitome of Navigation, with the necessary tables," &c. and "An Appendix concerning the Use of the Quadrant, Forestaff, and Nocturnal," octavo; "Oratio in Laudem Gassendi Astronomiæ, habita in Aula Edis, Christi, Oxon;" and of several unpublished MSS. on the Doctrine of Eclipses, Lunar Astronomy, Ship Building, &c.

GEMINI, the Twins, in astronomy, one of the twelve signs of the zodiac, the third in order, beginning with Aries. See ASTRONOMY.

GEMMA (REINIER), a learned Dutch physician and mathematician in the sixteenth century, was born at Dockum, in Friesland, in the year 1508. He was educated to the medical science, of which he became a professor in the university of Louvain. But he was particularly eminent for his proficiency in mathematics and astronomy, which he taught with distinguished reputation, and the character of being one of the best astronomers of his time. The fame of his great scientific knowledge, and of the excellent instruments which he made use of in the illustration of it, occasioned his being frequently invited to the court of the Emperor Charles V.; but he always modestly declined the overtures made to him, preferring the tranquillity of his literary retreat to the honours which he might expect from princely favour. He died at Louvain, in 1555, when only forty-seven years of age. He has sometimes had the surname of Friscius given him, from the country in which he was born. The most celebrated of his works were "Methodus Artihmeticæ ;" "De usu Annuli Astronomici;" "De Locorum describendorum Ratione, deque Distantiis eorum inveniendis;' ;""Libellus de Principiis Astronomiæ et Cosmographia," &c. "Demonstrationes Geometricæ de usu Radii Astronomici," &c.; and "De Astrolabio Catholico Liber."

The author had a son, named Cornelius, who was born at Louvain, in 1535, and died in 1579. He was a poet, philosopher, and physician, and taught the mathematical sciences at Louvain with consiVOL. V.

derable reputation. He was the author of "De Arte Cyclognomicæ," &c.; "De Naturæ divinis Characterismis, seu Cosmocritico;" and "De Prodigiosa Specie Naturaque Cometæ," occasioned by the extraordinary new star in the constellation Cassiopeia, in 1572, which disappeared after being visible for eighteen months; and other pieces.

The

GEMMA, in botany, a bud, a compendium of a plant seated upon the stem and branches, and covered with scales, in order to defend the tender rudiments inclosed from cold, and other external inju ries, till, their parts being unfolded, they acquire strength, and render any further protection unnecessary. Buds, together with bulbs, which are a species of buds, generally seated upon or near the root, constitute that part of the herb, by Linnæus called hybernacula; that is, the winter quarters of the future vegetable, as it is during that severe season that the tender rudiments are protected in the manner just mentioned. Plants, considered in analogy to animals, may properly enough be reckoned both viviparous and oviparous. Seeds are the vegetable eggs; buds, living fœtuses, or infant plants, which renew the species as certainly as the seed. In general, we may distinguish three kinds of buds; that containing the flower, that containing the leaves, and that containing both flower and leaves. first contains the rudiments of one or several flowers folded over one another, and surrounded with scales. In several trees, this kind of bud is commonly found at the extremity of certain small branches, which are shorter, rougher, and less gar- . nished with leaves than the rest. The external scales of this species of bud are harder than the internal; both are furnished with hairs, and in general more swelled than those of the second sort. The bud containing the flower, too, is commonly thicker, shorter, almost square, less uniform, and less pointed, being generally terminated obtusely. The second species of bud contains the rudiments of several leaves which are variously folded over one another, and outwardly surrounded by scales, from which the small stipule that are seated at the foot of the young branches are chiefly produced. These buds are commonly more pointed than the former sort. In the hazle nut, however, they are perfectly round; and in horse-chesnut very thick. The third sort of bud is smaller than either of the preceding, and produces both flowers and leaves,though not always in the same manner. Sometimes the flowers and leaves

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are unfolded at the same time. This mode of the flower and leaf-bud admits of the following distinctions from the sex of the flowers so produced with the leaves: male flower and leaf-buds as in the pine and firtree; female flower and leaf buds as in hazle-nut and horn-bean; hermaphrodite flower and leaf-buds as in the elm-tree, cornel-tree, mezereon, and almond tree.

GEMS certain stones, which, on account of their hardness, transparency, and beauty, when cut and polished, are highly esteemed, and, from their small size and scarcity, are valued at very considerable price. The gems have been placed among the siliceous fossils, as in some measure allied with them in external characters; and silex was supposed to be their principal ingredient. Bergman first shewed the error of this opinion, and proved, by analysis, that in the emerald, sapphire, topaz, ruby, and hyacinth, argil predominates; their other constituent principles, as discovered by his analysis, being silex, lime, and oxide of iron. Still, however, the old prejudice prevailed, and they have been generally ranked by mineralogists under the siliceous gems.

The specific distinctions of these fossils were not less osbcure; they were perplexed by the distinctions of the jewellers, drawn from very vague notions: the colour, in particular, being the property in which the gems differ most obviously, and which frequently gives them their mercantile value, served as a ground of distinction: hence the ruby, the sapphire, and the topaz, were considered as different, though essentially the same. Another circumstance, added to the confusion thus introduced, was, that other fossils, bearing a resemblance to these gems, had been classed with them; but, being inferior in lustre, transparency, and hardness, in order to distinguish between them, the epithet oriental was applied to those which were most perfect; and, by this contrivance, fossils were classed under one name, and regarded only as varieties of one species, which were totally different. The Oriental and the Saxon topaz, for example, were regarded under this point of view, or as varieties of one species, to which the common name of topaz belonged, though they are fossils altogether distinct. From these two circumstances, fossils were separated, which ought to have been associated, and others were connected, which were specifically different; and it has required much mineralogical discussion to disentangle the perplexity, and establish the proper species.

Romi de l'isle threw the first ray of

light on this subject, by disregarding the colour, and attending rather to the form of crystallization; in consequence of which he arranged together the principal gems named oriental, under the title of the ori ental ruby. Werner also has placed them under one species, to which he gives the name of sapphire. Hauy has adopted the same arrangement, distinguishing the species by the name of télésie; and, more lately, Bournon has still farther extended the relations of these fossils, by connecting them with the corundum, a fossil which had been brought from India, and which, analysed by Klaproth, was found to be composed principally of argillaceous earth. This, having in general little transparency or lustre, Bournon names imperfect corundum; while the other variety, possessing these qualities, and comprising the oriental gems, is distinguished by the appellation of perfect corundum : these arrangements have received the sanction of chemical analysis. The skill of Klaproth, of Vauquelin, and Chenevix, has been exerted in investigating the composition of these fossils, and they have proved to be argil nearly pure. See DIAMOND, CORUNDUM, TELESIE, RUBY, SAPPHIRE, TOPAZ, AMETHYST, EMERALD, EMERY, BERYL, CHRYSOLTE, CHRYSOBERYLL.

GENDARMES, or GENS D'ARMES, in the French armies, a denomination given to a select body of horse, on account of their succeeding the ancient gendarmes, who were thus called from their being completely clothed in armour.

GENDER, among grammarians, a division of nouns, or names, to distinguish the two sexes.

GENEALOGICA arbor, or tree of consanguinity, signifies a genealogy or lineage drawn out under the figure of a tree, with its root, stock, branches, &c. The genealogical degrees are usually represented in circles, ranged over, under, and aside each other.

GENEALOGY, an enumeration of a series of ancestors; or a summary account of the relations and alliances of a person or family, both in the direct and collateral line.

GENERAL of an army, in the art of war, he who commands in chief.

A general ought to be a man of great courage and conduct, to have great experience, and to be of good quality. His conduct appears in establishing his maga zines in convenient places; in examining the country, that he may not engage his troops too far while he is ignorant of the means of bringing them off; in subsisting them; and in knowing how to take the

most advantageous posts, either for fight ing or shunning a battle. His experience inspires his army with confidence, and an assurance of victory; and his quality, by creating respect, augments his authority. By his liberality he gets intelligence of the strength and designs of the enemy, and by this means is enabled to take the most successful measures. A general ought likewise to be fond of glory, to have an aversion to flattery, to render himself beloved, and to keep a strict discipline.

The office of a general is to regulate the march and encampment of the army; in the day of battle to choose out the most advantageous ground; to make the disposition of the army; to post the artillery; and, where there is occasion, to send his orders by his aids-de-camp. At a siege, he is to cause the place to be invested; to order the approaches and attacks; to visit the works; and to send out detachments to secure his convoys.

GENERAL issue, in law, is that plea which traverses and denies, at once, the whole declaration or indictment, without offering any special matter, whereby to evade it and it is called the general issue, because, by importing an absolute and general denial of what is alleged in the declaration, it amounts at once to an issue; that is, a fact affirmed on one side, and denied on the other. This is the ordinary plea upon which most causes are tried, and is now almost invariably used in all criminal cases. It puts every thing in issue, that is, denies every thing, and requires the party to prove all that he has

stated.

It is a frequent question, what can be given in evidence by the defendant upon this plea, and the difficulty is, to know when the matter of defence may be urged upon the general issue, or must be specially pleaded upon the record. In many cases, for the protection of justices, con-stables, excise officers, &c. they are by act of parliament enabled to plead the general issue, and give the special matter for their justification under the act in evidence.

GENERATING line or figure, in geometry, is that by which its motion produces any other plane or solid figure. Thus, a right line moved any way parallel to itself generates a parallelogram; round a point in the same plane, with one end fastened in that point, it generates a circle. One entire revolution of a circle, in the same plane, generates the cycloid; and the revolution of a semi-circle

round its diameter, generates a sphere, &c. See CYCLOID, SPHERE, &C.

GENERATION. See PHYSIOLOGY. GENERICAL name, in natural history, the word used to signify all species of natural bodies, which agree in certain essential and peculiar characters, and therefore all of the same family or kind; so that the word used as the generical name equally expresses every one of them, and some other words, expressive of the peculiar qualities or figures of each, are added, in order to denote them singly, and make up what is called the specific name. Thus the word rosa, or rose, is the generical name of the whole series of flowers of that kind, which are distinguished by the specific names of the red-rose, the whiterose, the apple-rose, &c.

GENEVA, gin, a hot fiery spirit, too much used by the lower classes of people in this country as a dram, and is unques tionably most injurious to their constitu tion and morals. A liquid of this kind was formerly sold in the apothecaries' shops, drawn from the juniper-berry, but distillers have now completely supplanted the trade of the apothecary, who sell it under the name of Geneva, or gin, in which it is believed juniper-berries make no part of the composition. It is composed of oil of turpentine and malt spirits.A better sort is said to be drawn off by a slow fire from juniper berries, proof-spirits, and water, in the proportion of three pound of berries to four gallons of water and ten of spirit. The celebrated Hollands geneva is manufactured chiefly at a village near Rotterdam, from the same materials, making use of French brandy instead of malt-spirits.

GENIOSTOMA, in botany, a genus of the Pentandria Monogynia class and order. Essential character: calyx turbinate, five-cleft; corolla one-petalled, with a villose throat, and a five-parted border; capsule oblong, two-celled, many seeded. There is but one species, a native of the isle of Tanna, in the South Seas.

GENISTA, in botany, a genus of the Diadelphia Decandria class and order. Natural order of Papilionacea or Leguminosa. Essential character: calyx twolipped, two and three-toothed; banner oblong, reflex downwards from the pistil and stamens. There are seventeen species.

GENIUS, in matters of literature, &c. a natural talent or disposition to do one thing more than another; or the aptitude a man has received from nature to per

form well, and easily, that which others can do but indifferently, and with a great deal of pains.

GENTIAN, in pharmacy, is to be found in many countries, but particularly in some parts of France, on the Alps, Pyrenees, and the mountainous districts of Germany. That used in this country is mostly brought from Germany. The roots are the only part of the plant made use of in medicine. Gentian stands at the head of the stomachic bitters.

GENTIANA, in botany, a genus of the Pentandria Digynia class and order. Natural order of Rotacea. Gentianæ, Jussieu. Essential character: corolla monopetalous; capsule superior, twovalved, one-celled, with two longitudinal receptacles. There are fifty-three species.

GENUS, among metaphysicians and logicians, denotes a number of beings, which agree in certain general properties, common to them all; so that a genus is an abstract idea, expressed by some general name or term.

A genus is an assemblage of several species: that is, of several plants which resemble one another in their most essential parts. Hence it is aptly enough compared to a family, all the relations of which bear the same surname, although every individual is distinguished by a par ticular specific name. In botany the es tablishment of genera renders the subject more simple and easy, by abridging the number of names, and arranging under one denomination, termed the generic name, several plants, which, though different in many other respects, are found invariably to possess certain relations in those essential parts, the flower and fruit. Plants of this kind are termed by botanists planta congeneres, that is, plants of the same genus.

Linnus's genera contain a description of each particular part of fructification, its various relations and different modes with respect to number, figure, situation, and proportion. Thus, all the different species of calyx, corolla, nectarium, stamina, &c. furnish the observer with so many sensible and essential characters. These characters the author denominates the letters or alphabet of botany. By studying, comparing, and, as it were, spelling these letters, the student in botany comes, at length, to read and understand the generical characters which the great Creator has originally imprinted upon vegetables: for the genera and species, according to Linnæus, are solely the work of nature;

whilst the classes and orders are a combination of nature and art Upon the principles, Linnæus, in his genera planta rum, determines the generical characters of all the plants there described.

GENUS, in natural history, a sub-division of any class or order of natural be ings, whether of the animal, vegetable, or mineral kingdoms, all agreeing in certai common characters.

GEOCENTRIC latitude of a planet, is its distance from the ecliptic as it is seen from the earth, which, even though the planet be in the same point of her orbit, is not constantly the same, but alters according to the position of the earth in respect to the planet.

GEOCENTRIC place of a planet, the place wherein it appears to us from the earth, supposing the eye there fixed: or it is a point in the ecliptic to which a pla net seen from the earth is referred.

GEODESIA, the same with surveying. See SURVEYING.

GEOFFROYA, in botany, so named in honour of Monsieur Geoffroy, a member of the academy at Paris, a genus of the Diadelphia Decandria class and order. Natural order of Papilionacea or Legu minosa. Essential character: calyx five. cleft; drupe ovate; nut flatted. There are two species.

GEOGRAPHY, is that science which exhibits the results of our investigations respecting the planet we inhabit, whether we consider its figure and the disposition of the lands and water upon its surface, or the subdivisions which the different nations who inhabit it have made, by which it is considered as forming king. doms and states.

The general curvature of the earth's surface is easily observable in the disappearance of distant objects; and, in par ticular, when the view is limited by the sea, the surface of which, from the com. mon property of a fluid, becomes naturally smooth and horizontal; for it is well known that the sails and rigging of a ship come into view long before her hull, and that each part is the sooner seen as the eye is more elevated.

On shore the frequent inequalities of the solid parts of the earth usually cause the prospect to be bounded by some ir. regular prominence, as a hill, a tree, or a building, so that the general curvature is the less observable.

The surface of a lake, or sea, must be always perpendicular to the direction of a plumb line, which may be considered as the direction of the force of gravity; and

by means either of a plumb line, or of a spirit level, we may ascertain the angular situation of any part of the earth's surface with respect to a fixed star passing the meridian: by going a little further north or south, and repeating the obser. vation on the star, we may find the difference of the inclination of the surfaces at both points; of course, su posing the earth a sphere, this difference in latitude will be the angle, subtended at its centre by the given portion of the surface, whence the whole circumference may be determined, and on these principles the earliest measurements of the earth were conducted. The first of these which can be considered as accurate, was executed by Picart, in France, towards the end of the seventeenth century.

But the spherical form is only an approximation to the truth. It was calculated by Newton, and ascertained expe rimentally by the French academicians, sent to the equator and to the polar circle, that, in order to represent the earth, the sphere must be flattened at the poles, and prominent at the equator. We may therefore consider the earth as an oblate elliptic spheroid; the curvature being greater, and consequently every degree shorter at the equator, than nearer the poles. If the density of the earth were uniform throughout, its ellipticity, or the difference of the length of its diameters, would be of the whole; on the other hand, if it consisted of matter of inconsiderable density, attracted by an infinite force in the centre, the ellipticity would be unly; and whatever may be the internal structure of the earth, its form must be between these limits, since its internal parts must necessarily be denser than those parts which are nearer the surface. If, indeed, the earth consisted of water or ice, equally compressible with common water or ice, and following the same laws of compression with elastic fluids, its density would be several thousand times greater at the centre than at the surface; and even steel would be compressed into one-fourth of its bulk, and stone into one-eighth, if it were continued to the earth's centre: so that there can be no doubt but that the central parts of the earth must be much more dense than the superficial.

Whatever this difference may be, it has been demonstrated by Clairaut, that the fractions expressing the ellipticity, and the apparent diminution of gravity at the equator, must always make toge

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; and it has been found, by the most accurate observations on the lengths of pendulums in different latitudes, that the force of gravity is less powerful by To at the equator than at the pole, whence the ellipticity is found to be of the equatorial diameter; the form being the same as would be produced, if about three-eighths of the whole force of gravity were directed towards a central particle, the density of the rest of the earth being uniform.

This method of determining the general form of the earth is much less liable to error and irregularity, than the measurement of the lengths of degrees in various parts, since the accidental variations of curvature produced by local dif ferences of density, and even by superfi cial elevations, may often produce considerable errors in the inferences which might be deduced from these measurements. For example, a degree measured at the Cape of Good Hope, in latitude 330 south, was found to be longer than a degree in France, in latitude 56 north, and the measurements in Austria, in North America, and in England, have all exhibited signs of similar irregularities. There appears also to be some difference in the length of degrees under the same latitude, and in different longitudes. We may, however, imagine a regular elliptic spheroid to coincide very nearly with any small portion of the earth's surface, a though its form must be somewhat dif ferent for different parts: thus for the greater part of Europe, that is, for England, France, Italy, and Austria, if the measurements have been correct, this oscillating spheroid must have an ellipticty of

130'

The earth is astronomically divided into zones and into climates. The torrid zone is limited by the tropics, at the distance of 23° 28' on each side of the equator, containing all such places as have the sun sometimes vertical, or imme diately over them: the frigid zones are within the polar circles, at the same distance from the poles, including all places which remain annually within the limit of light and darkness, for a whole diurnal rotation of the earth, or longer : the temperate zones between these, have an uninterrupted alternation of day and night, but are never subjected to the sun's vertical rays. At the equator, therefore, the sun is vertical at the equinoxes, his least meridian altitude is at the solstices, when it is 66° 32', that is, more