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agriculture, and political economy, the very name of which | air, are exposed to bright light during the short summiers was hitherto unknown, became quite a fashionable study. When in 1767 the Jesuits were exiled from the kingdom, the minister Tanucci consulted Genovesi as to a new plan for the organization of the schools and colleges of the kingdom, which he drew up accordingly. He continued to lecture and to write, although his health was greatly impaired for several years, almost to the day of his death, which occurred in September, 1769. The mind of Genovesi is exhibited in the following lines, which he wrote to his friend Angelo Pavesi in 1765: I am now getting old, and have nothing more to hope or to expect from this world; but my wish would be to leave my countrymen a little more enlightened than I found them, and also a little more affectionate towards virtue, which is the only true source of good. It is of little use to think about government, arts, or commerce, unless the morals of a nation are also reformed. As long as men will find their interest in being rogues, we must not expect much from our methodic labours.' A selection of Genovesi's familiar letters was published after his death, in two small volumes. He edited in his lifetime the Course of Agriculture' of Cosimo Trinci, to which he added notes and a preliminary discourse on the state of Neapolitan agriculture in his time. Galanti, one of Genovesi's best disciples, wrote an Elogio Storico,' or biographical notice of his master, and Fabbroni wrote another in Latin. Ugoni, in his Letteratura Italiana, devotes a long article to Genovesi.

GENSERIC, king of the Vandals, was the bastard brother of Gonderic, whom he succeeded A.D. 429. In the same year he left Spain, which had been partly conquered by the Vandals, and crossed over into Africa at the solicitation of Boniface, governor of that province, who had been induced, by the arts of his rival Aetius, to rebel against Valentinian III., emperor of the West. Boniface soon repented of the step he had taken, and advanced to meet the invader. But his repentance came too late. The Moors joined the standard of Genseric, and the powerful sect of the Donatists, who had been cruelly persecuted by the Catholics, assisted him against their oppressors. Boniface was defeated, and obliged to retire into Hippo Regius, where he remained till he obtained a fresh supply of troops. Having ventured upon a second battle, and being again defeated, he abandoned the province to the barbarians, and sailed away to Italy. A peace was concluded between Genseric and the emperor of the West, by which all Africa, to the west of Carthage, was ceded to the Vandals. This however did not long continue; and the city of Carthage was taken by the Vandals by surprise A.D. 439. The emperors of the West and East made great preparations for the recovery of the province; but an alliance which Genseric formed with Attila, king of the Huns, effectually secured him against their attempts.

Genseric's next object was directed to the formation of a naval power; an immense number of ships was built, and his fleets ravaged the shores of Sicily and Italy. Invited by the empress Eudoxia, he sailed up the Tiber (A.D. 455), and permitted his soldiers, for the space of fourteen days, to pillage Rome. In A.D. 460 he destroyed the fleet which the Emperor Majorian had collected for the invasion of Africa; and as his power increased his ravages became more extensive; the island of Sardinia was conquered, and Spain, Italy, Sicily, Greece, Egypt, and Asia Minor, were plundered every year by the Vandal pirates. Leon, the emperor of Constantinople, at last resolved to make a vigorous effort for the recovery of Africa. A great army was assembled, and the command was given to Basilicus. He landed at Bona, and at first met with considerable success, but was at length obliged to retire from the province. After this victory Genseric met with no further opposition, but remained undisturbed master of the sea till his death, which happened A.D. 477. He was succeeded by his son Hunneric. Genseric was an Arian, and is said to have persecuted the Catholics with great cruelty. (Procopius, De Bell. Vandal.; Gibbon's Decline and Fall, c. xxxiii.-xxxvi.)

GENTIA'NA, a genus of herbaceous plants, giving their name to the natural order Gentianaceæ, remarkable, as ornamental objects, for the brilliant colours and beautiful forms of their flowers, and most useful in medicine, on account of the pure intense bitter which they all contain. The species are extremely numerous, inhabiting the temperate parts of Europe, Asia, and America, chiefly in mountainous situations, where they breathe a pure and rarified

of such regions, and although fixed during winter in places
intensely cold, yet are so well prepared to resist it by the
warmth of their summer, and so much protected by the
snow that covers them, as to suffer no injury. These al-
pine plants are consequently difficult to cultivate, or even
uncultivable, from the impossibility of imitating their na-
tural atmosphere; and hence it is only a very small num-
ber that are ever seen in gardens. The prevailing colours
of their flowers are either an intense pure blue, or a bright
clear yellow: some idea may be formed of the brilliancy of
the former from that of Gentiana acaulis, a common species
in gardens, where it is much employed for making edging
to borders; the yellow species are equally represented by
Gentiana lutea, a tall kind, which thrives well in a common
American border. As the various plants comprehended in
the genus Gentiana, as defined by Linnæus, are extremely
different in appearance, and offer great diversities of struc-
ture in their flowers, some attempts have been made to
break the Linnæan genus up into several others. Botanists
however have not received these innovations favourably,
and therefore, although Dr. Grisebach's new arrangement
will probably be adopted, we shall still consider the species
as all belonging to one and the same genus.
The ornamental species that are found easily capable of
cultivation are G. lutea, with yellow, and G. asclepiadea,
saponaria, cruciata, septemfida, acaulis, and Pneumonanthe,
with blue flowers. Of these all require a good American bor-
der of peat-earth to grow in, with the exception of G. acaulis,
which prefers the hardest and stiffest clay. Many other
species are named in gardening books, but they generally
perish as soon as they are brought under the hands of the
cultivator. For medical purposes, the root of Gentiana lutea,
a native of the central parts of Europe, is principally col-
lected, especially for the French and English markets; but
Gentiana purpurea and punctata have roots that are still
more bitter, and the latter is said to furnish the chief part
of what is consumed in Germany and the north of Europe.
In the Himalayas the roots of Gentiana Kurroa are used as
a substitute, and the stems and leaves of G. cheretta.

GENTIA'NA LU'TEA, a perennial species, common in the mountainous and sub-alpine districts of Switzerland, Germany, &c. Though the whole plant is bitter, yet as this property is most concentrated in the root, that part only is officinal. This should be taken up in autumn, and is best when the plant is only one year old. It is generally cylindrical, often an inch thick at the summit, but below rather branched, of a dark or brown colour externally; internally fleshy and yellow. In commerce it is met with in pieces, cut longitudinally, from a half to one foot in length. A transverse section displays three distinct circles. The greater portion is procured from Germany; the specimens from Switzerland are generally thicker and darker coloured.

When fresh it has some smell, which is almost entirely lost by drying. The taste is at first somewhat sweet, then purely and strongly bitter. According to the analysis of Henry and Caventou, it contains a principle termed Gentianin, which is crystallizable; a volatile odorous principle, a greenish fixed oil, a free organic acid, uncrystallizable sugar, gum, colouring matter, &c.

Owing to its saccharine matter it soon moulds in a damp place, and should therefore be kept in a dry airy situation. From the abundance of the sugar, it is easily susceptible of fermentation, and from it is distilled a spirit, called Enziangeist, or 'bitter snaps,' much employed by the peasants on the Swiss Alps to fortify the system against the fogs and damps of these lofty regions.

Yellow gentian-root is often confounded with the roots of other species of this genus, a circumstance attended with no bad consequences, but unfortunately roots of very poisonous plants, growing in the same locality, are often taken up instead of the proper one these are, the Veratrum album (white hellebore), the leaves of which resemble those of gentian in their peculiar venation, but are alternate, while those of gentian are opposite-the root is very different, and besides this, it contains Veretria; and the Atropa Belladonna (deadly night-shade), which, besides differences in the physical characters, is devoid of the peculiar bitter of gentian, and acquires a bluish-black colour from tincture of iodine. The roots of Aconitum Lycoctonum and Ranunculus Thora are occasionally confounded with gentian-root.

Gentian-root is a pure and excellent bitter tonic, useful in all cases of debility, whether of the stomach only, or of

the system generally. It possesses facilities, from not being decomposed, of being administered along with many metallic salts. It yields its properties to water, particularly when warm, to alcohol, and to wine. The simple infusion, and not the compound, of the 'London Pharmacopoeia' should be employed, when any salts of iron are prescribed in the same formula. The extract is an eligible means of giving bulk, when several antispasmodic remedies, of which the dose is minute, are to be made into pills, such as oxide of zinc, protosulphate of iron, or extract of aconite. In the West Indies a preparation of gentian is used daily before meals, to give tone to the languid stomach.

In the East Indies several species are used as bitter tonics. The Chirayita, or Chiretta, in the form of a cold infusion, is much prized for its tonic and febrifugal virtues. Guibourt contends that this plant is the Calamus aromaticus of the antients, an opinion from which Dr. Royle dissents. (Flora of the Himalaya,' p. 277.)

GENTIANA'CEA, an extensive order of monopetalous Exogens, consisting of herbaceous plants, with opposite ribbed leaves, and flowers whose corolla is imbricated, the stamens alternate with the petals, the ovary superior, with two cells standing right and left of the axis of growth, and seeds containing a minute embryo lying in a mass of albumen. They are generally considered to be in the closest alliance with Scrophulariacea, but it is possible that their resemblance to that order is one of analogy rather than affinity. Along with Orobanchaceae and Monotropaceæ, they seem rather to belong to the albuminous group of Exogens, as has already been shown. [EXOGENS.] The flowers of these plants are usually coloured with pure bright yellow, red, or blue, and in many cases they are on this account among the most beautiful of flowers; but if we have a high development of form and colour in the majority of the species of this order, so we also have in the Guayana and Mexican plants belonging to the genus Voyra or Leiphaimos, the brown leafless habit and low development of Orobanche. This order is famous for its bitterness, which seems to pervade all the species. Gentiana itself furnishes all the officinal kinds; but Erythræa Centaurium, a beautiful wild-flower, common in many parts of England, is advantageously employed by country-people as a substitute;

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and the root of Frazera Walteri has been used as a means of adulterating the bitter Calumba root. GENTLEMAN, a corruption of gentilhomme, our Saxon ancestors having very early substituted mon,' or man,' for the corresponding term of the Norman-French, from which they originally received the term. Some form of this word (a compound of gentilis and homo) is found in all the Romance languages (gentil-homme in French, gentil-uomo in Italian, and gentil-hombre in Spanish), and it is undoubtedly one of the many traces of the great influence which the laws and polity of Rome have exercised upon modern society and civilization.

In the earliest form of the Roman constitution the populus, or ruling portion of the community, was divided into gentes, who were united by a common name, and the performance of certain sacred rights. Each gens was again subdivided into several familiæ, distinguished by a surname in addition to the common gentile appellation. Thus the gens Cornelia comprised the families of the Scipiones, the Lentuli, the Syllæ, &e. Now, in default of the Agnati, or of heirs in the male line, the property of the family reverted, not to the whole populus, in whom, of strict right, the sovereignty lay, but to the gens to whom it had transferred its rights.

This right of reversionary succession, especially in the case of usufructuary possessions of the public lands, being the most valuable and important privilege of the patricians, was brought prominently forward in the quarrels between the nobles and the plebeians, and the phrase gentem habere (Livy, x. 8) is often employed as distinctive of the former, Afterwards, when the members of the plebs obtained the right of intermarriage with patrician families, and an access to the honours of the state, which conferred the jus imaginum, or nobility, they also received the rights and privilege of gentes. To be admitted into a gens, became hence. forward equivalent to a patent of nobility in modern times, and gentilis is accordingly widely defined by Cicero (Top. 6), as denoting those who were of the same name and stock, free-born, in the full enjoyment of all the rights of citizenship, and whose ancestors were always free. Hence also, in an opposite sense, sine gente' is employed by Horace (Sat. ii. v. 15) and Suetonius (Tib. 1) for ignobly born and of servile parentage.

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This privilege of succession, which was called jus gentilitatis, or simply gentilitas (Cic., De Oratore, i. 38), and formed one of the enactments of the Twelve Tables, was gradually undermined by the encroachments of the prætors on the civil law, and finally disappeared (Gaius, iii. 25); but the name has survived in all the languages of Western Europe.

According to Selden (Titles of Honour, p. 852), a gentleman is one that either, from the blood of his ancestors, or the favour of his soveraigne, or of those that have the vertue of soveraigntie in them, or from his own vertue, employment, or otherwise, according to the customes of honour in his countrie, is ennobled, made gentile, or so raised to an eminencie above the multitude, that by those lawes and customes he be truly nobilis, or noble, whether he have any title, or not, fixed besides on him.' That the word was formerly employed in this extensive signification is clear, from a patent of Richard II., by which one John de Kingston is received into the estate of a gentleman and created an esquire (Nous lui avons resceivez en l'estate de gentilhome et lui fait esquier); and from another of Henry VI., who there, by the term nobilitamus,' creates one Bernard Angevin, a Bourdelois, a gentleman. And, according to Smith (De Rep. Ang., lib. i. c. 20, 21), under the denomination of gentleman are comprised all above yeomen, whereby noblemen are truly called gentlemen.

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In a narrower sense a gentleman is generally defined to be one who, without any title, bears a coat of arms, or whose ancestors have been freemen; and by the coat that a gentleman giveth, he is known to be, or not, descended from those of his name that lived many hundred years since.' (Jacobs' Law Dictionary.) There is also said to be a gentleman by office and in reputation, as well as those that are born such (2 Inst. 668); and according to Blackstone, quoting Sir Thomas Smith (1 Comm., p. 406), 'Whosoever studieth the laws of the realm, who studieth in the universities, who professeth the liberal sciences, and (to be short) who can live idly and without manual labour, and well bear the port, charge and countenance of a gentleman, he shall be called master, and taken for a gentleman.' VOL. XI.--R

The learned author must have been somewhat puzzled | with his definition of a gentleman, as understood in his time. Having defined a gentleman to be one who studieth the laws, &c., he adds (to be short), that he who can live idly and bear the port, &c. of a gentleman, is a gentleman; that is, if he can live idly, and if he can also do as a gentleman does (it not being said what this is), he is a gentleman. Perhaps a definition of the term, as now used, would not be easily made; it being extended by the courtesy of modern manners to many who do not come within the antient acceptation of the term, and denied by public opinion to many whose rank and wealth do not make up for the want of other qualifications.

GENTOOS. [HINDUSTAN.] GENUS, in physics, signifies a multitude or class of objects possessing some common quality or qualities: in logic it denotes the material part of the definition.

When we direct our attention to a particular object, we discover under its apparent unity a great variety of characters and qualities; and, upon an examination of several objects, we observe many points of agreement and difference between them. By the power which we possess of concentrating our faculties, we are enabled to consider these mutual relations and resemblances without any regard to their differences: we as it were draw the one away from the other; in short, we abstract them.

Now by abstraction we may either confine our view to a quality inherent in some object independently of that object; or else, neglecting the many points of disagreement which exist between a number of objects, we may seize upon the qualities that belong to all in common, in order to combine them into a single idea. In the former case the notion is simply abstract; in the latter it is abstract and general; and the multitude of objects to which we apply the general notion or common term constitutes a genus.

In this operation we may proceed continually by neglecting in succession a greater number of differences, and comprising under the common denomination fewer points of agreement and resemblance. In this manner we form a series of notions or genera of higher and lower order, until we ultimately arrive at the highest possible-that of being. In this co-ordination of genera, every intermediate genus is called a subaltern genus or species, being such in respect of different other terms; for that of which a higher genus is predicated is called a species, while relatively to all lower species it is itself a genus. Lastly, that which is not contained under any higher, is called the summum genus, and that under which individuals only are comprised is usually called the infima species.

These general notions and genera are the principles of classification and arrangement, and without them the knowledge of facts and nature itself would be, if not absolutely impossible, at best a confused mass of conceptions and objects without beauty, order, or coherence. But at the same time that we thus admit the utility of such general notions, we must remember that they are purely relative to human science and its objects; that even as such they are imperfect, and very far from conveying an adequate expression of the truth of nature, wherein there is nothing really corresponding to them, but only a something in the individual objects from which we derive them, which not only is the cause and the occasion of our forming them, but also transferring to them, as it were, a part of its own verity and existence, justifies us in according to them our confidence

in science and action.

GEOBDELLA. (Zoology.) [LEECH.] GEOCENTRIC (having the earth as centre), a term applied to the place of a planet, as seen from the centre of the earth, in opposition to its heliocentric place, as seen from the centre of the sun. [PARALLAX.]

GEOCICHLA, a genus of birds established by Mr. Gould for a pretty species resembling the Redbreast (Erithacus Rubecula, Swainson). It belongs, he observes, to an interesting group which was first characterized by M. Kuhl, and of which the Society's collection possesses four wellmarked species. (Zool. Proc. 1836.)

GEOCOCHLIDES, Latreille's name for the shell-snails: Trachélipodes colimacés of Lamarck; Limacinés of De Blainville; Limaçons of De Férussac.

GEODESY is that branch of applied mathematics which determines the figures and areas of large portions of the earth's surface, the general figure of the earth, and the

variations of the intensity of gravity in different regions, by means of direct observation and measurement.

Some of the antient philosophers, who lived several centuries before the Christian æra, were acquainted with the nearly spherical form of the globe, and even devised methods for measuring approximately a meridional circumference [ERATOSTHENES]. The Arabs long afterwards pursued the same object, and the Caliph Almamoun, in A.D. 814, ordered the measurement of a degree in the plains of Mesopotamia, an example which, after another long interval, was imitated by Snellius in Holland, Norwood in England, and by several French and Spanish mathematicians. Richer observed a variation in the length of the seconds' pendulum when sent to Cayenne by the French academy of sciences, the true cause of which phænomenon was explained by Newton; for the centrifugal force arising from the earth's rotation round its axis in twenty-four hours is directly opposed to the force of gravity at the equator, and in other latitudes the part of this force which acts in the direction of the plumb-line is nearly proportional to the square of the cosine of the latitude. He has also proved in his 'Principia' that a uniform fluid spheroid, in which the ratio of the centrifugal force to the attraction at the surface was the same as in the earth, would be in equilibrium when the axis of revolution was less than the equatorial diameter by 1-230th of the latter. From that time measurements have been undertaken under the directions of the various governments of Europe, to determine if the globe were really flattened at the poles, and also for the purpose of forming exact maps with respect to the latitude, longitude, and altitude above the level of the sea, of places in their respective dominions; and lastly, the French have deduced their legal metre of length from the measurement of a particular meridian Jacobi has lately shown that a revolving fluid ellipsoid, having three unequal axes, may also present a surface of equilibrium.

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The result of so many geodetic enterprises has not been as successful as could be wished. Some of the earliest attempts by the French were faulty in computation, and gave results directly contrary to Newton's theory, and some able mathematicians of that day appear to have been misled by a feeling akin to envy, to the extent of supporting these false conclusions by plausible reasonings. The error of that survey has been since discovered, and all the methods which have been employed in the numerous trials undertaken in this and the last century agree in proving that the polar axis of the earth is shorter than the equatorial by about1-300th; but they have served at the same time to demonstrate that the earth is not a spheroid, that it is not a solid of revolution, and that the figures of the northern and southern hemispheres are dissimilar. Hence if we suppose a solid of revolution having its axis in the same direction as that of the earth, and osculating the surface of the latter, the excentricity of this spheroid varies both with the latitude and the longitude of the place.

If the materials which compose the solid mass of the earth had equal capacities for heat and became liquid at equal temperatures, the spheroid of revolution would most probably be the figure assumed at the epoch when the cooling of the whole had rendered it solid. Such however is not the case; a great portion of the surface of the globe is yet liquid, and of the solid parts some must have assumed that state prior to others. It is also possible that the temperature of space is variable within the extent of the solar system, and therefore the conditions for the cooling of the northern and southern hemispheres may be different, and a very small difference would suffice to produce, in a long series of ages, a marked difference between the temperatures of the two hemispheres, and therefore a corresponding difference would arise relative to their forms. The general sphericity of the earth cannot be otherwise conceived than by its primitive fluidity, and the irregular cooling of its parts accounts sufficiently for the observed departures from the spheroidical shape, which would have been otherwise produced by the attraction of its parts and the centrifugal force of rotation. The other bodies of the solar system which have short periods of rotation present the analogous appear ance of unequal axes, the equatorial axis being always the longer.

In the trigonometrical survey of portions of the earth's surface, the extent or area may be computed more and more approximately by the suppositions of such portions being plane, spherical, spheroidical, and lastly of being

coincident with the osculating spheroid. We shall now explain the manner in which geodetic measurements are conducted, and the various corrections and reductions which it is necessary to apply to the principal calculations.

In order to measure an arc of the meridian, a series of stations are chosen near it in the most advantageous positions which the locality will permit; the lines which successively connect those stations form a series of triangles, in each of which, if one side and two angles, or two sides and one angle, are known, the remaining sides and angles are determinable by trigonometry; and if one extended line connected with the triangles, and called a base of verification, be measured, it serves at the different stages of the operation to detect any small errors which may have crept into the calculations. The stations should be so chosen that the triangles should be as nearly equilateral as possible, for then the errors of observation in the angles have the least influence in producing corresponding errors in the sides opposed to them. The measurement of the angles is effected by a theodolite, to which one or more telescopes are attached, with circles or arcs accurately graduated and accompanied by a vernier. The French, in the great survey between Dunkerque and Barcelona, which was conducted by Delambre and Mechain, employed Borda's repeating circle; while, in the English and Irish survey, a zenith sector, constructed by Ramsden expressly for this purpose, was used. It is of great importance that the telescopes should have a motion in azimuth as well as a vertical motion. In a Memoir on the Doctrine of Chances applied to geodetic operations (Connaissance des Tems,' 1820, p. 422), Laplace shows that it is in general an advantage to have the series composed of as few triangles as possible, and yet in the surveys we have mentioned a great number were employed without producing on an extended base any considerable error. When a country is deficient in spires, towers, or other lofty edifices, situated conveniently for stations, artificial ones are easily raised, and if illuminated at the top and provided with parabolic reflectors directed towards the observer, these, when employed by night, are found eminently successful. Care must however be taken that this station be estimated at the foot of the vertical passing through the object observed, and corrections must be applied for any eccentricity in the position of the telescope, or error in its line of collimation. The angles observed not being in the horizon, must be reduced to it by a formula given in most treatises on spherical trigonometry. (Woodhouse's Trigon., Appendix.) M. Delambre again reduced the latter angles to the angles between the chords of the spherical area between the stations, and therefore his series corresponded to the edges and faces of an inscribed polyhedron. (Base du Système Métrique.)

The three angles of any triangle in the series when added together are always greater than two right angles, which is a known property of every spherical triangle; but M. Legendre has shown that they may be treated as plane triangles by taking from each angle the third part of the excess of the sum above two right angles-a theorem of great use in geodesy, and which, as the same mathematician has proved, may be extended to spheroidical or other triangular portions of curved surfaces.

It happens not unfrequently that the instrument cannot be placed at the very centre of the station which forms the true angular point of the particular triangle in the series; by placing it as near to the centre as the observer conveniently can, a small correction, which is easily calculated, will reduce the observed angles to those which would have been observed had the centre been the point of observation: this correction is called the reduction to the centre. (Delambre, Déterm. d'un Arc du Mérid., p. 24.) A similar correction must be applied when the observed object is a tower with a polygonal base. Another source of error is the oblique illumination of the observed object, which it is most important to correct, many of the surveys of the last century being faulty by the observer either disregarding or being ignorant of the change of apparent position which is thus produced.

The actual measurement of the bases is one of the most delicate operation in geodesy, and requires the greatest precaution; it is best that they should be as long as possible and chosen on level ground, or at least that they may be in vertical planes, so as to correspond to arcs of a great circle when the earth is regarded as spherical. But the great difficulty is to determine their lengths in reference to a fixed

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unit of length; for whatever material may be employed for the chain or rod of measurement, the variations of temperature will produce sensible alterations in their length. These indications must be reduced to a fixed state of the thermometer, and if they are of a nature to be affected by moisture, it wil oe necessary also to make a correction for the hygrometric state of the atmosphere. The French employed for rods a species of metallic thermometer consisting of a copper rod placed on one of platinum, which had precisely the same length at a known temperature; as these metals expanded unequally by heat, the difference easily indicated the proper correction: the English in the late survey first employed glass at Hounslow-heath, and afterwards also steel rods, and applied the correction for temperature, which was small in the former case. The following table gives the proper corrections for the materials generally used; it must be however remembered that the linear dilatation is not always the same in the three dimensions of a body:

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General Roy in 1784 measured a base of five miles on Hounslow-heath, reducing his observations to the level of the sea and a temperature of 62° Fahrenheit, and formed a series of triangles between Greenwich and Dover. After his death (1790), Colonel Mudge extended it to Dunnose in the Isle of Wight; a verification-base being measured on Salisbury Plain; and the same great survey has been extended to Ireland and Scotland under Lieutenant-Colonel Colby, by whom a base of seven miles was measured near Londonderry.

The irregular figure of the earth is the cause that the geodetic meridian is not a plane curve. If through a point on the earth's surface and the axis of the earth a plane be drawn, this plane intersects the celestial sphere in a great circle, which is the celestial meridian of the place. Conceive verticals to be drawn parallel to this plane; the points where these verticals meet the irregular surface of the earth have evidently a common celestial meridian, and since the radius of this circle is indefinitely great, the locus of all these points forms a geodetic line. If another section of the surface be taken perpendicular to this, the radii of curvatur of these two curves at their common intersection are suffi cient to give that of any other section made through the same point by a plane of known inclination to either, whatever be the figure of the earth's surface, and the sum of the curvatures of any two rectangular sections through the same point is constant. The geodetic line possesses the singular property of being the shortest route between any two points taken in it; the equations to this curve of double curvature may therefore be found either by the differential calculus, if we consider the points of which it is the locus, or by the calculus of variations, if we regard the above-mentioned property. When the surface is one of revolution, this line is in the plane of the celestial meridian of the place, and is the same as the curve of revolution; but as it is not a plane curve, it follows that the earth is not a solid of this nature.

The refraction of light by the atmosphere is very great when the visual ray is nearly horizontal; and hence arise great errors in the measurement of angles, whether the observed objects are in the same level or not. These errors are generally remedied by an empirical law for terrestrial refraction, but all such laws fail to apply in the varied states of rarefection or of moisture in which the lower strata of the atmosphere are found; the best remedy is to seize the most propitious opportunities, when the heat of the surface of the earth has undergone no sudden changes, and when the atmosphere is fair and free from fogs

All the preceding corrections being made so as to ensure | the accuracy of the observations, it is necessary to reduce all to one level: for this level the mean surface of the sea, between its ebb and tide, or that which would be its level if there were no tide, is selected. The barometer must then be used to determine the altitude of the place of observation above this level, and a formula given by Laplace, and inserted in most treatises on hydrostatics, being applied, will give the altitude of the place, which with its latitude and Longitude are all the co-ordinates necessary to determine its position. The preceding remarks are probably sufficient to give an idea of all the difficulties and delicate processes necessary for an exact trigonometrical survey.

The results of the most careful geodetic measurements show that the earth is compressed at the poles and extended at the equator. The lengths of a degree diminish regularly in the following different countries, to which we have annexed the names of the surveyors: Sweden, Melanderhielm; England, Mudge; Cape of Good Hope, Lacaille; France, Delambre; Italy, Boscovich; Pennsylvania, Mason; Peru, Bouguer; India, Lambton: but in distances which are small compared with the surface of the earth, the alteration in the length of the degrees is very irregular, as is strikingly manifested by the English base of General Roy, connected with the French of Delambre prolonged by Biot and Arago. Another method has been adopted to determine the ellipticity of the earth by means of a seconds' pendulum, which, as well as direct measurement, indicates the flattening of the earth towards the poles. The following table is taken from the Mécanique Céleste,' tom. ii. No. 42; the latitudes are expressed in grades, and the length of the pendulum at Paris is adopted as the unit; the seconds are centesimal and of mean time:

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Arengsberg. 64.72 Petersburg. 66×60 Lapland . 74.22

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1.00137 Academicians. By employing the method of least squares, and assuming the figure of the earth to be nearly spheroidical, Mathieu has deduced from these observations, as the fraction which expresses the eccentricity; the theory of the lunar inequalities make it to be,, which differs but little from the former, and still less from that which corresponds to the southern emisphere; the English observations would give •

In the Philosophical Transactions' from the year 1819 to 1830, may be found the experiments on the length of the pendulum by Captains Kater, Sabine, Foster, and Mr. Fallows; an account of which, together with those of Freycinet and Duperrey, is inserted in the 7th volume of the Memoirs of the Royal Astronomical Society,' by Mr. Baily. Besides the works already quoted, the reader may consult with advantage the article on the Figure of the Earth, by Mr. Airy, in the Encyclopædia Metropolitana,' and the Traité de Geodésie,' by Puissant.

GEOEMY'DA. [TORTOISES.]

GEOFFRÆA INERMIS, or ANDI'RA INERMIS, is a tree inhabiting the tropical parts of America, and yielding a bark, with emetic, drastic, purgative, and narcotic properties, and in large doses poisonous. It acts as a powerful anthelmintic. The leaves are pinnate, and covered with a rusty down; the leaflets are oblong-lanceolate, or ovate-lanceolate, acuminate, and the flowers are arranged in terminal and axillary ferruginous panicles, very showy, with reddish lilac petals. Legume the size of a large plum. An account of it by Dr. Wright will be found in the Philosophical Transactions for 1777, p. 512, t. 70.

GEOFFREY OF MONMOUTH, otherwise named ARTHUR, the well-known British historian, was born in the town from which he took his name, and is supposed to have received his education at the Benedictine monastery in its vicinity. Tradition still points out a small apartmen

in the remains of that monastery which is designated as his study. He was made archdeacon of Monmouth, and on the 24th February, 1152, consecrated bishop of St. Asaph. Robert, earl of Gloucester, natural son of Henry I., and Alexander, bishop of Lincoln, were his chief patrons. Walter Mapes, at that time archdeacon of Oxford, a diligent inquirer for his day after the works of antient authors, is said, whilst journeying in Armorica, to have met with a history of Britain written in the British tongue, the translation of which, upon his return to England, he recommended to Geoffrey of Monmouth, who undertook the task and completed it with great fidelity. At first he divided it into four, but afterwards into eight books, to which he added the book of Merlin's Prophecies,' which he had also translated from British verse into Latin prose. Numerous fabulous and trifling stories are inserted in the history, to an extent which has induced some authors, and among them Buchanan, to consider the whole as fiction; but others, among whom are Archbishop Usher, Leland, &c., consider that parts of his history are true, and that the work is not to be rejected in the gross. The best Welsh critics seem to consider that Geoffrey's work was a vitiated translation of the 'History of the British Kings,' written by Tyssilio or St. Talian, bishop of St. Asaph, who lived in the seventh century. Geoffrey's omissions, additions, and interpolations are very numerous; and his Latin for British appellations frequently very difficult to understand.

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Several editions of Geoffrey's history are extant in Latin; the earliest is in 4to., printed by Ascensius at Paris in 1508; reprinted, 4to., 1517. It was also printed by Commeline at Heidelberg, in folio, 1587, among the Rerum Britannicarum Scriptores vetustiores et præcipui. A translation of it into English, by Aaron Thompson, of Queen's College, Oxford, was published in London, 1718, in 8vo. (Tanner, Bibl. Britan. Hib., pp. 305, 306; Nicholson's Hist. Library ; Chalmers's Biog. Dict., vol. xviii., p. 488-492.)

Copies of Geoffrey of Monmouth's history, in manuscript, are not unfrequent in our great libraries: several, of an age very near his time, are preserved among the manuscripts of the Old Royal Library in the British Museum; one formerly belonging to the library of Margan Abbey is believed to be the best. Geoffrey of Monmouth died about the year 1154.

GEOGRAPHY (a term derived from the Greek yewypapia, geographia) is a science the general object of which is to describe the surface of our globe. Its more special object is to ascertain and describe such physical peculiarities in each country as tend to promote or retard the increase of population and the arts of civilized life.

The political condition of a nation and the changes to which it is subject are in a great degree dependent on the character of the country which it inhabits, or of those countries which surround it. The difference in civilization observed in nations living near one another may also in a great degree be ascribed to the same cause. Accordingly we find that as soon as men began to apply themselves to the explanation of such changes and differences, they were obliged to look to the particular character of the countries inhabited by those nations whose history it was their object to investigate. Geography is coeval with history. It is as impossible to form a just idea of the events which have been most decisive in the history of a nation without a knowledge of their country, as it is to understand the movements of two armies on a field of battle without knowing the nature of the ground which is the scene of their operations.

Herodotus, the father of history, is likewise the father of geography. His geographical descriptions are short and general, but always clear and sufficient to show how far the physical peculiarities of each country influenced the changes and events which he had undertaken to commemorate. When he found that a country was characterized by striking peculiarities he described them at considerable length. An instance of this is his description of Egypt in the second, and his description of the Scythians and their country in the fourth book.

There is however something vague in the descriptions of Herodotus, for want of a means of referring to the position of places as determined by astronomical observations. Herodotus indeed was apparently not fully acquainted with the state of science, and particularly astronomical knowledge, as it existed in his age. Thales had some time before calculated an eclipse of the sun, and from his epoch astronomy attracted the attention of the Greek philosophers, and facts

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