Imágenes de páginas

Ptolemy. Strabo makes mention of the terrestrial globe : and a contemporary of his, Propertius, refers directly to depicted worlds; and Claudius, who describes Archimedes's glass sphere, evinces great knowledge of the constructions of an orrery, spheres, &c. that then existed among mathematicians. Among the improvers and makers of globes may be subsequently ranked the following as chief: Tycho Brahe, Regiomontanus, Schonerus, Gemma Fricius, Gr. Mercator, J. Hondius, Johnsonius, Wm. Saunderson, Wm. Bleau, &c.; some of whoma wrote learnedly on their uses: but, in this respect, the preference is certainly due to our countryman, Mr. Robert Hues, whose Latin treatise was afterwards published by Handinus, and then by Pontanus, with figures and notes. This work was afterwads translated into English by J. Chilmead, in the year 1639. No globes had any pretensions to accuracy, taste, or elegance, till the time of Mr. John Senex, F. R. S.; who, about the year 1739, delineated and engraved sets of plates for globes of nine, twelve, seventeen, and twenty-eight inches in diameter, which he used with the globes then manufactured by himself,and making these instruments more accurate and useful than any former maker. The terms and names of places on the globes of seventeen and twenty-eight inches in diameter were in Latin. About the year 1759, and just after the decease of Mr. Senex, Mr. Benjamin Martin,alearned optician,became possessed of Mr. Senex's plates, and continued for many years to manufacture the globes, but with additional skill, and with various improvements. About the year 1765, the late Mr. George Adams caused new plates for eighteen and twelve inches globes to be engraved. The terms and names of these, like the larger ones of Senex, wereprinted in Latin. Instead of horary circles fixed on the meridian, with moveable indices for computation of time, Mr. Adams contrived circular wires, to envelop the globe about the equinoctial circles, with sliding brass points; so that, as the globes were revolved on their axes, the time by these was pointed out on the graduations of the above great circle ; which consequently gave a more extensive and conspicuous scale of time, than could be had by means of the smaller horary circles. He also applied to each globe a semi-circular slip of brass, connected at the poles,

having on the terrestrial a sliding compass bearing circle ; and on the celestial a small sliding sun. The brass slips were graduated each way from the equinoctial, so that the positions for rhomb-lines, right ascensions, and declinations, could be better and more readily obtained. The horary, or hour circle of the globes being usually attached to the external edge of the meridians, prevented a free and uninterrupted motion of the meridians, with their poles through the horizons of the globes, to admit of an universal position of the axis with respect to the horizon, for all latitudes of places. Mr. James Harris, of the Mint, in the year 1740, contrived a method of fixing the brass horary circles at the poles, under the meridians; i.e. between the surface of the globes and interior edge of the meridian, and to be occasionally moveable,independent either of the globe or meridian. In this manner the globes were rendered completely useful for the solution of problems in all latitudes. About the year 1785, Mr. G. Wright contrived a moveable index, applicable to the poles of a globe, to act in a similar manner to the circle of Mr. Harris, which pointed to a circle of hours engraved round the poles of each globe. This he considered a method of obviating the great friction, or adherence, that sometimes inconveniently takes place between the surfaces of the circle and globe. From the lapse of years, the numerous astronomical and geographical discoveries, and the Latin terms: adopted in the larger globes of Senex and Adams, these globes became inconvenient, embarrassing, and finally obsolete. A short time before the year 1800, sets of new and accurately engraved plates were suggested, and considered as a desideratum in astronomy, by the Astronomer Royal, Dr. Maskelyne, Sir Joseph Banks, Professor Vince, and others; and conformably to this object, in the year 1800 were completed and produced a set of entirely new plates for globes of eighteen inches in diameter, and under the denomination of the “New British Globes.” The graduations and lines are laid down in the most correct manner, and with much greater accuracy than in any former globe plates. The drawing from which the terrestial

is engraved was an entirely new one,

from the hands of Mr. Arrowsmith, an eminent geographer. The latitudes and longitudes of places are rectified from the latest and best authorities; and there are likewise inserted all the authentic discoveries to the present time. The celestial globe contains a description of a complete catalogue of stars, clusters, planetary, nebula, &c to the amount of nearly 6,000, from the observations and communications of Dr. Maskelyne, Dr. Herschel, Rev. Mr Wollaston, &c., and inserted from calculations made by Mr. W. Jones, optician of Holborn, in their exact positions, to the present period. To the prinicipal stars are annexed Bayer's Greek letters of reference ; and the whole are circumscribed by well-designed figures of the constellations, faintly engraved. The great circles are divided into twenty minutes of a degree, and the equinoctial in addition into two minutes of time, so that, by estimation, the solution of problems may be obtained to five minutes of a degree, or half a minute of time; a degree of accuracy sufficiently useful, not only for all the common problems, but most of the trigonometrical ones. As the reading off of time is found to be a ready and convenient nethod by hour circles attached to the micricians, the horary circle has been contrived to admit of being slid away from its pole, upon the exterior edge of the meridian; this is done by making the extremity of the pole which carries the index of the horary circle moveable, by unscrewing. The hovary circle being attached to the meridian barely by springs, when the index is unscrewed, the circle may consequently be slid to any part of the meridian. This contrivance is necessary only for the circle of the north pole of Messrs. W and S. Jones's terrestrial globe, who have aiopted this circle, and at the south pole of the globes have applied the interior brass index, or circles above mentioned. Plates for the British globes of twelve inches diameter have been reduced and abridged, from the eighteen inches above mentioned. Plates for globes of nine, twelve, and twenty-one inches diameter, have been engraved by Mr. Cary, of the Strand. The stars of the celestial globe are not circumscribed with the figures of the constellations. GLOBULAR chart, a name given to the representation of the surface, or of some part of the surface of the terrestrial globe upon a plane, wherein the parallels of latitude are circles nearly concentric, the meridian curves bending towards

the poles, and the rhomb-lines are also cut Wes. Globu LAR sailing. See SAILING. GLOBULAR1A, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Aggregatae. Lysimachiae, Jussieu. Essential character: calyx common, imbricate; proper tubular inferior; corollets the upper lip twoparted ; lower three-parted ; receptacle chaffy. There are eight species. G.I.ORIOSA, in botany, a genus of the Hexandria Monogynia class and order. Natural order of Sarmentaceae Lilia, Jussieu. Essential character : corolla six-petalled, waved, reflex; style oblique. There are two species, viz. G. superba, superb lily, and G. simplex. GLOSS, in matters of literature, denotes an exposition or explication of the text of any author, whether in the same language or any other; in which sense it differs little from commentary. G10.SSOMA, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of 18hamni, Jussieu. Essential character: calyx turbinate fourtoothed, superior; corolla four-petalled ; anthers almost united, with a membranaceous scale at the end ; stigmas four, drupe. There is only one species, viz. G. guianensis, a native of Guiana, flowering in September. Voto.nita is the vernacular name. GLOSSOPETALTM, in botany, a genus of the Pentandria i'cntagynia class and order. Natural order of Rhamni, Jussicu. Essential character : calyx very small, five-toothed ; petals five, with a strap at the tip of each berry. There are two species, both lofty trees, natives of Guiana and Cayenne. GLOT TIS, in anatomy, the mouth or aperture of the larynx, through which the air ascends and descends in respiring. lt can be dilated or contracted at pleasure, and by the various vibratory motions of which the tones of the voice are modified. The name was applied by the ancients to an additional moveable part of the flute, which they placed between their lips in performance, and which is supposed to have bec n similar to our reed. GLOW storm. See LAMPY It Is. G1.0X1N1A, in botany, so called in honour of Ben. Petr. Gloxin, of Colmar, a genus of , he Didynamia Angiospermia class and order. Natural order of Personatae. Campanulaceae, Jussieu. Essential character: calyx superior, five-leaved; corolla bell-shaped, with the border oblique; filaments, with the rudiment of a fifth, inserted in the receptacle. There is only one species, viz. G. maculata, spotted gloxinia, a native of South America. “ GLUCINA, in chemistry, an earth lately discovered by Vauquelin, while he was analyzing the beryl, to ascertain whether its constituent parts were the same as those of the emerald. See Betty L. In this experiment he found the glucina, which is so named from its sweetish kind of taste. Glucina in the form of powder, or in fragments, is almost three times as heavy as water; it is infusible in the fire; it does not contract, like alumina, by great heat, and it has no effect on vegetable colours. The oxygen, nitrogen, and hydrogen gases have no action on it; nor is it acted upon by carbon, sulphur, or phosphorus. It combines with sulphurated hydrogen. Et is insoluble in water, but combines with acids, making with them soluble salts, distinguished by a sweet and slightly astringent taste.

GLUE, among artificers, a tenacious viseid matter, which serves as a cement to bind or connect things together. Glues are of different kinds, according to the various uses they are designed for, as the tommon glue, glove glue, parchment glue, isinglass glue, &c.

The common or strong glue is chiefly used by carpenters, joiners, cabinet-makers, &c. and the best kind is that made in England, in square pieces, of a ruddy brown colour, and next to this the Flanders glue. It is made of the skins of animals, as oxen, cows, calves, sheep, &c. and the older the creature is, the better is the glue made of its hide. Indeed, whole skins are but rarely used for this purpose, but only the shavings, parings, or scraps of them; or the feet, sinews, &c. That made of whole skins, however, is undoubtedly the best; as that made of sinews is the very worst.

In making glue of parings, they first steep them two or three days in water; then washing them well out, they boil them to the consistence of a thick jelly, which they pass, while hot, through ozier baskets, to separate the impurities from it, and then let stand some time, to purify it further: when all the filth and ordures are settled to the bottom of the vessel, they melt and boil it a second time. They next pour it into flat frames or moulds, whence it is taken out pretty hard and solid, and cut into square pieces or cakes. They afterwards

dry it in the wind, in a ‘sort of coarse net; and at last string it, to finish its drying. The glue made of sinews, feet, &c. is managed after the same manner; only with this difference, that they bone and scour the feet, and do not lay them to steep. The best glue is that which is oldest; and the surest way to try its goodness is, to lay a piece to . steep three or four days, and if it swell considerably without melting, and when taken out resumes its former dryness, it is excellent. A glue that will hold against fire or water may be made thus: mix a handful of quick lime with four ounces of linseed oil, boil them to a good thickness, then spread it on tin plates in the shade, and it will become exceedingly hard, but may be dissolved over a fire, as glue, and will effect the business to admiration. Glue, niethod of proparing and using. Set a quart of water on the fire, ti, on put in about half a pound of good glue, and boil them gently together till the glue be entirely dissolved, and of a due consistence. When glue is to be used, it must be made thoroughly hot; after which, with a brush dipped in it, besmear the faces of the joints as quick as possible; then clapping them together, slide or rub them lengthwise one upon another, two or three times, to settle them close ; and so let them sand till they are dry and firm. Give, parchment, is made by boiling gently shreds of parchment in water, in the proportion of one pound of the former to six quarts of the latter, till it be reduced to one quart. The fluid is then to be strained from the dregs, and afterwards boiled to the consistence of glue. Isinglass glue is made in the same way; but this is improved by dissolving the isinglass in alcohol, by means of a gentle heat. See CEMEN’rs. GLUME. See BotANY. GLUTA, in botany, a genus of the Pentandria Monogynia class and order. Essential character: calyx bell-shaped, deciduous; petals five, glued at bottom to the column of the germ ; filaments inserted into the tip of the column; germ sitting on an oblong column. There is only one species, viz. G. benghas, a native of Java. GLUTEN. With the fecula and saccharine matter which compose the principal part of nutritive grain, is another substance, approaching more nearly in its characters to animal matter than any other product of the vegetable system. From

the resemblance in its properties to the animal principle formerly called gluten, but now described under the term F1 hai N, (which see,) it has received the name of vegetable gluten. It is obtained in largest quantities from wheat, amounting to the twelfth part of the whole grain, by kneading the flower into paste, which is to be washed very cautiously, by kneading it under a jet of water, till the water carries off nothing more, but runs off colourless; what remains is gluten: it is ductile and elastic ; it has some resemblance to animal tendon or membrane : it is very tenacious, and may be used as a cement for broken porcelain vessels. It has a peculiar smell, with scarcely any taste. When exposed to the air it assumes a brown colour, and becomes apparently covered with a coat of oil. When completely dry it resembles glue, and breaks like glass. It is insoluble in water, alcohol, and ether; but the acids dissolve it, and the alkalies precipitate it. It has a strong affinity for the colouring matter of vegetables, and likewise for resinous substances. When kept moist it ferments, and emits a very offensive smell; the vapour blackens silver and lead. Its constituent parts are oxygen, hydrogen, carbon, and azote. It exists, as we have observed, most abundantly in wheat, but it is found in large quantities in many other plants. It is gluten that renders wheat so useful in the art of bread making. GLYCINE, in botany, a genus of the Diadelphia Decandria class and order. Natural order of Papilionaceae, or Leguminosae. Essential character: calyx twolipped; corolla the keel turning back, the banner at the tip. There are twenty-five species. There are eleven species enumerated by Muhlenberg, as natives of the United States, and a twelfth was discovered by Dr. W. P. C. Barton, professor of botany in the University of Pennsylvania, viz. G. parabolica. It is described by him in his Prodromus Flora Philadelphica. GLYCYRRHIZA, in botany, English liquorice, a genus of the Diadelphia Decandria class and order. Natural order of Papilionaceae, or Leguminosae. Essential character: cal: x two-lipped, upperlip three-parted, lower undivided; legume ovate, compressed. There are four species. These are tall growing perennial, herbaceous plants, with the stalk somewhat woody at bottom. The stipules are distinct from the petiole; the flowers in a head or spike from the axils and at the ends of the branches; seed vessel a legume or pod, smooth, hairy, or prickly.

GLYPH, in sculpture and architecture, denotes any canal or cavity, used as an ornament.

GLYSTER, or ClystER, among physicians. See Clyst ER.

GMELINA, in botany, so called in honour of Joh. George Gmelin, professor of natural history at St. Petersburgh, afterwards of botany at Tubingen, a genus of the Didynamia Angiospermia class and order. Natural order of Personatae. Witices, Jussieu. Essential character: calyx slightly four-toothed; corolla four-cleft, bell-shaped ; anthers two parted, two simple ; drupe with a two or three-selled nut. There is but one species, viz. G. asiatica. GNAPHALIUM, in botany, a genus of the Syngenesia Polygamia Superflua class and order. Natural order of Compositae Discoideae. Corymbiferas, Jussieu. Essential character: calyx imbricate, with the marginal scales rounded, scariose, coloured; down feathered ; receptacle naked. There are sixty-six species: the numerous species of this genus are chiefly under shrubs or herbs; the leaves are alternately placed, generally hoary : the flowers usually terminate the stem and branches in globes or corymbs. The calyx is permanent, with yellow or white scales. There are nine American species.


GNEISS. in mineralogy, is composed

rincipally of felspar, quartz, and mica,

...; plates, laid on each other, and seÉ. by thin layers of mica. It differs rom granite by being shistose; though, like that, it sometimes contains shorl and garnet. The beds of gneiss sometimes alternate with layers of granular limestones, shistose, hornblende, and porphyry. It is rich in ores, almost every metal has been found in gneiss rocks, either in veins or beds. Mr. Jameson mentions four kinds of gneiss: 1. That which approaches to the granular structure. 2. The waved or undulated. 3. The common; and, 4. The thin slaty ; and he says the order of their transition is also that of their relative antiquity,consequently the more granular the structure, the older the rock; and, on the contrary, the more slaty, the newer it is. In the last member of the series is the smallest portian of felspar, and largest of mica; hence its texture is more completely slaty than that of any of the others. The other extremity contains much felspar, and but little mica. The common contains a nearly equal quantity of felspar and quartz.

GNETUM, in botany, a genus of the Monoecia Monadelphia class and order. Natural order of Piperitae. Urtica, Jussieu. Essential character: male an ament with scales; corolla none; filaments one, with two anthers: female an ament with scales; corolla none; style with a bifid stigma; drupe with one seed. There is only one species, viz. G. gnemon, a native of the East Indies, where the leaves, male catkins and fruits, are eaten. GNIDIA, in botany, a genus of the Octandria Monogynia class and order. Natural order of Vepreculae. Thymeleas, Jussieu. Essential character: calyx funnel-form, four-cleft; petals four, inserted into the calyx; seed one, somewhat berried. There are eleven species. GNOMON, in dialing, the style, pin, or cock of a dial; . by its shadow, shows the hour of the day. The gnounon of every dial represents the axis of the world. See Dial and DIALING. GNoxon, in geometry. If, in a parallelogram (Pl. VI. Miscel. fig. 1) the dia. meter A C, be drawn; also two lines, EF, HI, parallel to the sides of the parallelogram, and cutting the diameter in one and the same point, G, so that the parallelogram is, by these parallels, divided into four parallelograms, then are the two parallelograms, D G, BG, through which the diameter does not pass, called complements; those through which the diameters pass, E H, F I, are called the parallelograms about the diameter; and a gnomon consists of the two complements, and either of the parallelograms about the diameter, viz. G D + H E + E I, or G D + F I + G B. GNorton, in astronomy, a stile erected perpendicular to the horizon, in order to find the altitude of the sun. Thus in the right angled triangled A B C, fig. 2, are given A B, the length of the stile, B C, the length of its shadow, and the right angle, A B C. Hence, making C B, the radius, we have this analogy for finding the angle, A C B, the sun's altitude, viz. BC: A B : : radius: tangent of the angle C. By means of a gnomon, the sun's meridian altitude, and consequently the latitude of the place, may be found more exactly than with the smaller quadrants. By the same instrument, the ho of any object, G H, may be found; for as D F, fig. 3, the distance of the observer's eye from the gnomon is to D E, the height of the stile, so is FH, the distance of the observer's eye from the object, to G H, its height.

the Gnomon may be made useful in taking the meridian altitude of the sun, and thence finding the latitude of the place. Having a meridian line drawn through the centre of the gnomon, mark the point where the shadow of the gnomon terminates when projected along the meridian line, and measure the distance of that point from the centre of the gnomon, which will be the length of its shadow ; then, having the height of the gnomon, and the length of the shadow, the sun's altitude is easily found. Thus, if A B be the gnomon, and A C the length of the shadow, then in the right angled triangle, A B C, we have A B and B C given; hence the angle C is easily found, for C B : B A : : radius: tangent of the angle C ; that is, as the length of the shadow is to the height of the gnomon, so is radius to the tangent of the sun's altitude above the horizon. Ear. We learn from Pliny, at the time of the equinoxes, that the shadow was to the gnomon as 8:9, therefore we say as 8:9::R # =1.125, the tangent of an angle of 48° 22', which is the height of the equator at Rome, and its complement 41° 38' is therefore the height of the pole, or the latitude of the place. This method, however, requires correction for the sun's parallax, and for refraction. GNOMONICS, the art of p1 ALING, which see. From the shadow of a rod, perpendicularly or obliquely placed on a plane, may be determined a triangle, by drawing from the top of the rod a line that shall touch the luminous body, forming with the rod the least possible angle. The sides of the triangle will be, first, the part of this line comprehended between the top of the rod and the given É. then the rod itself; and lastly, the ine drawn from the bottom of the rod till it meets the other line already mentioned. This last line will be the shadow relatively to the given plane : it will increase and decrease in proportion as the sine of the angle, whose summitcoincides with the summit of the rod, shall be greater or less, that is, in proportion as the luminous body shall descend or ascend with respect to the given plane; and if that body move to the right or the left of the position first occupied by the triangle, that determines the shadow, which will move on the o in a contrary direction; and on these principles the art of dialing consists. GNOSTICS, in church history, a sect of Christians, so called from their preted

« AnteriorContinuar »