« AnteriorContinuar »
verdigris moistened with water, will answer the purpose. Copper, and the alloys formed by its combination with zinc, are gilded nearly in the same manner as silver; but as their affinity for mercury is coniderably less that that of silver, it would be difficult to made the amalgam of gold adhere to the burnished surface of these metals by the same means, and with the same evenness, as takes place in the case last described. To obviate this inconvenience, advantage is very ingeniously taken of the action of nitric acid to facilitate the adhesion of the copper and mercury, in the following manner. A piece of copper, a button for example, is cleaned, by steeping it in acid, and subsequent washing, and is then burnished, either in a lathe, or by any other means: after this it is dipped in a neutralized solution of nitrate of mercury, and in the space of a few minutes, owing to the strong affinity of nitric acid for copper, the mercurial salt is decomposed, the copper takes the place of the mercury, and at the same time the mercury is deposited in the metallic state on the surface of the copper, covering it entirely, and strongly adhering to it; the gold amalgam is now applied, and the rest of the process goes on as already described. By this method of proceeding, a given quantity of gold may be made to cover a larger surface than in any other way of gilding on metals; five grains of gold will completely gild both the upper and under surfaces of one hundred and fortyfour copper buttons, each of them an inch in diameter. There is no metal, the gilding of which presents so many difficulties as iron, or rather steel. If the method of simple burnishing down is had recourse to, the heat requisite for this purpose will, in many cases, bring the temper of the steel too low : on such occasions, the way already described for gilding on copper is sometimes practised; that is, the parts of the ...? to be gilded are pencilled over with nitrate of mercury, by which they are covered with a slightly adhering coat of mercury; then the amalgam is applied, and the gilding finished in the usual way. The objections to this mode of proceeding are, first, that a considerable heat is required, though inferior to that requisite for burnishing down; and, secondly, that, even with all possible care, the gilding is apt to be rough and scale off. A very considerable improvement in this way of gilding is, to trace the figure of the gilding on the steel first of all, with a
brush charged with a strong solution of sulphated copper, in consequence of which a pretty thick plate of this metal is deposited on the steel, to which it may be made to adhere, with considerable firmness, by means of the burnisher; thus the gilding is, in fact, performed upon the copper. A new method of gold-gilding upon steel has lately been published, possessed of many advantages over the others, and probably in time may attain to a very high degree of perfection. It depends upon the well known fact, that, if sulphuric ether and nitro-muriate of gold are mixed together, the ether will, by degrees, separate from the acid nearly the whole of the gold, and retain it for some time in solution, in nearly a metallic state. If ther, thus charged with gold, is spread, y means of a pen or fine brush, on the surface of highly polished steel, the ether F.'. evaporates, leaving the gold beind in close contact with the steel, and the adhesion is considerably improved by the subsequent application of the burnisher. The dearness, and especially the rapid volatility of ether, are, at first, objections of some moment, but may be got over by using the best oil of turpentine instead of ether, which has nearly the same efficacy in decomposing the nitromuriate of gold, and is both cheaper, and not so very quickly evaporable. Gold-gilding upon silver is, we believe, at present entirely disused. It was performed in the following manner: a saturated solution of gold, in nitro-muriatic acid, was poured upon some linen rags, and, when they were become dry, they were heaped in a plate, and touched with a hot coal. The fire gradually spread through the mass, and reduced it to a heavy black ash. A soft cork, being moistened in water, was dipped in thisash, to which a part of it adhered, and was then rubbed on the surface of polished silver, upon which the minute particles of gold became fixed, and covered it with an extremely thin coating, which, when burnished, exhibited the genuine colour and lustre of the precious metal. .shikin's Dict. GILL, a measure of capacity, containing a quarter of a pint.
GILT varnish. See WARNIsh.
GIMBALS, in sea affairs, the brass rings by which a sea compass is suspended in its box, so as to counteract the ef. fect of the ship's motion, and keep the card horizontal.
GIMBLETING, a term applied to the anchor, to denote the action of turning it round by the stock, so that the motion of the stock appears similar to that of the handle of a gimblet when it is employed.
GIN. See GEN Eva.
GIN, in mechanics, a machine for driving piles, fitted with a windlass and winches at each end, where eight or nine men heave, and round which a rope is reeved that goes over the wheel at the
top. GINANNIA, in botany, a genus of the Enneandria Monogynia class and order. Natural order of Lomentaceae. Leguminose, Jussieu. Essential character: calyx double, both one-leafed ; petals three, fringed, spreading; germ pedicelled, with a membranaceous wing at top ; legume. There is but one species, viz. G guianensis, ashrub about fifteen feet high; a native of the forests of ruiana. GINGER, in botany. See ZINzimer. GINORA, in botany, so called in honour of the Marquis Carlo Ginori, a genus of the Dodecandria Monogynia class and order. Natural order of Salicariae, Jussieu. Essential character: calyx sixcleft; petals six ; capsule one-celled, fourvalved, coloured, containing many seeds. There is but one species, viz. G. Americana, an elegant little shrub about four feet high ; it is a native of Cuba, grows by river sides, and called there rosa del rio, or river rose. GINSENG, in botany. See PANAx. GIRDERS, in architecture, some of the largest pieces of timber in a floor. Their ends are usually fastened into summers and breast-summers, and joists are framed in at one end to the girders. The size of girders and summers, upon the rebuilding of London, were fixed, by act of Parliament, to be in length from ten to twenty-six feet, in breadth from eleven to seventeen inches, and in depth from eight to fourteen inches. It was also fixed by the same statute, that no girder or summer should be less than ten inches in the wall, and that their ends should be laid in loam ; as also that they be of good oak, as free from knots as may be, because the least subject to breaking, and may with more safety be relied on in cross and transverse work. GIRT, in the measuring of timber, is the circumference of a tree, though some use this word for the fourth part of the circumference only, on account of the use made of it. The square of the fourth part is considered as equal to the area of the section of the tree, which square therefore, multiplied by the length of the
tree, is accounted the solid content. This content is about one fourth less than the true quantity, being nearly equal to what it will be *:: the tree is hewn square, and is probably intended to make an allowance for the squaring the tree. GIRT, in naval affairs, the situation of a ship which is moored so tight by her cables as to be prevented turning to any change of the wind or tide, to the current of which her head would otherwise be directed. The cables, to produce this, are extended by a strong application of mechanical powers within the ship, so that as she veers, or endeavours to swing about, her side bears upon one of the cables, which interrupts her in the act of traversing. G|RTH line, a rope passing through a single block on the head of the lower masts, to hoist up the rigging, and the persons employed to place the rigging and cross-trees on the mast heads. The girth-line is the first rope employed to rig a ship, after which it is removed till the ship is to be unrigged. GISEKIA, in botany, so named in honour of Paulus Dietericus Giseke, a genus of the Pentandria Pentagynia class and order. Natural order of Succulentæ. Por. tulaceae, Jussieu. Essential character: calyx five-leaved; corolla none; capsule five, approximating, roundish, one-seeded. There is but one species, viz. G. pharnacioides, trailing Gisekia, an annual plant, and a native of the East Indies. GlVEN, among mathematicians and philosophers, the same with data. If a magnitude be known, or we can find another equal to it, it is said to be given in magnitude. Or when the position of any thing is known, it is said to be given in position : when the diameter or radius of a circle is known, the circle is given in magnitude. The circle is given in position, when the position of the centre is given. See DATA. GLABRARIA, in botany, a genus of the Polyadelphia Polyandria class and order. Essential character: calyx five-cleft; pe. tals five; nectary composed of bristles the length of the calyx; stamens thirty, always in sixes ; pericarpium a drupe. There is but one species, viz. G. tersa, a large tree resembling the champhor tree, the wood of which is very light and pale coloured, and not being liable to rotor to be injured by insects, it is much used for building both houses and ships. It is a native of the East Indies. GLACIERS. Those vast piles of eternal ice, with which it has pleased the author of nature to crown the immense chasms between the summits of the Alps, objects more grand, sublime, and terrific, than are any others of the phenomena of nature which remain stationary. These tremendous spires and towers, of uncertain and brittle fabric, seem to forbid the attempts of travellers to explore the depth between them, or even the rocks and rich vallies around them ; but courage and perseverance have been attended with commensurate soccess, and we are enabled by their labours to learn previously concealed wonders,and toreason upon the causes which produced them. In treating on this subject, it must be remembered, with satisfaction, that great part of our information is derived from the exertions of several gentlemen distinguished for patient investigation and intrepid exploration. M. Bourrit, Precentor of the Cathedral Church at Geneva, mentions, in the relation of his journey to the glaciers of Savoy, the enterprise of Messrs. Windham and Pocock, in 1741, who, inspired by the artless relations of the peasants, descriptive of the sublimity of their country, when they descended with honey and chrystals for sale, determined to visit those frightful regions of ice which had received the appellation of Les Montagnes Maudites, or the accursed mountains; the gentlemen alluded to took every precaution for securing their safety; but entertaining many well-grounded fears, naturally arising from a first attempt, they did not reach any considerable distance beyond the edge of the ice in the valley of Montanvert, yet their example operated so powerfully as to induce several others to imitate them, and proceed to the boundary whence they returned; at length M. de Saussure had the resolution and courage to penetrate across the ice to the very extremities of the vallies; Mr. Coxe followed soon after; and every possible information may be obtained from their publications which the nature of the subject will permit. The most astonishing phenomenon attending the glaciers is their near approach to the usual vegetation of summer; for what can be more wonderful than to view wheat ready for the sickle, parched brown by the rays of the sun, separated only by the intervention of a few feet from the chilling influence of an endless bed of ice, which seems impenetrable to its rays. Many systems and theories have been ingeniously suggested to ascertain the first cause of the glaciers, their maintenance, and whether they increase or diVOL. VI.
minish in extent, of which Grunner's, im. proved and illustrated with actual obser vation by M. de Saussure, is the most rational and probable, and Mr. Coxe implicitly adopts it. Admitting that a person could be raised sufficiently above the summits of the Alps of Switzerland, Savoy, and Dauphine, to comprehend the whole at one view, he would observe a vast chaos of mountains and vallies, with several parallel chains, the highest of which are situated in the centre, and the remainder gradually lessening as they retire from it. The central chain he would find to be surmounted by stupendous fragments of rock, towering in rude masses, which bear vast accumulations of snow and ice, where they are not decidedly perpendicular, or do not overhang their bases ; on each side he would see the intervening chasms and gulphs, filled with ice, descending thence even into the verdant vallies rich with foliage and cultivation. The inferior ranges of mountains, next the central, present the same appearance in a lesser degree, but in those more remote the snow and ice is confined to the most elevated points; and others, still further removed, are covered with grass and plants, which, in their turn, give place to the hills and vallies common in any part of the world. Mr. Coxe divides the glaciers, in the above general survey, into two classes; the first occupy the deep vallies situated in the bosom of the Alps, and the second adhere to the sides and summits of the mountains. Those in the vallies are far more extensive than the upper glaciers, some are several leagues in length, and that of Des Bois is three miles broad and fifteen long ; but they do not communicate with each other, and there are few parallel to the central chain; their upper extremities are connected with inaccessible precipices, and the lower proceed, as already mentioned, quite into the vales; the depth of these astonishing accumulations of frozen fluid vary from eighty to six hundred feet, and they generally rest on an inclined plane; urged forward by their own enormous weight, and but weakly supported by the pointed rocks inserted in their bases, they are universally intersected by yawning chasms, of dreadful aspect to the curious investigator, who beholds fanciful representations of walls, towers, and pyramids, on every side of him ; , but upon reaching those parts where the glacier rests upon an horizontal plane, his progress is seldom impeded by considerable fissures, and he B
walks in safety over a surface nearly uniform, and not so pe:fectly polished as that of ponds and rivers suddenly and violently frozen. The absence of transarency, the various marks of air bubles, and the general roughness, so perfectly resemble the congelation of snow, when half restored to fluidity, that M. de Saussure was immediately led to form the following probable theory of the formation of the glaciers. Snow is constantly accumulating in the recesses or depths of the mountains, during nine months of the year, by the usual fall of moisture, and the descent of vast masses borne down by their weight from the precipices and crags above. Part of this is necessarily reduced to water by slight thaws and casual rains, and being frozen in this state, the glacier is composed of a porous opaque ice. The upper glaciers, Mr. Coxe subdivides into those which cover the summits, and those which extend along the sides of the Alps; the former originate from the snow frequently falling, and congealing into a firm body, though not becoming actual ice, which the brilliancy, of the projections has induced some philosophers to suppose it. M. de Saussure, having explored Mont Blanc, ascertained that the top was encrusted with ice, which might be penetrated by a stick, covering a mass of snow on the declivities, so chilled and dry as to be incapable of coherence. The sides of the Alps support a congelation of half dissolved snow, which is different from the pure snow of the summits and the ice of the lower glaciers. Two causes operate to produce this ef. fect ; the first is the descent of watcr from the higher regions, where a dissolution of the snow sometimes occurs; and the second arises from the more favourable situatian of these parts for reflecting the rays of the sun, and the consequent melting of the snow. From hence downwards the ice adhering to the cavities becomes gradually more solid by the freezing of the snow-water, then nearly divested of that air, which in the less dissolved portions renders the ice, formed from it, porous, opaque,and full of bubbles. Considerable difference of opinion has prevailed amongst philosophers, whether the masses of ice and snow in these regions of endless winter increase, decrease, or remain nearly stationary; Mr. Coxe seems inclined to think they vary in their size; that gentleman observes, that the glacier of Montanvert is generally
bordered with trees; near the base of this vast body of frozen matter the ice is excavated into an arch, perhaps one hundred feet in height, whence the Arveron rushes with impetuosity and in a large sheet of water. As he approached the ice he passed through a forest of firs, those near the arch were very ancient and about eighty feet high, the trees between them and the glacier were evidently younger, from the inferiority of their size and other intrinsic marks; others, still less, had been enveloped by the ice, and many were thrown down; arguing from this gradation in the appearance of the firs, he concludes that the glacier has originally extended to the full grown ancient trees, and, dissolving, young ones have grown on its former site, which have been overturned by a fresh increase of 1ce Thisinference seems almost conclusive, but it is still further supported by the fall of large pieces of granite, called moraine by the inhabitants, which, borne along by the ice, sinks through it as it dissolves, and, falling into the plain, form a border along its extremity; those have been urged forward by the pressure of new ice, and cxtend even to the place occupied by the large firs. Exclusive of these circumstances, Mr. Coxe discovered, that the glacier of Grindelwald had diminished, at least, 400 paces between the dates of his two visits in 1776 and 1785; and in the valley of Chamouny, the Muraille de Glace, which he had described as forming the border of the glacier of Bosson, in 1776, no longer existed in 1785, and young trees had grown on the site of the edge of the glacier of Montanvert. In opposition to the evidence thus adduced, it is argued that the operations observable in the vallies, arising from the concentration of solar heat, form no data for judging of those on more elevated situations, where, it is asserted, a greater quantity of snow falls and becomes ice than can possibly be dissolved annually ; and experience proves, beyond doubt, that mountains have been covered, passages obstructed, pastures and habitations destroyed by the ice, within the memory of man. In replying to these arguments, the result obtained is extremely satisfactory. The rain and sleet falling during summer not only thaws the ice and snow, but forms various channels in it, the water descending must wear and carry along part of the frozen sides and depths, and prepare the way for separating and throwing down large masses of each, which are termed avalanches in some parts of the country, and lauwine in others; those tremendous bodies, accumulating by adhesion in their progress, overwhelm every thing in their way, and rush to the vallies from the highest summits, whence various other causes serve to detach them. Here the traveller often meets a dreadful and instantaneous fate; but humanity has endeavoured to obviate it as far as human abilities will permit. Matthison, who visited the monastery of St. Bernard, founded for the relief of those who cross the Alps,speaks thus of the lauwine or avalanche, and the excellent canons of St. Bernard: “In the very worst seasons, as often as it snows, or the weather is foggy, some of these benevolent persons go forth with long poles, and, guided by their excellent dogs, seek the highway, which these sagacious animals never miss, how difficult soever to find. If then the wretched traveller has sunk beneath the force of the falling lauwine, or is immersed in the snow in a benumbing swoon, how deeply soever he may be buried, the dogs never fail of finding the place of his interment, which they point out by scratching and snuffling, when the sufferer is dug out and carried to the monastery, where every possible exertion is used for his restoration. The number of those who lose their lives in the field of battle is known to all Europe, but no one could give me an account what number have thus had the gift of life conferred on them a second time. Yet, notwithstanding all the care and attention of these reai friends of mankind, and their faithful dogs, scarcely a year passes, but, as the snow melts away in summer, the dead bo‘lies of travellers are discovered, who, remote from their homes, and all that was dear to them, have perished here unnoticed and unknown. As the ground for a considerable extent round the monastery is solid rock, the dead bodies are collected together in a chapel, lying on its eastern side, which is made to admit a thorough draft of air by openings in the walls, guarded by large iron bars. The sight of so many unfortunate persons, probably collected from various parts of the world, yet, howsoever remote from each other in life, brought hither by an unfortunate similarity of fate to rest together in death, afflicted my inmost soul. They are all covered with palls, and as in this frozen region no exanimate body moulders, but only gradually shrivels and dries away, so the features remain undis
figured for a considerable length of time, and some have even been recognized by friends and relations, after having lain here for two or three years. The bodies are not disposed one over the other, but are all placed upright, and each fresh corpse leans its head on the breast of the former; this disposition has something familiar in it, and gives them the semblance of being united only in a general slumber. Four rows of these slumberers already rest here, from the faces and hands of many of whom the palls have slipped off and left them uncovered: these have all a perfect mummy-like appearance.” The fall of the avalanche nceessarily reduces the quantity of snow and frozen fluid on the summits, and the transfer of it to a warmer region must facilitate its dissolution. The lower glaciers, though not subject to equaily precipitate descent, cannot otherwise than gradually advance towards the vallies, which may be inferred from the constant passage of torrents under them, and their own enormous weight; besides the chasms that universally intersect them lainly evince, that their foundations being partially undermined, they glide slowly downward, overturning trees, and pushing stones before them; the edges obtruded on the earth by this means naturally dissolve far more rapidly than if they remained stationary. Another means of dissolution arises from evaporation, which takes place on every portion of the globe, however elevated. Exclusive of the above causes, are the constant play of the beams of the sun on the surfaces of the glaciers, which being capable of redoubled heat by concentration and refraction in some favourable positions, must produce very violent thaws; the air heated by this and similar means, in other parts of the mountains, often meets the traveller in streams, which seem as if proceeding from a hot bath, consequently those projections subject to their influence must vanish rapidly; but a more certain and regular change occurs from the mean temperature of the earth, where the transition from summer heats to winter colds cannot ever take place. “This mean temperature,” says Mr. Coxe, “termed by some philosophers the internal heat of the earth, is always above the freezing point, as is evident from the heat of the springs which issue from the bowels of the earth. In winter, therefore, or in those high regions of the globe, where the cold is usually below the freezing