Different and far more troublesome methods of working are necessary with those materials now to be considered, that are much harder, and in which the existence of stratification is considered but rarely and imperfectly to exist; namely, in the compact and cemented porphyries, principally from Egypt and Sweden; the crystalline granites, abundant in Devonshire, Cornwall, and Aberdeenshire, and the elvans of Cornwall, which latter, and some other varieties, appear to merge from the porphyries to the granites, are used for similar purposes, worked by the same means, and ask for an intermediate position*.

"In converting the rude masses of granite, for example, to their intended forms, the line of the proposed division is first marked, and holes from two to three inches deep, and four to five inches asunder, are bored upon this line, by means of an iron rod terminating at each end in chisel-formed edges of hardened steel, with a bulb in the middle to add weight; this tool, called a jumper, is

*The harder crystalline rocks last referred to, rarely occur in stratified beds, although a remarkable case has of late years been found to exist in the Foggintor Quarry, worked by the Haytor Granite Company, on Dartmoor, the subjoined particulars of which have been kindly communicated to me by William Johnson, Esq., who read a paper before the British Association at Plymouth, 1841, on the Railways and Machinery connected with this interesting work. See the Report of the

Athenæum, p. 654.

"The distinct beds of the Foggintor Quarry follow pretty nearly the natural slope of the hill, and are thicker and more nearly horizontal the deeper they are situated. The beds are intersected by heads or natural joints, which are nearly perpendicular fissures, (sometimes solid seams,) of unknown depth, that separate the horizontal floor of the quarry into irregular figures, and the continuity of the beds is often broken at the different headings. The quarry, in some parts 100 feet high, is worked in benches, or like a huge irregular flight of steps.

"In detaching the masses of granite rock from their natural beds, the points of least resistance are first determined by an experienced eye, and holes are sunk at those points, vertically or inclined as circumstances may require the diameters of these holes vary according to the mass and the amount of resistance; and their depths according to the thickness of the blocks to be detached.

"The holes are made with an iron rod, terminating at foot with a chisel-formed edge of hardened steel; the tool is held by one man, who changes its position at every blow received from sledge-hammers worked by other men who stand around. When the holes are thus made sufficiently deep, they are charged with gunpowder, in order to effect a separation of the mass by blasting. The ordinary process of stamping confines the powder, and the fuse communicates the blast. The art of the quarryman consists in placing the blast, (or shot,) where the smallest amount of powder will remove the largest mass of rock with the least breakage, simply dislodging or turning it over ready for converting. Three and even five thousand cubic feet have been removed by one discharge.".

GRANITE, PORPHYRY.

171 made to fall on one spot, it rebounds, and is partially twisted round to present the edge continually in a different angular position; and in this manner a very expert workman will bore about a hundred holes in a day. Each of the holes is then filled with two half-round pieces of iron called feathers, with an iron pointed wedge between them; the wedges are progressively and equally driven until the stone splits, and the fissure will be in general moderately flat, even should the mass be four or six feet thick, although in such cases the holes are sometimes continued round. the ends also."

“The scouters, the next class of men, employ the jumpers' feathers and wedges for removing any large projections, by boring holes sideways, and thus casting off large flakes; the spallers employ heavy axe-formed or muckle-hammers, for spalling or scaling off smaller flakes; and the scabblers use heavy pointed picks, and complete the conversion, so far as it is effected at the quarry, ready for the masons employed in erecting the buildings for which the blocks are used, who complete their formation on the spot." All these materials are likewise used in the ornamental arts.

Porphyry is worked in the lathe with remarkable perfection, and many excellent specimens from Sweden, of vases, slabs, pestles and mortars, and bearings intended for the gudgeons of heavy machinery, may be seen in London*. I learn that they are first worked as nearly as possible to the required forms with the pick, &c., they are then mounted in lathes driven by waterpower, and finished by grinding them with other lumps of porphyry, supplied with emery and water; the machinery is kept going day and night, and the gangs of men relieve each other at certain intervals.

Granite is incapable of being turned in the lathe; it is

* Various works in porphyry are contained in the Polytechnic Exhibition, Regent Street. Amongst them is a table five feet ten inches diameter, tastefully inlaid with a great number of pieces of various colours; the pedestal is fluted with great exactness, apparently by mechanism: it is said the construction of the table occupied five men during seven years. The sum asked for it is 5007.

The Emperor of Russia had four splendid vases of porphyry five or six feet high, two of which he gave to the Duke of Devonshire; these are at Chatsworth House. This manufacture has in a very few years converted the barren valley in which the porphyry is found, to a scene busy with the hum of industry.

The Saloon of Egyptian Antiquities, British Museum, contains various sculptures in porphyry, the work of almost inconceivable labour and perseverance.

therefore treated like porphyry, that is, shaped with heavy picks, and finally with smaller points used with a hammer; it is afterwards ground with circular or reciprocating motion, according to the figure, by means of iron plates fed with sharp sand, next with emery, progressively finer and finer, upon wooden rubbers, the endways of the grain; and lastly, the polish is perfected with felt rubbers and crocus. The process is tedious, and difficult from the unequal hardness of the particles; in this respect granite is inferior to porphyry.

Of late years a variety of vases and other circular and ornamental objects have been admirably executed in polished granites and elvans, which occur of various colours and degrees of hardness; when decomposed they are friable, and furnish the china stones extensively used in porcelain and china works, and also the material for very refractory crucibles*.

AGATE, JASPER, CHALCEDONY, CARNELIAN, ETC. (7)

Are all composed of silex nearly pure; they break in general with a conchoidal fracture, and to divide them into plates it is necessary to resort to the lapidary process. They may be slit with emery, but it is far more economical to employ diamond powder, as the time then required is only one third of that called for when emery

* Four beautifully polished granite columns may be seen in the King's Library, British Museum; and in the Museum of Economic Geology, there are also many ornamental polished works in granites, elvans, marbles, serpentines, &c., and likewise six inch cubes of a great variety of the same, and of numerous other building stones, collected under the "Commission appointed to visit the Quarries, and to inquire into the qualities of the stone to be used in building the New Houses of Parliament." The various materials are most elaborately detailed in the Parliamentary Report of the Commission, together with every information respecting cost, supply, and transport; the dates of various buildings in which the stones have been used, and remarks on their present states of preservation; followed by the chemical and physical examination of the stones, and other points which concern the civil engineer, architect, and builder, to whom the report would appear to be almost invaluable. Ultimately four varieties of magnesian limestones were selected for the erection of the national edifice.

The Museum of Economic Geology also contains a most interesting series of specimens of all the metallic ores found in Great Britain and Ireland; and in many instances there are associated with them, geological maps and models of the mines; working models of the machinery for mines; and also specimens explanatory of the various processes of manufacture from the ore to the marketable metal. For an infant museum it is most rich, interesting, and instructive.

The Institution of Civil Engineers also has a collection of granites.

TOPAZ, SAPPHIRE, RUBY.

173

is used; these stones are always ground with emery, and polished with rotten-stone, as will be explained.

Agate is used as the bearing planes for the knife edges of delicate balances, for pestles and mortars, burnishers for gilders, bookbinders, &c. and some other purposes in the mechanical arts; and the whole are largely employed for the purposes of jewellery, the handles of knives, snuff-boxes, and a variety of ornaments.

TOPAZ (8). SAPPHIRE, RUBY (9).

These may be split with plane surfaces through their natural cleavages, and which method is continually employed; otherwise, they can be only slit with the diamond powder. The first and similar stones may be smoothed with emery, but this, being in hardness only equal to 9, produces but little effect upon topaz, &c., upon sapphire and ruby it is almost inert, and on diamond quite so; the sapphire and ruby, therefore, are always polished with diamond powder.

On account of the peculiar interest attached to the mechanical applications of the hard gems, I propose to depart a little from the subject and order of these pages, to advert to some few of their uses, which may not be generally understood. The sapphire, the ruby, and also the diamond, are commonly used for the construction of certain parts of the best time-pieces and watches, such as the pivot-holes, pallets, and other parts of the escapements.

Fig. 58 represents upon a true, although very enlarged scale, the jewelled pivot-hole for the verge, or the axis of the balance of a marine chronometer, a is the hardened steel pivot, which is turned with a fine cylindrical neck, and made convex at the end ; the jewelling consists mostly of two stones, the one, commonly sapphire or ruby, is turned to the form of the black figure b, that is, convex above and concave beneath, of two different

174

APPLICATIONS OF THE RUBY.

sweeps, to thin it away at the part where it is to be pierced with the hole, which is made a little smaller in the middle to lessen the surface bearing.

The other, which is called the "top-stone" or "end-stone," is generally a ruby, in the form of a plano-convex lens, or else it is a diamond cut into facets; the flat side of this touches the end of the pivot.

Each stone is burnished into a brass or steel ring, like some of the lenses of telescopes, &c., and the two stones, (separated a slight distance for the reception of oil,) are inlaid in a countersunk recess c, d, in the side plate, the cock, or other part of the watch, and retained therein by two side screws as represented, although unimportant variations are made by different artists in the shapes and proportions of the parts.

The delicacy of these jewelled holes will be imagined, when I add that in the axis above referred to, the side plate, e, e, is only one-tenth of an inch thick; the rings from c, to d, one-sixth, and the pivot the one two-hundredth part of an inch diameter; in pocket-watches, and more particularly the flat Geneva watches, these measures, especially the first two, are amazingly reduced, although the same number of parts are nevertheless employed in each hole that is jewelled.

The wire for making the pendulum springs for chronometers, is sometimes drawn through a pair of flat rubies with rounded edges, as represented in fig. 59; the stones are cemented into the ends of metal slides having screw adjustments, not represented. Sometimes two pairs of rubies are placed one before the other, to constitute a rectangular hole of variable dimensions, for equalizing the wire both in width and thickness.

Rubies and other gems, are drilled with holes conical from both sides, as in fig. 60, for drawing the fine silver gilt, and silver wires, for the manufacture of gold and silver lace; the wires are afterwards flattened, wound spirally upon silk, and woven into the lace. Ruby holes are also employed for rounding the leads of ever-pointed pencils; but for this use they are chamfered from the one side only, and the lead is pushed through from the small side, the ruby is then used as a cutting tool; whereas the hole in the draw-plate is slightly rounded upon the ridge, and acts more as a burnisher or compresser; the action of the wire, which is pulled through in the direction of the arrow, tends to draw