d Fig. 1939. into the water, so that as the wheel and axle revolve the water rises in the spiral tubes, and makes its escape at the top through the holes o o, which are set in a broad close ring on the top of the axle, into which ring the water is delivered from the upper open ends of the screw tubes, and falls into the open cistern c. The lower end of the axle a' turns on a gudgeon in the water in c; and the spiral tubes in that axle take up the water from c and deliver it into a similar cistern just below the top of a', on which there may be such another wheel as w" to turn a third axle by such a wheel upon it. In this way water may be raised to any given height provided there be a stream of sufficient power to act on the float-boards of the first wheel. handed if the first wheel is turned in the direction of | Mechanics: "If we conceive that a flexible tube is the arrow, and left-handed if the stream turn the rolled regularly about a cylinder from one end to anwheel the contrary way: also, the screw on the axle other, this tube or canal will be a screw or spiral, of A must be cut the contrary way to that on the axle which we suppose the intervals of the spires or threads A', because the 2 axles turn in contrary directions. to be equal. The cylinder being placed with its axis The screws must be covered over with boards, and in a vertical position, if we put in at the upper end of they will then form spiral tubes: or a tube of stiff the spiral tube a small ball of heavy matter, which leather may be wrapped round shallow grooves in the may move freely, it is certain that it will follow all the axle, as in Fig. 1939. The lower end of the axle A turnings of the screw from the top to the bottom of revolves constantly in the stream which gives motion to the cylinder, descending always as it would have done the wheel, and the lower ends of the spiral tubes open had it fallen in a right line along the axis of the cylinder, only it would occupy more time in running through the spiral. If the cylinder were placed with its axis horizontally, and we again put the ball into one opening of the canal, it will descend, following the direction of the first demi-spire, but when it arrives at the lowest point in this portion of the tube it will stop. It must be remarked that, though its heaviness has no other tendency than to make it descend in the demi-spire, the oblique position of the tube, with respect to the horizon, is the cause that the bail, by always descending, is always advancing from the extremity of the cylinder whence it commenced its motion to the other extremity. It is impossible that the ball can ever advance more towards the further, or, as we shall call it, the second extremity of the cylinder, if the cylinder placed horizontally remains always immoveable: but if, when the ball is arrived at the bottom of the first demi-spire, we cause the cylinder to turn on its axis without changing the position of that axis, and in such manner, that the lowest point of the demi-spire on which the ball presses becomes elevated, then the ball falls necessarily from this point upon that which succeeds, and which becomes lowest; and since this second point is more advanced towards the second extremity of the cylinder than the former was, therefore by this new descent the ball will be advanced towards that extremity, and so on throughout, in such a manner that it will at length arrive at the second extremity by always descending, the cylinder having its rotatory motion continued. Moreover, the ball, by constantly following its tendency to descend, has advanced through a right line equal to the axis of the cylinder, and this distance is horizontal because the sides of the cylinder were placed horizontally. But if the cylinder had been placed oblique to the horizon, and we suppose it to be turned on its axis always in the same direction, it is easy to see that if the first quarter of a spire actually descends, the ball will move from the lower end of the spiral tube, and be carried solely by gravity to the lowest point of the first demi-spire, where, as in the preceding case, it will be abandoned by this point as it is elevated by the rotation, and thrown by its heaviness upon that which has taken its place; whence, as this succeeding point is further advanced towards the second extremity of the cylinder than that which the ball occupied just before, and consequently more elevated, therefore the ball, while following its tendency to descend by its heaviness, will be always more and more elevated by virtue of the rotation of the cylinder. Thus it will after a The water-screw, Fig. 1940, resembles the above, only the spiral projections are detached from the external cylinder within which the screw revolves. Fig. 1940. This want of perfect contact between the screw and the cover occasions a loss; and, in general, at least 1-third of the water runs back, and the axis cannot be placed at a greater elevation than 30°: it is also easily clogged by impurities in the water. When, however, the lower end is immersed to some depth in the water, it has been found to raise more water than the screw of Archimedes, so that if the height of the surface were liable to any great variations the water screw might be preferred to the Archimedian screw. The mode of operation of the screw of Archimedes is thus illustrated by Dr. Gregory, in his Treatise on certain number of turns be advanced from one extre- | man's hand sustains 50 lbs. weight, he will be a mity of the tube to the other, or through the whole by means of this jack, to sustain a weight or for length of the cylinder; but it will only be raised 11,000 lbs., or about 5 tons weight. The mac through the vertical height determined by the obliquity in some cases open from the bottom nearly up t of the position of the cylinder. wheel o to allow the lower claw, which in such cis turned up as at B, to draw up a weight. W the weight is drawn or pushed to the required i-. it is prevented from going back by hanging then of the hook s, fixed to a staple, over the curved p-of the handle at h. The jack is also sometimes :.nished with a pall and ratchet, to stop the motic: the machine as soon as it begins to run back. SCREW PILE-See LIGHTHOUSE. SCREW PRESS-See CARD, Fig. 477-PRINT.Y SCREW PROPELLER-See STEAM. SCULPTURE, MECHANICAL PROCESSES OF. S ture is the art of carving or cutting any material a proposed form or shape, for the purpose - 1, representing entire figures, as in statues and gr called by artists the round; 2, of making figures et in high or low relief (alto or basso rilievo), the ot being more or less raised, but not entirely detach from the back ground; 3, of cutting or sinking a ground, so as to make the object represented be. the plane of the original ground. Sculpture is a defined as the art of representing objects by m (in contradistinction to painting, in which coless a introduced,) and has thus been applied to carting, modelling or the plastic art, to casting in metal, a to gem-engraving in hard and soft stones. [See SE ENGRAVING.] "Instead of the ball, let us now consider water, as entering by the lower extremity of the spiral canal, when immersed in a reservoir: this water descends at first in the canal solely by its gravity; but the cylinder being turned, the water moves on in the canal to occupy the lowest place; and thus, by the continual | rotation, is made to advance further and further in the spiral, till at length it is raised to the upper extremity of the canal, where it is expelled. There is, however, an essential difference between the water and the ball; for the water, by reason of its fluidity, after having descended by its heaviness to the lowest point of the demi-spire, rises up on the contrary side to the original level; on which account more than half one of the spires may soon be filled with the fluid." SCREW-JACK, an instrument in common use for raising timber or heavy weights through short lifts. It consists of a powerful combination of teeth and pinions enclosed in a strong wooden stock or frame, BC, Fig. 1941, and moved by a winch or handle H P. In Fig. 1942 the rack-work is shown, the stock being removed. In the figure a very short rack is shown: A H Fij 1942. it should be at least 4 times as long in proportion to the wheel o, and the teeth be 4 times more numerous, or have about 3 in the inch. If the handle H P be 7 inches long, the circumference of this radius will be 44 inches, which is the distance or space through which the power moves in one revolution of the handle; but as the pinion of the handle has but 4 leaves, and the wheel o, say 20 teeth or 5 times the number, therefore to make one revolution of the wheel o requires 5 turns of the handle, in which case it passes through 5 times 44 or 220 inches: but the wheel having a pinion R of 3 leaves, these will raise the rack 3 teeth or 1 inch in the same space; hence the handle or power moving 220 times as fast as the weight, will raise or balance a weight 220 times greater; and if a The materials used by the sculptor are very B rous, but probably more so in ancient times than a present. "For modelling, clay, wax, and stones plaster, appear to have been universally used: clay, after having been worked into the proposed fore was frequently baked, acquiring by that process a hardness not inferior to stone: in this state, too. often served for moulds, into which soft clay w squeezed, and thus the object became easily mult plied. A considerable number of ancient specime of statues, bassi rilievi, lamps, tiles, and architectur ornaments in this material (called terra cotta) have been preserved. Marbles, stones, and woods of a kinds, as well as ivory, were employed by the carvers. and all the known metals, wax, plaster, and eve pitch, were used for the different processes of casting. There was a statue of amber of Augustus; and at the celebration of Funeralia, as in those of Sylla, at public exhibitions, or on other extraordinary occasions, we read of statues having been made of aromatics, and of materials of the most combustible nature; and amongst the odd conceits of the ancient artists, may be mentioned a statue of Venus, which attracted a Mars of iron. The combination of different materials for the purpose of producing variety of colours, either for drapery or ornaments, was termed polychroms" sculpture (Toλùs, many, xpua, colour), and those works which were composed of a variety of stone or marble were, in like manner, called polylithic (rodis, many, and Xíos, a stud). This mixture of materials, which modern taste disapproves, was continually resorted to by the most celebrated artists during the | the end of the nose, it is then removed to the correbest period of art in Greece, particularly in colossal sponding position of the other scale, and the workman works." As it would be quite impossible within our limits to describe the modes of working up all the materials enumerated in the above passage, we propose to notice briefly the mechanical processes adopted in the production of a work in marble. Many of the processes above alluded to are described under separate heads. The sculptor first expresses his idea in a sketch on paper, and then makes a small model, generally in clay, or, for greater accuracy and perfection, a model of the size in which the marble, bronze, or wood, &c., is to be executed. The figure is first modelled naked, and in its proper action and form: he then lays on the drapery either from studies made after the living figure, or drapery placed on a lay figure or mannikin. The clay model, if large, is supported by a frame-work of iron, and the masses of clay are kept together by small wooden crosses attached to the iron frame-work, by wires of different lengths dispersed in different parts of the clay. The modeller's tools are of wood or ivory, with ends pointed, rounded, square, or diagonal, with which he forms his model, marks out the hollows and dark parts, and does whatever his unaided fingers cannot effect. The clay must be occasionally sprinkled with water to prevent the model from shrinking and cracking, and when left for some hours it should be covered with a damp cloth. The clay model being finished, it is moulded and cast in plaster, the plaster being supported by iron bars cemented, to prevent the rust coming through. The method of taking plaster casts is described under GYPSUM. If the work is to be executed in bronze, particular attention must be bestowed on the mould, to enable it to bear the weight of the fluid metal. The model is copied in marble in the following manner :-A number of small black points are marked upon the model in every principal projection and hollow, so as to give the distances, heights, and breadths, sufficient for copying with the marble from the model. The ancients did this by considering every 3 points on the figure as a triangle, which they made in marble to correspond with the same 3 points in the model, by trying it with a perpendicular line or some other fixed point, both in the marble and the model. The modern method is this:-After having ascertained by rough measurement that the block of marble is sufficient to make the statue of the size of the model, it is fixed on a stone base or wooden bench, called a banker, in front of which is a long strip of marble divided into feet and inches. A similar strip is placed in front below the model, together with a wooden perpendicular rule the height of the whole work: this rule can be taken from the marble graduated scale under the model to the marble scale under the marble block. The rule being first placed upon the scale of the model, and the exact distance taken from it to any prominent part, as, for instance, (1) Article SCULPTURE-Encyclopædia Metropolitana. VOL. II. cuts away the marble to the same distance from the perpendicular at the same height; that is, until he has arrived at that portion of the block which is to form the tip of the nose. He then proceeds in the same way with some other prominent part, such as the top of the head, until at length he has produced a rough representation of the whole figure. Machines have also been contrived for producing the same effect with greater convenience and rapidity. One of them, called a pointing instrument, consists of a pole or standard, to which a long brass or steel needle is attached, so as to admit of being extended and withdrawn, loosened or fixed, and moved in every direction by means of a ball and socket-joint. This instrument being made to touch a particular part of the model, it is removed to the banker, and the marble is cut away until the needle reaches as far into the block as it had been fixed at upon the model. A pencil mark is then made upon the two corresponding parts of the model and block, and a point is thus said to be taken. By a frequent repetition of the process, the various points at fixed depths, corresponding with the surface of the model, give a rough copy of the intended work. The work is sometimes performed by drilling. For example, the workman measures how far any particular part of his model, such as the tip of the nose, is from the front of the banker, and having found the proper position of the corresponding point of the block of marble, he drills a hole to the same depth from the front, as in the model. Other prominent points are, in like manner, measured, and holes are drilled to their proper depths from the front, until at length the block of marble presents a honeycombed appearance from the nume rous drillings. The portions of marble between the holes are cut away with the chisel, care being taken not to chip away any of the stone below the drillhole. Mr. Behnes has contrived an improved machine on this principle. When at length the figure has thus been roughed or blocked out, the mechanical art merges into the fine art. The sculptor now takes the dead mass in hand, and imparts to it that artistic life which conveys to the beholder beauty of form, mental and anatomical expression, and a well-defined purpose. How this is done cannot be told in words. The mechanical aids are steel chisels, varying in breadth from an inch to a mere point, and for deep parts drills are also used. The sculptor goes over every part of the surface of the marble, urging the chisels with hammers of from 2 to 4 lbs. each. In this work, however, the sculptor is frequently assisted by a superior workman, called a carver, who knows by the pencil marks how far he can penetrate into the marble. After the chiselling the surface is gone over with rasps, and then with sharp files. The smooth parts are rubbed up with pumice stone or grit stone, cut to suit the various forms of the surface. SEAL-ENGRAVING is the art of sinking, in intaglio, armorial bearings on gems and hard stones. ૨૨ When the subjects are of a more artistic kind, the | feather on the tools, which prevents them from si, ping round. The pulley is about 14 inch dumeand is generally made in one piece with the q art is properly called GEM-ENGRAVING. When the design is engraved in relief, it forms a third division of the art, viz. CAMEO-CUTTING. All three branches have, however, in practice, a great affinity, and the tools and processes are similar in all. The tools are small revolving wheels, the edges of which are charged with a fine abrasive powder, applied by means of oil or water: the object to be engraved is held in the fingers of the artist, and thus applied to the lower edges of the small wheels, and is moved about into the positions favourable to the production of those fine lines, grooves, and hollows, which are in fact counterparts of the small wheels or tools themselves. For hard stones the wheels are of iron charged with diamond powder by means of oil of bricks, the polishing being performed by means of copper wheels charged with rottenstone and water. For engraving glass, similar but larger tools of copper are used, charged with emery and olive oil, the polishing being effected by means of leaden tools charged with pumice-stone and water. All gems inferior in hardness to the diamond admit of being operated on by the seal-engraver, and even the diamond itself has been engraved. The sapphire is cut very slowly, but smoothly; the ruby is cut slowly, and is apt to break off in small flakes; carnelian and bloodstone are of close structure, and can be cut slowly. The softer stones can be cut with greater rapidity, but the effect is not so smooth as in the case of the harder stones. The amethyst is as soft a stone as can be engraved smoothly: when such soft substances as glass or marble are engraved, the tools soon become deteriorated in consequence of the diamond powder becoming imbedded in the material and reacting on the tool. Stones consisting of lamine of different degrees of hardness require care in the cutting, to prevent the tool from sinking more deeply into the softer parts. When the device is seen from the surface in the colours of the lower stratum the seal is called a nicolei. The seal engraver's tools are furnished with long conical stems for fitting into the hollow mandrel or quill of a small lathe head or engine, Fig. 1944, mounted on a table, Fig. 1943, which is hollowed out in front, and furnished below with a light foot-wheel for driving the engine with a steady motion. The engine, shown separately in section, Fig. 1944, is a brass pillar about 6 inches high, with a bolt at the bottom for passing through the bench, where it is retained by a nut. At the upper part of the pillar are two openings which cross at right angles: these are for the reception of the pulley and the bearings of the quill. The bearings are usually cylindrical, and are made of tin or pewter cast upon the quill, fitting it by a set screw, which passes through a brass cap at the top of the pillar. The quill is of steel, about 2 inches long and inch diameter: it passes through the bearings, and has two small beads upon it for preventing end play. The quill is hollow throughout its length, and slightly conical, and on one side of the perforation is a small groove, into which passes a Fig. 1943. SEAL ENGRAVER'S LATHE GE The top of the pillar is covered with a small cap f keeping away dust and grit from the bearings, and a used as a rest for steadying the hand of the engraver The tools are of soft iron wire carefully annealed. Around the stem of each tool is cast a conical plug of tin, pewter, or other soft metal, for fitting it into the quill of the engine. As it is of great importance that the tools should run true, they are fixed in the quill and turned to the proper forms: the rest for turning the tools is passed through a mortice in the brass standard. The forms of the tools are very various, but the general form is that of a small disk more or less rounded on the edges, which is the part used in cutting. For cutting fine lines the edge is nearly as thin as that of a knife: a thicker and more rounded edge is used for thicker lines. For sinking large shields the tools Fig. 1944. are considerably rounded, and in some cases almost spherical. The rounded tool cuts more rapidly than one with a nearly flat edge, and is commonly used for removing the chief bulk of the material, while the flatter edge is used for smoothing the surface. To allow the tool to be applied to sunken flat surfaces without the stem interfering with its action, the edge is made conical, as at e, Fig. 1945. The tools bede are seldom larger than 4th inch diameter, and they are made so small as not to exceed th inch diameter, when the tool can scarcely be distinguished by the unassisted eye from its stem. These very a b Fig. 1945. h -0 small tools cannot be formed by the file alone; but | is used; this is called a sharp or knife tool. The outwhen made as small as possible by that means they line being dotted out with this tool, a thicker tool, are used on works of larger size until worn down with a rounded edge, such as d, may be employed for small enough to be used for making small dots and perfecting the outline: a thicker and larger tool, such markings in the figures of men or animals, the full as f, is next used for removing the bulk of the matelengths of which are not more than inch. The rial within the outline. The surface, when sufficiently lowered, is smoothed or stippled with a smaller and flatter tool, such as e. In roughing out the work the engine is driven rapidly, and the stone applied with moderate pressure. A slower speed and a less pressure are used when the smaller tools are applied; and, with the smallest tools, such as are used for cutting the details, the pressure is slight, and the engine is driven still more slowly. Curved lines and rounded forms are, from the circular forms of the tools, more easily engraved than straight lines. Fine lines, with sharp curves, such as the hair-strokes in writing, are difficult to engrave; but the bolder lines, in German-text initials, are far more easy of execution. "The cutting of the fine parallel lines on the field, called colour lines, presents considerable difficulty, as they are very shallow, and to give them a uniform hand. To assist in cutting these lines equidistant, appearance requires much care, and a light but steady a tool is used, having 2 knife-edges, c, Fig. 1945, and called a colouring tool. The front edge of this tool is used to cut the first line to the required depth, and the second line is at the same time marked out by the back edge: at the next process the second line is cut to the full depth, while the third line is marked in the same manner, and so on; the lines being cut in succession from right to left, in order that the operator may be enabled to watch the progress of the tool throughout, and the stone is held in an inclined position to cause the greater penetration of the front edge of the tool."1 surfaces of the tools must be smooth, i. e. free from creases, as the hollows are called, one of which, in a thin tool, such as or d, will be likely to chip instead of cut. The formation of creases is prevented by the frequent use of a fine file. The mode of preparing diamond powder is described under LAPIDARY-WORK, Fig. 1272. It is brought into a pasty condition by mixing with olive-oil, and the paste is kept in a small conical cup, which every now and then is applied to the tool, or the engraver may wear on the forefinger of the right-hand a ring made of a strip of tin, to which are soldered 2 little hollow discs about inch diameter, one of which contains a very small quantity of diamond paste, the other 1 or 2 drops of the oil of bricks. The diamond paste is applied to the extreme edge of the tool while in slow motion: the tool is then moistened with the oil of bricks, and the cutting is proceeded with until the brick-oil is evaporated. The tool must not be allowed to become too dry, or the diamond paste would become detached from the tool, which would then be cut instead of the stone. Sperm-oil is sometimes used instead of brick-oil. The engraver watches his work during the cutting through a lens of from 1 to 2 inches focus, which is mounted in an adjustable stand directly over the tool. The work is brushed from time to time to allow of its being seen distinctly; but the engraver depends very much on the sense of feeling for placing the work in the proper position with respect to the tool, and upon that of hearing for judging of the progress of the tool. He occasionally takes a proof of his work in blue modelling clay, or in a black wax made by mixing fine charcoal powder with bees'-wax. The stones to be engraved are brought to their general form by the lapidary, and are often set by the jew eller before being engraved. They are then mounted in a handle about 5 inches long and inch diameter. If the stone is not set, it is fixed with lapidary's cement upon a wooden handle, the cement being coated with sealing-wax to prevent the cement from adhering to the fingers. If the stone is set, it is inserted in a notch made in a piece of cork or bamboo. If the stone is hard and polished, the surface is roughened by rubbing it upon a soft steel plate charged with a little diamond powder and oil, or if the stone be soft, upon a leaden plate charged with emery. The tools are less liable to slip, and penetrate better on a rough than on a smooth surface: and the outline of the device can also be better sketched out upon the rough surface. The general outline is first carefully drawn upon the stone with a brass point: the entire surface within this outline is then sunk: the details of the design are next sketched in and sunk in succession. For forming an outline, the small tool b, Fig. 1915, resembled those of the modern engravers. It is of great importance that the artist should have his hands perfectly steady, and placed so as to be moved about in all directions with freedom. For this purpose, it is usual for him to rest the palm of the left-hand on the cap of the engine, Fig. 1944, while the forefinger and thumb embrace the revolving tool, (1) Holtzapffel, "Mechanical Manipulation," vol. iii. The valuable chapter on "Gem and Glass Engraving," describes, for the most part, the practice of Mr. W. Warner in seal-engraving, and of Mr. Henry Weigall in gem-engraving. In Rees's CycloA treatise on "The Ancient Method of Engraving on Precious Stones compared with the Modern," by L. Natter, Engraver on Gems, was published, in French and in English, in 1754. It appears, from this treatise, that the methods of engraving in use among the ancients closely padia is a very interesting article on GEMS. o o o |