of this upper slide is to move the cutting-tool which is fixed upon it parallel to the centre line of the lathe; but it can be so placed, by aid of the swivel-joint, as to cause the tool to advance at any required angle with the centre line of the lathe. Thus two direct movements are obtained: the first to set the tool to the work, and the second to move it either to the right or left, in a line parallel with the work, or at any given angle with the first.

The slide-rest principle enters largely into the construction of all kinds of machinery, from the most minute to machines of vast magnitude, where by its aid ponderous masses- such for instance as railway turn-tables 36 feet in diameter-are operated upon with a precision unattainable by any other

means.

Fig. 1.

Slide-valve, in locomotive engines, the valve placed in the steam-chest to work over the steam-ports. It regulates the admission of steam to the cylinder from the boiler, and the escape of the steam from the cylinder to the atmosphere. Its form is that of an arch in the centre, with a flat face all round to keep it steam-tight on the face of the steam-ports. It is by the arched part that the steam escapes to the atmosphere. It is a very simple valve, and answers its purpose well, with one draw-back, namely, the pressure of the steam upon it being unbalanced by any counter-pressure. Numerous attempts have been made to relieve this pressure, some of which it is hoped will be successful. In stationary engines the contrivances differ materially, as shown below. Fig. 2. t

Fig. 3.

k

k

d

k

Fig. 1 represents in section the cylinder, piston, and slide: S is the mouth of the steam-pipe coming from the boiler; is the pipe leading to the condenser; t is the rod which is attached to the slide, moving through a stuffing-box, m n. This slide is represented in longitudinal section, separately, in fig. 3, and in transverse section in fig. 4. In the position of the slide represented in fig. 1, the steam passing from the boiler enters at S, and passes to the bottom of the cylinder through the opening b, where it acts below the piston, causing it to ascend. The steam which was above the piston escapes through the opening at a, and descending through a longitudinal opening in the slide behind the mouth of the steam-pipe, finds its way to the pipe e, and through that to the condenser.

When the piston has reached the top of the cylinder, the slide will have been moved to the position represented in fig. 2. The steam now entering at S passes through the opening a into the cylinder above the piston, while the steam which was below it escapes through the opening and the pipe e to the condenser.

Fig. 4.

d

d

e

The form of the valve, from which it derives its name of D-valve, is represented in fig. 4. The longitudinal opening through which the steam descends then

e appears in section of a semicircular form. The packing at the back of the slide is represented

at k; this is pressed against the surface of the valve-box.

Slide-valve lap, Outside, in locomotive

engines, that portion of valve which would overlap the steam-ports when placed over them. If the steamports measure 8 inches over all the ports, and the valve be 10 inches broad, this would be an overlap of 1 inch on each side of the ports,

and is called the 'lap' of the valve. Expansion was formerly regulated by the extent of lap only, but it is now regulated by both the lap and the expansion gear, which gives greater scope in doing so. Inside lap is the portion of the valve face which would overlap the inside of the steam-ports when placed over them; for if the steam-ports were 4 inches from inside of the one port to the inside of the other port, and if the arched part of the valve only measured 33 inches across, this would give of an inch lap on each side, which is called inside lap.

Slide-valve lead, in locomotives, the width which the steam-port is opened by the slide-valve when the piston is at the end of the stroke. It varies from to an inch, according to the work required. The lead is obtained by fixing the eccentric on the axle, a little in front of the crank, by which arrangement the steam-port is opened in front of the direction in which the piston is moving before the latter has completed its stroke. By these means the steam-port is thrown quickly open when the piston commences its return stroke, and has at once the full pressure of the steam against it.

Slide-valve travel, the distance which the slide-valve travels in one direction for each stroke of the piston. This is from 4 to 54 inches generally, but is reduced by each variation of the expansion gear, and its travel is taken for both the front and back strokes of the piston for each notch where the handle is fixed. The up and down quarterrevolutions of the crank do not equally draw the piston the half length of the cylinder, and this and the expansion gear so far affect the working of the slide that it is necessary to take the front and back stroke working separately.

Setting the slide-valve: the eccen

tric is brought as much before the crank as to give the required lead to the slide-valve when the crank is on the centre, that is, in a straight line with a cylinder. The

handle is then moved to each separate notch, and the position of the slide and piston carefully taken for each variation. These are then recorded in the following manner :

Slide-valve rod guide, in locomotive engines, a bracket fixed to the boiler, the lower end of which is fitted for the slide-valve rod to work through. A set-screw is useful in this guide for fixing the rod when the valve has to be disconnected.

Slide-valve rod and frame, in loco

motive engines: the frame is fitted on to the top part of the valve,

and the rod connects the frame with the slide-block, or rockingshaft, according to the description of valve-gear of the particular engine

Slide-valve motion, in locomotive engines, a short motion similar to the piston-rod motion, connecting the quadrant to the slide-valve by parallel guides

Sliding rule, a rule constructed with

logarithmic lines, formed upon a slip of wood, brass, or ivory, inserted in a groove, in a rule made to slide longitudinally therein, so that by means of another scale upon the rule itself the contents of a surface or solid may be known Slimes, mud containing metallic ores, mud or earthy particles mixed with the ores

Slit deal, a name for inch and a quar

ter-inch deal cut into two boards Sloop, in navigation, a small onemasted vessel, the mainsail of which is attached to a gaff above, and to a long boom below. The word is also applied to any small ship.

Sluice, in hydraulics, a water-gate, a

flood-gate, a vent for water Smelting is the process of separating

metals from the earthy and other matters with which they are combined in the state of ore. Of this operation, as conducted upon the ores of iron, copper, lead, and tin, the following is a brief description:

Smelting Iron. The reduction of iron ore is effected in a furnace in which, the required intensity of heat being obtained by a current of air driven rapidly into the furnace, has received the name of a blastfurnace. The kind of furnaces employed, the quantity of ore or mine, as it is termed, reduced at each heat, and the peculiar method of conducting the operation, vary widely in different countries and counties, and have some reference in detail to the precise quality and composition of the ore to be treated. Previous to the year 1740, the smelting of ores of iron was, in England, performed solely with the charcoal of wood, the ores operated upon being principally the brown and red hematites, or rich ores, that is, containing a large proportion of metal with a small quantity of earthy materials. In the treatment of this class of ores, it may be observed that very little improvement has yet been effected, the modern

processes having been chiefly applied to the leaver ores, such as blackband, &c. The expensiveness and comparative scarcity of charcoal as a fuel for the smelting of iron ores, induced those engaged in the art to attempt the substitution of coal for wood-charcoal; and by the year 1788, these attempts had so far succeeded, that there remained only 24 out of 59 charcoal furnaces, while 53 furnaces had been established in which coal, burned into the form of coke, was used for the smelting of the ore. Since that date, the extension of this process has proceeded rapidly, and the total quantity of metal produced has experienced a corresponding augmentation. At the present time, the Backbarrow Iron Company are nearly the only smelters of iron with wood-charcoal in the kingdom. The two principal seats of the iron manufacture in Great Britain are in Staffordshire and South Wales. In the former district, comprising the neighbourhoods of Dudley, Bilston, Wednesbury, &c., the smelting or blast furnaces are constructed almost wholly of bricks. They are usually of a conical form externally, and sometimes pyramidal, the plan being a square or rectangle. In the interior they are mostly circular in form, except in that part called the hearth. The fuel and the ore to be smelted are fed into the furnace from the top, and its height being from 40 to 50 feet, an ascending platform or inclined plane is constructed for wheeling up the barrows in which the materials are conveyed. The pipes through which the air is driven into the furnace (by a steam engine) are called the tuyères, and are usually two, but sometimes three in number. The relative quantities of coal, ironstone, and limestone, which are put into the smelting furnaces of Staffordshire for the production of each ton weight of iron produced, are about 50 cwt. of coal, 50 cwt.

of mine, previously calcined, and from 12 to 16 cwt. of limestone, the latter material being added as a flux to promote the fusion of the mass. The Conegree furnace, near Dudley, may be instanced as a good example of a blast-furnace adapted for the economical smelting of iron ores. It is 54 feet in height, 5 feet in diameter on the hearth, and 12 feet above, widening upward to a diameter of 13 feet 9 inches, and reduced to 8 feet above the platform, on which the charges are delivered. The quantities of materials employed in this furnace to make one ton of pigiron, are of coal 2 tons and 5 cwt., or of coke 37 cwt., charred mine 2 tons 5 to 10 cwt., limestone 13 to 16 cwt.

Each charge delivered into the furnace consists of 94 cwt. of coke, 12 cwt. of charred mine, and 4 cwt. of limestone. At this furnace 115 tons of pig-iron have been made in one week. The cylinder from which the air is blown through the tuyères into the furnace is 72 inches in diameter, and the stroke is 7 feet in length. Originally there were five tuyères for the introduction of the blast, one muzzle being 24, two others 2, and the other two 2 inches in diameter. Subsequently these were changed to four muzzles, of the respective diameters of 34, 24, 2, and 2 inches.

Smelting Copper. Copper ore, as smeltedin South Wales and other places, usually consists of pyrites (composed of sulphuret of copper and sulphuret of iron in nearly equal proportions), and vein-stone. The earthy matters combined with the pyrites are commonly silicious, and the process of smelting consists in alternate roastings and fusions. The first of these operations is, calcining the ore in furnaces in which the heat is applied, and increased gradually, till the temperature be as high as the ore can support without melting or agglutinating,

when the ore is, thrown into an arch formed under the sole of the furnace. The second operation, or fusion of the calcined ore, is performed in a luted furnace, the ore having been spread uniformly over the hearth, and fluxes, such as lime, sand, or fluor-spar, being added when required, although the necessity for this addition is sought to be obviated by a careful admixture of ores of different qualities, the several earthy components of which shall serve as fluxes in the fusion of the mass. These two processes of calcination and fusion are repeated alternately until the ore is completely freed from all the earthy materials, and pure metal is obtained.

Smelting Lead. The ores of lead, after being sorted, cleansed, ground, and washed, are roasted in furnaces, which are without any blast or blowing apparatus, the ores being separable from the metal by its great fusibility. Several of the furnaces are usually connected with one chimney-stalk, to which a series of flues about 18 inches square conduct. The melted lead runs freely from the ore and is drawn off into the moulds in successive quantities, the ore being repeatedly turned over, and a small quantity of coal added over the burning mass at each drawing.

Smelting Tin. This process consists of the calcining or roasting of the ores after they have been cleaned, sorted, stamped, and washed. The calcining is performed in a reverberatory furnace from 12 to 15 feet long and 7 to 9 feet wide. The hearth of these furnaces is horizontal, and they have only one opening, which is in the front, and closed by an iron door. The sulphureous and arsenical vapours which arise from the ore are conducted by chimneys over the doors of the range of furnaces into horizontal flues, in which the acid is condensed. In the process of cal