ing. They are prevented from falling off by a couple of screws sunk into holes in A and B, provided with spiral springs under the heads to allow for the motions of the jaws D, and covered in with grub screws. The curved springs underneath the jaws raise them after releasing the work, in readiness for another job. By this device the hammering down of work usually done in ordinary vices is avoided, and a better seating is secured. There is a clear space down between the jaws, owing to the absence of a long central screw, so that drills or other tools may clear through the work freely. Vices of this type are also made with swivelling bases and tilting arms, to angle the work into any position. The loose jaws may be shaped specially to suit repetition work if necessary.

There is another vice, the Jordan, in which rapid adjustment is effected by means of grooves on the base, into which a flat strip is inserted, and retains a push jaw from which set-screws press against a sliding jaw, which is prevented from rising by a couple of bolts.

Fig. 177 illustrates a swivel vice, with graduated base, used especially for milling and grind ing machines. The loose jaws are held on with long screws passing through from the rear. The sliding jaw is prevented from tipping by screws and blocks underneath.

When very long work, beyond the capacity of the ordinary vice opening, has to be held upon a table, two or more loose jaws pushed up with screws are placed at suitable distances apart and clamped to the table. Some vices have reversible jaws to point backwards, and so in combination with a second vice form an opening of any desired width.

A loose pair of pivoted jaws is often inserted between those of an ordinary vice, to accommodate tapered work, instead of making the actual jaw to swivel. A pair of small point centres are also sometimes screwed on the top of each jaw of a planer or shaper vice, by which circular pieces are held with their countersunk holes in the ends.

MacNaughting.-A term which was common between about 1850 and 1870, to signify the conversion of old type low-pressure condensing beam engines into compound. The

term was derived from the name of the inventor, W. M'Naught. A new high-pressure cylinder was fitted at the opposite end of the beam from the low-pressure cylinder, the steam after doing work in the first passing into the second and thence to the condenser. This divides honour with the rather earlier compound engine of Simms, as putting the principle of compounding on a practical basis.

Magazine Feeds. When pieces of work cannot be produced in continuous fashion from a rod, or bar, or sheet, or strip of metal in automatic machines, some form of magazine is necessary to carry and feed the single detached pieces into position for cutting or stamping. There are several types of feeds, including shutes, and rotating discs for carrying a line of pieces either in actual contact with each other, or placed in separate pockets.

In automatic screw machines there are two classes of articles which are fed with magazines -studs and pins of various kinds that cannot be completely finished at one operation, and must be reversed end for end, and castings and stampings which are too large to be produced from bar. In the first case the feeding-in may be done through the hollow spindle; in the second, the pieces may be passed vertically down to the nose chuck at the front.

Power presses usually have a circular dial, provided with a number of holes in a circle, each of which receives a partially cupped or otherwise partially finished article.

Part rota

tion of the dial is effected by a ratchet device operated from the crankshaft of the press, and the only duty of the attendant is to keep the pockets supplied with pieces. It would be impossible, or at least dangerous, to attempt to place the objects directly in position under the ram, so that it is preferable to let the dial feed them there instead. Locking of the dial is effected by positive mechanism, to ensure that all the holes come into accurate alignment with the ram punch.

Magnalium. The name given to various alloys of magnesium with aluminium. With less than 10 per cent. of magnesium the alloy rolls well. With 10 parts of magnesium to 100 of aluminium the material has the mechanical properties of rolled zinc, with 15 parts it

resembles cast brass, with from 20 to 25 parts the alloy resembles hard drawn brass. Sodium, carbon and nitrogen, in the aluminium cause the alloys to be of no value. The presence of tungsten, nickel, and copper does not harm beyond increasing the specific gravity of the alloys. The addition of 10 to 15 per cent. of antimony raises the melting point from 700° C. to a white heat. Magnalium can be turned, drilled, and milled. When being filed it resembles soft brass. It takes a permanent silvery polish. It is harder than pure aluminium, and shows a finely grained surface on fracture, somewhat resembling that of steel. It can be soldered. The sp. gr. of magnalium is about 2.5. Its tensile strength is from 20 to 24 kilos. per square metre, or 127 to 15.2 tons per square inch.

Magnesite. An impure magnesia with small quantities of silica, oxide of iron, alumina, and lime, with carbonic acid. It is used to a slight extent for the linings of basic Siemens furnaces, but is more costly than silica bricks.

Magnet. The property possessed by magnetite, Fe,O,, of attracting small pieces of iron and steel, was known to the ancients. In the tenth or twelfth century it was also found that when suspended by a thread these natural magnets pointed north and south; hence the name "lodestone" or "leading stone," applied because of its use in navigation. This ore of iron is found in Arkansas, Spain, and Sweden, though not always possessing magnetic properties. Artificial magnets are made of iron, steel, cobalt, and nickel; cerium, chromium, and alloys of copper, aluminium, and manganese are also magnetic. Pieces of these metals rubbed with a lodestone or another magnet become magnets themselves.

The attractive

power is situated at the two "poles," these being distinguished as north and south, according as they turn to these points when the magnetised needle or bar is free to move. The imaginary line joining the poles is the magnetic

axis. A magnet cannot have one pole only. Moreover, if a magnet be broken into many short lengths, each individual piece becomes a separate magnet, with its two opposite poles. If two magnets be brought together, it is found that a north pole attracts a south pole, and

vice versa, but similar poles repel each other, i.e., unlike poles attract each other, like poles repel each other. And the force exerted between two magnetic poles is proportional to the product of their strength, and inversely proportional to the square of the distance between them. (A unit magnetic pole is one which exerts unit force-one Dyne-on a pole of equal strength placed at unit distance-one centimetre-from it.)

The lines of forces between the poles and in the neighbourhood of a magnet are rendered visible by placing a sheet of thin glass over a magnet, and sprinkling iron filings over the glass. On tapping it, these filings at once fall into an interesting series of curved lines from the north pole to the south pole--magnetic phenomena are, like electric phenomena, always circuital. These curves are an actual plan of the stresses existing in the neighbourhood of the poles of the magnet, and the area over which they are spread is called the magnetic field. The magnetic force is most intense in the immediate neighbourhood of the pole, and falls off inversely as the square of the distance from the pole.

The total number of lines of force proceeding from a pole is called the magnetic flux of that pole, and the flux density, denoted by the letter B, is the number of lines of force per unit of area crossing a surface taken at right angles to the direction of the lines of force. If B is known, the pull between two flat magnet poles may be found from the formula :B2 × A 72,134,000,

P =

diamagnetic materials, which are repelled from a magnet, and in which a magnetising force is able to induce fewer lines of induction than in iron and air. Bismuth, phosphorus, antimony, zinc, lead, silver, copper, gold, sulphur, water, and air are more or less diamagnetic.

A magnet may be "super-saturated," that is, it will lift a greater weight immediately after magnetisation than it will habitually do after wards, when it falls to the point of "saturation." When a magnet loses its magnetism either slowly or rapidly, as in the case of soft iron, there always remains a small quantity of "residual" magnetism.

The extent to which various bodies retain their magnetism varies. The retentivity of hard-tempered steel is very great, and that of soft iron very small, although the former is magnetised with more difficulty than the latter. Moreover a tap or rough usage is sufficient to destroy the slight retentivity of soft annealed iron. Temperature also considerably affects magnetism. Great heat destroys the magnetic properties of most metals; in the case of cast iron heating to a red heat only, is sufficient. Chilling increases strength, although severe cooling destroys the magnetism of steel magnets.

Permanent, powerful magnets are built up with a number of thin sheets or laminæ, each separately magnetised. Such a magnet is in reality a collection of magnets, in fact, a magnetic magazine or battery. As regards shape, magnets are bar-shaped, horse-shoe shaped (a more permanent form, as the lines of force have less tendency to return through the metal), and ring-shaped.

But the vast and undreamt of commercial possibilities of magnetism were not thought of until the development of the electro-magnet. It was found in 1820 that very powerful magnets could be artificially made by causing a current of electricity to circulate through a spiral coil of wire enclosing a bar of iron or steel, the wire being insulated by being overspun with silk or cotton. Such an electromagnet is more powerful than the ordinary magnet, and has the additional advantage of acting magnetically only as long as the current flows, ceasing to act when the circuit is broken; moreover it can be operated at any distance.

Various forms of electro-magnets are in use. The iron core may be in the form of a horseshoe, the coils being divided into two parts wound on bobbins. A similar type is used for electric bells: two cores are connected with an iron cross-piece or "yoke," the bobbins bearing the wire being slipped over the cores, and the wires connected. A very powerful pull is obtained with a short cylindrical electromagnet, surrounded by an external iron jacket.

The amount of magnetism in the core depends on the intensity of the magnetising force, and the quality, length, form, and sectional area of the iron. Many formulæ are in use for calculating the magnetism of an electromagnet, denoted by the letter m. The Lenz and Jacobi formula is manC, in which a is a constant, varying with the form, quality, &c., of the iron, n the number of turns of wire in the coil, and C the current. According to Lamont and Frölich, m = = B where B

nc 1+ onC and σ are constants varying with the quality, &c., of the iron, σ being the reciprocal of the number of ampère turns which brings the magnetism up to half saturation.

A rather interesting development in the use of magnets has taken place in recent years. Cranes are now equipped with powerful magnets for lifting and carrying ingots, iron and steel plates, &c. See Lifting Magnet, Dynamo.

Magnetic Brake. A device for producing a braking effect by the pull of an electromagnet; either to operate a system of levers, straps, or friction discs, or to produce magnetic friction between two attracted surfaces, or a combination of both effects. Magneticallyoperated brakes are much used on electricallydriven machinery, such as plate rolls, planers, &c., and also on electric cranes where it is desirable to bring the motor to rest for reversing the machines driven, or to sustain a suspended load. In such cases the braking is by mechanical means. Straps, blocks, or friction discs are operated by springs or weights, the magnet being used to release the machine by pulling against the holding power of the brake. The magnet coils are connected in series with, or as a shunt circuit to the motor, according to requirements (see Electric Brake), but in