steam chest by the internal steam pipe; a thin brass pipe extending across the steam chest and closed at the end, but having fine slits cut in its upper surface for the passage of the steam. This fitting is intended to check the passage of water. Stop valves now act automatically; that is to say, when a regulating stop is screwed back, the valve is free to open or close according as the pressure is greater in the boiler or in the main steam pipe; thus the passage of steam into the boiler from the main steam pipe is prevented by the valve becoming closed. This is intended to operate in the event of a rupture occurring in one of a set of boilers in communication, the sudden fall of pressure within causing the valve to close and thus isolating the injured boiler.

Blow out Cocks or Valves, for discharging water from the boiler into the sea. On opening the bottom blow out cock or valve on the boiler and the blow out Kingston, water is discharged from the bottom of the boiler by the bottom blow out pipe.

On opening the surface blow out or brine cock and Kingston, surface water is discharged from the water level by the surface blow out or brine pipe. The lighter foreign matter may thus be discharged from the surface, and the heavier matter from the bottom.

The bottom blow out apparatus is used also for running up the boiler from the sea before raising steam.

Feed valves, main and auxiliary, for regulating the admission of feed water.

The main feed valve regulates the feed supplied by the main feed pumps from the condensers, which is available while the engines are working. The auxiliary feed valve regulates the auxiliary feed, which may be supplied from the sea whenever necessary, by the auxiliary feed or donkey pump. These valves are usually simple lift valves which close automatically whenever the main feed pumps are stopped, thus acting as non-return valves. An internal feed pipe is fitted to deliver the water clear of the plates.

Glass Water Gauges for showing the water level in the boiler; usually two to each boiler. The gauge consists of a glass tube, packed at the ends, and placed in communication at the upper end with the steam and at the lower end with the water in the boiler, the water level is then visible in the glass.

The glass is now fitted on a steady pipe which communicates with the upper part of the steam chest and with the lower part of the water space; this conduces to steadiness within the glass. The centre of the glass is at the ordinary working level.

Water Gauge Cocks, for use in the event of the glass gauges being broken or unserviceable. These are cocks placed at different levels, which may be opened to ascertain if the water is above or below them respectively.

Steam Pressure Gauges (Bourdon's) two to each boiler. These act on the principle that a coiled tube of flattened section tends to straighten under internal pressure. One end of such a tube communicates with the steam chest, the other end is closed, but free to move, and its movement is shown by a pointer on a dial.

The pressure indicated is the excess of the steam pressure above the atmospheric pressure in lbs. per square inch.

MARINE ENGINES.

The Cylinder.-Fig. 10 is a section of an ordinary horizontal navy cylinder. A is the cylinder proper, bored out to the correct diameter and fitted with the piston P. B is a rough cylindrical shell surrounding A, but standing clear of it so as to leave a steam-tight space about an inch deep around its circumference; this space jjj is the steam jacket. At the ends are the cover and bottom, C and D, which are made double and supplied with steam to complete the steam jacketing; the cover C is a separate piece secured by nuts.

The construction now adopted for cylinders is as follows:

The working barrel or liner A is a separate casting; the outer shell is cast with one end, the bottom, complete, and the liner is afterwards secured in place by a flange i at the inner end, which makes a steam-tight joint with the bottom. At the outer end the liner fits into the outer shell, and steam tightness is obtained by packing the small space left beyond. The ring rr secures the packing in its place.

With this construction the liner is free to expand or contract lengthways under changes of temperature. It may also be made of harder metal than the rest of the structure, and may easily be renewed if necessary.

The outer shell is of cast iron, the liner is of harder cast iron, or of steel, to withstand wear. Whitworth's compressed steel is now very generally used.

The steam enters and leaves the cylinder by steam passages cd, leading from orifices ab at the ends to steam ports 8 8, tt in the cylinder face, a

plane surface on the outside of the casting on which the slide valve works. In the centre of the cylinder face is the exhaust port e, communicating by the exhaust passage h with the condenser. The action of the slide valve, which opens and closes these communications, will be explained later. It works on a false face fff of hard cast iron secured to the cylinder face, and is situated usually at the side in horizontal cylinders, its weight being taken on a guide surface beneath.

The cylinder cover is at the back end; at the front end the cylinder is secured to the engine main frames, which are bolted down to the bearers. The cylinders, slide casings, steam pipes, etc., are clothed in felt encased in wood to prevent radiation.

Cylinder Fittings.-Escape Valves for affording an outlet for any water which may find its way into the cylinder when the engines are at work. One is fitted at each end, opening outwards against the resistance of an adjustable spring. The valve is forced open and the water discharged by the impact of the piston on the water.

Relief or Drain Cocks, for draining the cylinder of water at any time; one at each end. They are opened by hand, and are used before and during starting, and whenever necessary.

Auxiliary Starting Valve, for assisting in starting the engines. This is a small slide valve moved by hand over a face with ports and passages similar to those for the main slide valve. By moving this valve steam may be admitted to either side of the piston and exhausted from the opposite side. These valves are also used for warming and clearing the cylinders of water before starting.

Piston. The piston should be steam tight, yet work with as little friction as possible. For steam tightness the piston must be packed at its circumference; for this purpose metallic packing is used.

The ordinary construction for navy pistons is as follows:

A A the body of the piston (Figs. 11 and 12), is a hollow cast iron disc having internal ribs RRR for stiffness. On one face is a flange ƒƒ at the circumference; on the opposite face the junk ring JJ is secured, forming a Corresponding flange, and between these flanges is held the packing ring p p. This is a cast iron hoop which, after having been turned to the correct diameter, has been cut through at one part (Fig. 14), and is thus free to be pressed out against the cylinder, for which purpose the steel springs 8 8 8 (Fig. 11) are placed behind it. The springs of navy pistons are generally of the "coach