to the furnace crown at intervals, and passed up through stuffing boxes in the shell. The curve assumed by the furnace was not regular, for the gauge rod at a quarter of the length of the boiler from the front end showed as much rise, and in one caseth more rise than a rod placed midway in the length. The furnace tubes rose in. when the flames passed round the brickwork flues in the ordinary way, and in. when they were led directly into the chimney without heating the outer shell.

The bottoms of Lancashire boilers are subject to strain due to the fact that the temperature is lower there than elsewhere, and the expansion of the plates there is consequently less than that of the flue tubes. When cold feed water is delivered at the bottom of the boiler, the evil is intensified, and rips of the transverse seams have frequently occurred by reason of this straining.

Many cases have come under the notice of boiler inspectors of the blow-off cocks, and the front end plates of horizontal boilers having been so encased in brick-work that their condition could not be seen, and so corrosion has gone on for a long while without being detected, while the same parts have also been subjected to severe strains through being bound fast, instead of being left free to move with the expansion of the boiler.

Grooving and furrowing are liable to occur in longitudinal lap-jointed seams, being due to the tendency of the internal pressure to impart a truly cylindrical form to the shell. The tendency is then to assume a kink at the seam. On release of the pressure the seam returns to its original position, and this constant working starts a crack or fissure, which is liable to increase in dimensions, and to be enlarged by incessant straining, and the corrosion of acids. For this reason it is desirable to use butt joints, double strapped,

this being the only way in which a truly circular form can be maintained. In small boilers subject to moderate pressure only, lap seams are frequently used, but such a form of joint is quite inadmissible in any boiler, large, or small, subject to high pressures.

Grooving is apt to occur also at the edges of the circumferential ring seams of horizontal boilers, caused by the working, due to the difference of expansion which takes place. It also happens around the roots of the angles, or of flanges by which the shells are riveted to the end plates. In Cornish and Lancashire boilers it often happens around the attachment of the flues to the end plates, due also to the cambering caused by the difference of temperature between the top and bottom of the flue. In vertical boilers it is found around the angle iron rings, or the flanges which unite the uptake to the shells, and close to the foundation ring.

Grooving occurs more seriously in boilers of small than in those of large diameter. Thus, while the shells of Lancashire boilers are relatively free from this, it is apt to occur in the barrels of locomotive boilers, and the records of explosions show a large proportion attributable to this cause. And it must be remembered that such failures would be much more numerous but for the frequent and severe examinations. to which such boilers are subject.

There are two types of grooving, the difference in which was pointed out by Mr Fletcher, one broad, the other narrow,-a mere nick, the first being apparently due to single riveting, the second to double riveting. In one case mentioned by Mr Fletcher the grooving occurred in the steam space, and not in the water space as generally happens. Such narrow nick grooves are difficult of detection.

Furrowing is partly chemical, partly mechanical. It does not occur with all waters, and it seldom happens above the water line. Furrowing may be commenced by mechanical action, as when a plate bends or springs at a seam. Or it may begin at a joint where the caulking tool has been driven into the metal, cutting below the scale or skin. In either case clean metal becomes exposed to the action of acidulated water which corrodes it. The evil is intensified

by the mechanical action of repeated bendings, until a deep and narrow furrow results. In order to diminish the risk, all seams should, as far as practicable, be placed above the water line. Lap seams should be avoided below the water line, butt joints with covering straps being used.

Probably the most injurious form of boiler deposit and scale is that which has only been known since the introduction of high pressure steam. It is derived primarily from the lubricating oils of animal and vegetable origin which are used in the cylinders, by reason of the dryness and non-lubricating character of the superheated steam. These also pass over into the boiler, being carried with the exhaust steam into the feed water. They float on the water as scum, until becoming entangled with solid particles of carbonate and sulphate of lime, the scum becomes of the same specific gravity as the water, and is then carried about with the circulating currents until it fastens on the surfaces of the plates. The portions which become deposited on the crowns of the flues cause overheating.

That familiar type of corrosion known as pitting is due to galvanic action, and is the result in the first instance of minute specks of foreign matters, as carbon, or manganese originally present in the iron or steel, and which being electro-negative to the metal induces local corrosion. Then the oxide of iron formed is electro-negative to the metallic, and induces further corrosion, with an increase in the size of the pit both in diameter and depth. Pitting once started must almost inevitably increase, because each portion of oxide as it forms increases the quantity of electro-negative substance which acts upon the iron.

Over-straining leads to many explosions. This is due to the effects of heat acting in different degrees upon different parts of a boiler. The parts in contact with the fire expand most. The expansions of metal produce strains of many tons per square inch, and a boiler therefore, which is designed so badly that these become excessive, will in some cases fail directly. In most instances, however, the period of rupture is long delayed, and does not happen until certain vital sections have become deeply grooved or

furrowed by reason of the incessant working action produced by the movements due to heat.

Boiler Feed.-The supply of water introduced to a boiler. It is introduced cold, or hot, is pumped, or injected automatically. The old practice was to bring in cold feed near the bottoms of horizontal boilers where the circulation is sluggish, with the risk of the cold water causing contraction of plates and leakage of rivets. The proper way, now universal, is to introduce it about midway between the flue crown and the working level of the water. It is also carried along some way and distributed through a perforated pipe. The feed is brought in through the front plates of Cornish and Lancashire types, in verticals at any convenient position round the boiler, but at a horizontal location about midway between the fire box crown and the normal water level.

See also Boiler Efficiency, Boiler Feed Pump, Feed Waters, Injector.

Boiler Feed Water.-See Feed Waters.

Boiler Feed Pump.-This class of pump includes several types. The commonest and simplest is one that is driven by an eccentric from the crank shaft, and is of short stroke. An old type, less used now, is driven from the crosshead, and has therefore the same stroke as the engine piston, and is not suitable for high-speed engines. Pumps of these types are not adapted for large engines, the boilers of which are preferably fed by steam pumps, either of simple or of compound types, and independently of the working of the engine. Any pump must be of large capacity. It ought to be able to supply double the quantity of water evaporated by the boiler in order to have a margin of security against leakage and irregular working, blowing off, blowing the whistle, or using auxiliary engines. For a condensing engine, the supply is taken from the hot well, and in the usual surface condenser is fresh water.

Two examples of feed pumps are given in Figs. 238, 239, both by Messrs J. P. Hall & Sons, Ltd., of Peterborough.

Fig. 238 is a direct-acting single cylinder double-acting pump of 18-inch stroke, and having an engine cylinder of 6-inch bore. It makes 13 strokes per minute, and is equal to

12 inches. The number of strokes per minute is 14, and the duty is 2,200 gallons per hour. The valves are of circular section. The upper valve is the high-pressure one, and the lower or low-pressure valve is operated from this by a lever seen in the external elevation. The valves are controlled by the auxiliary valve on the front of the steam valve casing, and this is operated from the motion of the piston rod of the engine by the rocking lever and connecting rods seen. The lever which connects the two valves has a handle, the purpose of which is to start the pump, effected by placing the high-pressure valve in the opposite position to that it would occupy if the pump were working. This has the effect of filling the highpressure cylinder with steam, so that when the positions of the valves are reversed, the steam is exhausted into the low pressure, and the action is continuous. Cushioning is effected in the low-pressure cylinder by casting the main ports at some distance from the end, so bringing the piston slowly to rest, and allow the pump valves to reseat themselves before the commencement of the next stroke. The small amount of steam compressed escapes through a very small port at the end.

Generally feed pumps are fitted in duplicate, so that one set can be overhauled and repaired while the other is working. The feed pumps require frequent attention, since if they should cease to work, or work inefficiently, damage may ensue to the boiler in a short time. The valves must be examined periodically. Trouble often arises due to the presence of hot water, though the advantages of using water as hot as possible are obvious. Hot water has more elasticity than cold, and is therefore able to suffer compression before it can overcome the boiler pressure.

Boiler Fittings.-These are the manhole doors, mudhole ditto, furnace bars, bearers, doors, dampers, and any other parts which are usually considered distinct from mountings.

Boiler Flue Flange Drilling Machine. -A special machine for drilling the rivet holes in the flanges of the furnace flues of horizontal boilers. A couple of flue lengths are placed in position, with the axis vertically, on a chuck, and clamped, or held with tacking bolts. The

chuck is rotated by worm gear and hand wheel, and pitching is effected by change wheels. There are two standards for the drilling heads in a complete machine. They are bolted to extensions of a bed, the centre of which carries the chuck. The drilling heads have vertical adjustments on the standards, and radial adjustments on the saddle, with a good range. Some of these machines also have a horizontal drilling spindle for drilling the rivet holes in the sides of the flues for cross tubes.

Fig. 240, Plate XVI., shows a doubledrilling machine for flue flanges, having a special form of drive to the drill spindles, so that they may work while the flanges are placed together, as shown, with the caulking ring in situ. The capacities of the machine are as follows:-Maximum drill centres 5 ft., minimum ditto 2 ft. 1 in., horizontal range of each drilling head 1 ft. 51⁄2 in., maximum height admitted, from top of chuck to drill point, 5 ft. 1 in., minimum ditto 1 ft. 3 in., the chuck grips from 2 ft. 11 in. to 4 ft. 7 in. diameter.

In another form the flue, carried on a horizontal chuck as before, is flanked with standards which carry a cross rail along which the drilling heads have horizontal adjustment, the cross rail being vertically adjustable. Some of these are much elaborated to combine provision for turning as well as drilling, and for tapping, and tube-hole cutting. They resemble a planing machine in having cross slide, tool boxes, and bed, along which the saddle of the chuck slides, provision for running the chuck for turning, for pitching by change wheels and worm gear, and in having counterbalanced heads fitted for drilling and tapping, with variable feeds, and a separate tool box for turning, with another tool box on one of the housings. An example is shown in Fig. 241, Plate XVI., taking flues from 1 ft. 10 in. to 4 ft. 6 in. diameter, and 1 ft. 10 in. to 4 ft. 6 in. height. The pitching is effected by the hand wheel seen to the left, which through the medium of change wheels rotates the chuck through a portion of a circle at each complete turn of the hand wheel.

Boiler Flue Flange Turning Machine. -A type used for turning the flanges of the furnace flues of horizontal boilers in readiness for drilling and riveting them up in lengths,