THE present tendency of steam engineering, in the effort to increase the work performed in return for every pound of fuel consumed, is to employ steam of very high pressure. The greater the initial pressure of the steam, the greater are the advantages to be derived from its expansive principle. To resist successfully the enormous aggregate of pressure to which locomotive boilers are subjected, a well-constructed strong boiler is absolutely necessary; and the various railroad companies throughout the country meet the required conditions in an admirable manner, as is evidenced by the remarkable exemption of such boilers from serious accidents. Although the locomotive is the most intensely pressed boiler in common use, that supreme disaster, an explosion, is of rare occurrence, considering the vast number of boilers doing service all over the States. This result is due to constant care in the construction, in the maintenance, and in the management, of the locomotive boiler. Like the conservation of liberty, eternal vigilance is the price of safety.

There is perfect safety in using a boiler so long as a good margin of resisting power is maintained above the tendency within to tear the sheets asunder. This margin is very low for locomotive boilers generally, hence the greater necessity for care in maintenance and management. Years ago the mechanical world established by practice a rule making one-fifth of the ultimate strength of a boiler its safe working-pressure. That is, a boiler carrying 140 pounds working-pressure should be capable of withstanding a tension of 700 pounds to the square inch before rupture ensues. Locomotive practice in this country does not provide much more than half of that margin of safety. When deterioration or accident reduces this margin, danger begins.

Certain mechanical empirics and impractical quasi-scientists have at various times attempted to surround the cause of boiler explosions with a halo of mystery. But our most accomplished scientists who have made the subject a special study, and our best mechanical experts who have devoted years of patient experiment and research to the investigation of boiler explosion, attribute the terrible phenomenon to intelligible causes alone. The conclusions of the practical part of the mechanical world are well summed in one sentence in one of the annual reports of the Master Mechanics' Association. It says, "Explosions originate from over-pressure: it matters not whether the whole boiler, or a portion of it, is too weak to resist the pressure."

The preservation of a boiler depends very much upon the care and attention bestowed upon it by the engineer, and no other person is so much interested in its safety. To prevent undue strains from being put upon the boiler, the engineer should see that the safety-valves and the steam-gauge are kept in proper order. To secure this, the steam-gauge should be tested at least once a month. The rule established on well-conducted roads, prohibiting engineers from interfering with safety-valves, is a very judicious one; and no persons are more interested in its strict observance than the engineers themselves.

Some men are idiotic enough to habitually screw down safety-valves, that the engine may be able to overcome heavy grades without doubling. This is criminal recklessness, and all train men are interested in its suppression. Low water has often been blamed falsely as the cause of disaster to boilers; a theory having prevailed, that permitting the water to become low led to the generation of an explosive gas which no sheet could withstand. That theory was exploded long ago; but, nevertheless, it is certain that low water paves the way for explosions by deteriorating the fire-box sheets, and destroying stay-bolts. A careful engineer watches to prevent his engine from getting "scorched" even slightly; for the smallest scorching may yield a harvest of trouble, even after many days. The danger of scorching is most imminent when an engine is foaming badly from the effects of impurities in the feed-water or in the boiler. At such a time the water rises so lavishly with the steam, that the gauges are no indication of the true water-level. The steam must be shut off to find the true level of the water. Where this trouble is experienced, the engineer should err on the safe side, ever, though untold patience is needed to work the engine along with the boiler full of water.

Mud within the boiler, and scales adhering to the heating surface, are dangerous enemies to the preservation of boilers; and engineers should strive to prevent their evil effects by rooting them out so far as practicable. Much can be banished by washing out frequently; and scales can, to some extent, be prevented by selecting the softest water on the road. If water in a tank is so hard that it makes soap curdle instead of lather when a man attempts to wash with it, that tank should be avoided as far as possible.

The sudden cooling down of boilers, by blowing them off while hot, is a most pernicious practice, which is responsible for many cracked sheets and broken, stay-bolts. It also tends to make a boiler scale the heating surfaces rapidly. Every time a boiler is blown out hot, if the water contains calcareous solution, a coat of mud is left on the heating surfaces, which dries hard while the steel is hot. If a piece of scale taken from a boiler periodically subjected to this blowing-out process be closely examined, it will be found to consist of thin layers, every one representing a period of blowing off, just as plainly as the laminae of our rocks indicate the method of their formation. When a boiler must be cooled down quickly for washing out or other purposes, the steam should be blown off, and the boiler gradually filled up with water. Then open the blow-off cock, and keep water running in about as fast as it runs out until the temperature gets even with the atmosphere. The boiler may now be emptied without injury. Or another good plan is to blow off about two gauges of water under a pressure of forty or fifty pounds of steam, then cool down the boiler gradually, to prepare for washing.

Although the dangers of blowing off hot boilers, and then rushing in cold water to wash out, are well known and acknowledged, yet the practice is still followed on many roads where more intelligent action might be expected.

Should it happen from any cause that the safety-valves fail to relieve the boiler, and the steam runs up beyond a safe tension, the situation is critical; but the engineer should not resort to any method of giving sudden relief. To jerk the safety-valve wide open at such a time is a most dangerous proceeding. A disastrous explosion lately occurred to a locomotive boiler from this cause. The safety-valves had been working badly; and, while the engine was standing on a side track, they allowed the steam to rise considerably above the working-pressure. When the engineer perceived this, he threw open the safety-valve by means of a relief lever, and the boiler instantly went into fragments. Cases have occurred where the quick opening of a throttle-valve has produced a similar result. The proximate cause of such an accident was the violent motion of water a within the boiler, induced by the sudden diminution of pressure at one point; but the real cause of the disaster was a weak boiler, —a boiler with insufficient margin of resisting power. The weakest part of a boiler is its strongest point. This may seem paradoxical, but a moment's reflection will show that the highest strength of a boiler merely reaches to the point where it will give out. Hence engineers should see that a boiler is properly examined for unseen defects so soon as signs of distress appear. Leaky throat-sheets or seams, stay-heads dripping, or incipient cracks, are indications of weakness; and their call should be attended to without delay.

When an engineer finds the steam rising beyond a safe pressure, he should reduce it by opening the heaters, starting the injector, dampening the fire, or even by blowing the whistle. The whistle offers a convenient means of getting rid of superfluous steam, and its noise can be stopped by tying a rag between the bell and the valve opening.

Should any boiler attachment, such as a check-valve or blow-off cock, blow out or break off, no time should be lost in quenching the fire. That is the first consideration. A bursted flue will generally save an engineer the labor of extinguishing the fire. In this case an engineer's efforts should be directed to reducing the pressure of steam as quickly as possible, so that he may be able to plug the flue before the water gets out of the boiler. Flue-plugs and a rod for holding them are very requisite articles; but, in driving flue-plugs, care must be exercised not to hammer too hard, or a broken flue-sheet may result. Plugs are often at hand without a rod to hold them. In such an emergency, a hard wooden rail can be used; the plug being fastened to the end by means of nails and, wire, or even wet cord. Where no iron plug is available, a wooden plug driven well in, away from the reach of the fire, may prevent a bursted flue from leaking, and enable the engine to go along; but wooden plugs are very unreliable for such a purpose. They may hold if the rupture in the flue should be some distance inside; but, should the cause of leaking be close to the flue-sheet, a wooden plug will burn out in a few minutes.

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