Suppose we have come to the station and are commissioned to locate the difficulty. We go into the engine room and find everything in good order. The engine is a fine piece of mechanism; it has no loose joints, no leaky valves, yet it seems to lack power; is overloaded. Inquiry shows there are no more lights than formerly, while the service was satisfactory. You go at once to the boiler room. It may also be in good order so far as appearances go, but you look at the steam gage and find the pressure is low. "Yes," says the fireman, "I simply can't keep the pressure up. I shovel in coal and keep the drafts on so that I have a roaring fire, but, in spite of all, my steam pressure runs down." Look into the furnace (the stomach) of this leviathan! If the grate-bars are clean; if there is no accumulation of ashes, cinders, or clinkers to interfere with the combustion (digestion) of the black provender fed to it, you may close the furnace door and open another. Look into the fire tubes (the lungs) of the laboring monster that has shown signs of weakness! If the fire tubes are clean, free from soot and dust, the trouble is not there. [Sidenote: "Scale," like an irritated mucous lining] [Sidenote: "Scale," the cause of dim light] We have now gone almost the full course; there is but one place left to explore and that is closed. The trouble is inside the boiler. It is lined with scale deposited from the water evaporated in producing steam. This scale, which may be likened unto an irritated mucous lining of the stomach, or the intestines, forms a coating upon the lower inside of the boiler, and the upper side of the fire tubes, just as it is deposited on the bottom of a teakettle, and it shuts out the heat from the water. The heat being the source of energy, and the steam only the means of applying it, the power-plant is crippled. Seldom does it happen that so great a thickness of scale is to be found in a boiler as may be seen in almost every household teakettle, yet the effects (symptoms) are found in the dimmed lights miles away, and if the difficulty is not dealt with, it will rapidly increase until the service becomes intolerably inefficient. [Sidenote: Difficulty in dealing with the "scale"] Had we found the grate-bars choked with ashes, cinders, and clinkers, and the fire tubes (lungs) smothered with soot and dust, we should have instructed the fireman to keep them clean and free. This is not a difficult thing to do, requiring only careful daily attention, but the scale inside the boiler is not so easily dealt with. It is completely enclosed, and there is no possibility of getting at it except by extinguishing the fire and letting the boiler cool--by making the boiler "dead," or "killing" it, as firemen term it. [Sidenote: Treating the "dim light" dis-ease]