All About Engines

^4 All About Engines cylinder, 5 or 6 feet in diameter and 20 to 25 feet long, through which passes a narrow cylinder, 15 to 24 inches in diameter. The grate and ashpit are formed in the first 5 or 6 feet, and the remainder of the smaller cylinder forms the flue. The whole boiler is set in brickwork, and the hot gases, after emerging from the internal flue, pass through a flue in the brickwork under the boiler, divide at the front end, and return through brick flues on both sides of the boiler to the chimney. Sometimes the internal flue is placed a little to one side in order to improve the circulation, and as the hot gases flow, to a large extent, through a wide flue without com- municating their heat to the water, the flue is crossed by “ Galloway tubes ” (Fig. 28). These are water tubes which, crossing th« wide Fig. 28.-Gall„„ay tube. flue> intercept hof gases> while the water in them becomes heated, is rendered lighter, and flows out of the upper end. This causes steam to be raised much more quickly, and by equal- ising the temperature in different parts of the boiler tends to prevent undue strain. The Lancashire boiler (Fig. 27) is exactly similar in principle, but is larger—6 to 8 feet in diameter and 25 to 30 feet long, with two or three furnaces of cor- respondingly larger size. The flues are often made in sections, which are often joined by Adamson’s