All About Engines

92 All About Engines at least, which acts upon a different principle. It consists of a small bulb divided into an upper and lower half by a partition, Fig. 47, Plate 5. The lower half is in communication with the steam and water spaces of the boiler, and the upper half is connected by a fine tube with the valve by which the feed water enters the boiler. The upper half and the thin tube are completely filled with dis- tilled water. The apparatus is fixed so that the partition is at the correct level of the water surface in the boiler. If the level falls, steam gains admission to the lower chamber. As this is hotter than the water, the distilled water in the space above expands, the valve rod is depressed, and water enters the boiler more freely. When water is entering the boiler more rapidly than it is being evaporated, steam is cut off from the lower chamber, the water in the narrow tube contracts, and the supply is imme- diately decreased. It is hardly possible to imagine a more beautiful contrivance than this. It is extremely sensitive, and while the engine is taking steam the valve is never at rest. It feels the pulse of the boiler with unerring accuracy, and never allows the level of the water to vary by an inch. For a task like this no man is equal to a machine. The Delivery of Dry Steam Between the engine and the boiler are the steam pipes, stop valves, reducing valves, dryers, traps, upon which engineers have been prodigal in their in-