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118 which great care must be taken not to “nick” or damage the plates, or subsequent fractures are liable to result.

The steel boiler and firebox casing plates are liable to corrosion and “pitting,” cracks, and “grooving.” The corrosion takes the form of a uniform wasting of the plates, whereby they become thinner, and eventually unable to withstand the pressure required. This form of corrosion is usually found on a longitudinal belt from 1 ft. to 2 ft. wide near the water level. Pitting is the more usual form of corrosion, in which the plate is honeycombed with small cavities either isolated, or running into one another to form depressions of considerable size. This defect is probably caused by combined chemical and galvanic action due to dissolved acids in the water. The scale deposited by the feed water, if thin, helps to protect the plates, but the alternate expansion and contraction of the plates, as they are heated and cooled, helps to detach pieces of the scale, leaving the plates exposed to the action of any acid in the water. It may here be remarked that most boiler and firebox troubles are caused by the alternate expansion and contraction of the different parts, and that its wear and tear are due to causes quite different from those which operate in the cylinders and motion. “Grooving,” usually found on the smokebox tubeplate and at the foundation ring which unites the copper firebox to the firebox shell, is also due to the same cause. As the tubes expand