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 they tend to push out the centre of the tubeplate, whilst it is rigidly held at the edges, especially where it is jointed to the flange of the cylinders. The constant bending and unbending causes a slight crack in the plate which ultimately develops into a groove, the latter being enlarged by the action of acid in the water.

A badly grooved or wasted plate is renewed, but if the defects are not so serious, the plate is repaired by riveting a patch over the defective portion. Rivet heads are liable to corrosion, and defective rivets are knocked out and replaced.

Firebox. The firebox is particularly subject to defects. The copper plates, especially at the firehole or at the flanges where the plates are united, are wasted away by the action of the flame.