and dirt in the surface are things that may destroy the work of the painters. No surface of structural steel ought to be coated with paint unless it shows the grayish-white, natural color of the metal.

Now, it may be noted at times immediately after the removal of the scale from the surface of steel by means of a sand-blast, that the parts are soon covered with rust. In reply to a direct question of the author, Prof. Henry M. Howe recently wrote: "The reason for this is that the sand-blast roughens the surface, and that on a rough surface of iron, as we all know, rust forms much more rapidly than on a smooth one. You can leave a razor lying in a drawer for years without its rusting at all, because it is so highly polished, whereas a piece of rough, unpolished steel lying beside it will rust very rapidly.