that are over railroad tracks, or for a moist climate, especially near salt water. Some paints are a good protection against corrosion but do not stand the weather, while others have exactly the opposite properties. Therefore, the first or priming coat should be of the first-mentioned variety and the finishing coats of a paint that is not readily affected by exposure to the weather. The most commonly used and satisfactory paint for the priming coat is pure red lead ground in linseed oil. Sometimes linseed oil alone is used and sometimes paint with a Portland cement base. For the finishing coats, graphite paints, graphite and silica paints, or paints of other carbon pigments, such as lamp black with oxide of iron, or oxide of iron paints are most frequently employed. The latter are not suitable for the priming coat, owing to the fact that these pigments promote oxidation, a condition that is often ignored in practical work.

As to the application of the paint-this must be well done, if satisfactory results are expected. Metal should never be painted when it is damp, in freezing weather, or over rust. In repainting old structures probably the most important consideration is the cleaning of the metal. Paint applied over rusted surfaces will not be durable. Further, there is likelihood of corrosion spreading underneath the paint, and then the protection will soon break down. As previously noted, some parts of the structure are subject to faster deterioration of the paint than other parts. This is true of the tops of stringers, floor beams, and compression chords, and of flat surfaces exposed to weather, especially surfaces beneath the floor, such as lateral plates, battens, etc., which give lodging to cinders and other materials that will hold moisture. For this reason a very satisfactory method to use in painting an old bridge, where the paint is bad on certain parts and fair on others, is to clean thoroughly the metalwork and paint over the parts where it is exposed with a coat of red lead or some other good priming paint, and then give the whole structure a good finishing coat.

As to the method of cleaning rust from the metal-this is done by cutting with chisels and hammers and by scrubbing off with wire brushes, or with a sand blast. The latter method is most effective; however, it is expensive and is not recommended unless the bridges are in a bad state of corrosion. Also it must be used with a great deal of caution so that in removing the rust an excessive amount of the surrounding metal will not be cut away at the same time.

To obtain satisfactory results, a great deal of care must be taken in applying the paint; it should be brushed out thoroughly on the metal. Recently painting by air-spraying machines has come into extensive use, but the employment of these machines on bridgework is not common. It is doubtless a labor saver and, if properly handled, is not very wasteful of paint. The manufacturers of these devices claim that the work they do is superior to hand painting; and, when skilfully used, it is certain that good results can be obtained. However, for durability, it is probable that such coatings are not as good as paint well brushed on by hand; still it might be