expense. Sometimes a jacket of reinforced-concrete can be built around an old pier and the top protected with a water-proof coating.

Concrete has largely displaced stonework for the material of sub-structures in bridges. Piers made of iron or steel cylinders and filled with concrete provide a cheap construction where conditions are suitable. They were formerly largely used for railroad bridges, but their employment is now confined mostly to a light class of highway bridges or to railroad bridges carrying only light loading. If this type of pier is very high, it is subject to vibration under traffic; and this vibration will produce an injurious effect on the superstructure as well as on the piers. Many piers of this class are kept in service by encasing them in concrete, thus increasing their weight and stability; and this can usually be done without putting the bridge on falsework and removing the old piers, because the spans will be supported upon the old cylinders while the new work is in progress.

Steel-frame structures are subject to deterioration by corrosion; and to prevent this the surface must be always covered by a protective coating, usually paint. Since corrosion will gradually eat away metal which cannot be replaced, it is evident that this protection is of the utmost importance. In many cases it does not receive the attention it deserves. Many bridges have been seriously damaged by rust, even to the extent of having to replace them, all of which expense could have been prevented by keeping the structures properly painted. Railroads usually recognize the importance of painting, and seldom allow the rust to accumulate to the extent of weak- ening the structure. However, even on railroads, bridges are found that have been materially damaged by corrosion. There are parts specially subject to rust, such as the top flanges of deck girders and of stringers that are more or less hidden by the ties, and which sometimes become badly corroded while the paint on the main body of the structure is still in a good state of preservation. Also parts of highway bridges underneath the floor are frequently in bad condition, owing to the fact that they cannot be noticed by anyone crossing the bridge. However, if proper provision for inspection at regular intervals is made, there is no excuse for this condition resulting in damage to the structure.

There are many varieties of paint recommended for steel bridges. The best are usually the most expensive but will be the most economical in the end. The application of paint being about twice as costly as the material, a saving of a considerable percentage in cost of paint will result in only a small percentage of economy on the entire job. The result is that the cheaper paints will last probably two or three years less than will the more expensive ones. The subject of bridge paints is highly technical and cannot be gone into in detail here; however, it is important to note that different paints should be used for different conditions. For instance, a paint that would give satisfactory results in a dry climate would not be suited  for  a  structure  subject  to acid fumes or engine blasts,  as on bridges