The preservation of bridges against rapid deterioration is just as important a matter as ensuring that they are properly proportioned and constructed—yes, even more important, for what behooveth it the owner of a steel structure to take the utmost care in its designing and building, if he neglect to protect it effectively against the ravages of rust? The life of a metal bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long, but if badly neglected is often quite short, especially when it is exposed to acid fumes, such as those contained in the smoke from locomotives passing through or beneath. It is evident, therefore, that the subject of economics of metal protection is one of consequence and deserving of the most thorough consideration.

It may appropriately be divided into two topics, viz., the general question of economic expediency in guarding the structure against injury by the expenditure of considerable money, and the cheapest ways of effecting satisfactory protection.

The first topic may readily be disposed of by the statement that it is in the line of true economy to spend whatever amount of money (within, of course, the bounds of reason) that is found to be necessary to prevent the starting of any rusting of metal whatsoever. If, as many people think, the life of a steel bridge is limited to two or three decades, the economic question would arise as to how much money it would pay to spend on painting or other protection, in order to prolong the said life a few years; but such is by no means the case, because, as previously indicated, a modern steel bridge ought to last for centuries.

The second topic covers a wide field, and requires to be treated in detail. The principal subjects that it includes are the following:

1. Best kinds of paint to use in shop and field.

2. Best vehicle for pigments.

3. Use of driers.

* After this chapter had been finished for some time it was submitted for criticism to the veteran paint specialist, Dr. A. H. Sabin, Consulting Chemist to the National Lead Company, and the acknowledged dean of American paint men. Dr. Sabin very kindly prepared some memoranda on certain points; and, in order not to necessitate a re-writing of the entire chapter, his suggestions have been incorporated as foot-notes.

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