taken from the "Compromise Standard System," and that the "Equivalent Loads" thereof are to be employed.

In this "System" will be found from "Class Z" to "Class T," inclusive, a close approximation to any live load that an engineer is likely to want to use; and if, for a certain car-load, some engineer should prefer a heavier or lighter engine-loading, he can obtain practically what he wishes by specifying that one class is to be used for floor systems and primary-truss members, and another class for main-truss members. The author does not advise this, however, except in the case, of double-track bridges, where it would be advantageous to use a certain class for floor systems and primary-truss members, and a lighter class for the trusses, because the chances of there being two, full, maximum train-loads on the span at the same time are generally very small. It might be well to carry this idea even further by specifying, for instance, "Class V" for stringers, "Class W" for floor-beams and primary-truss members, and "Class X" for main-truss members of double-track bridges. Such a method would be in accordance with the theory of probabilities; but it would not apply to single-track bridges, for which the locomotive and car loads of the "Compromise Standard System" have been properly adjusted.

The "Equivalent Uniform-Load Method" reduces to a minimum the labor of making computations of stresses in bridges. The correctness of this statement will be rendered evident by the ensuing explanations of the use of the method. As for its exactness, if any-one has any doubt whatsoever about its closeness of approximation to the theoretically correct method of wheel-concentrations, let him read the author's letter in the Railroad Gazette of July 28, 1893. An inspection of Table I of that communication shows that no reasonable man can object to the "Equivalent Uniform-Load Method" because of its want of exactness.

In designing a bridge, one commences naturally with the stringers, then passes to the floor-beams; and afterwards to the trusses; so let us follow this order.