rate system of tests of actual intensities of working stresses for all main members of modern steel bridges under live loads, applied with varying velocities, is probably more urgently needed at the present time by the engineering profession than is any other series of experiments.

In the specifications of this treatise the effect of impact is provided for, how correctly only such experiments as those just referred to can demonstrate. As pointed out in Chapter I., the determination of the various amounts of impact was made solely by adopting a few fixed intensities of working stress and varying the percentages of impact so as to make the structures designed thereby agree as nearly as may be with the best general practice. If the impact formulae adopted are ever proved to be incorrect, it will be a simple matter to correct them in a later edition.

Principle X.

In making the general layout of any structure, due attention should be given to the architectural effect, even if the result be to increase the cost somewhat.

There is no feature of bridge-designing which has been ignored in America to such an extent as has this; and it is only of late years that even a few American engineers have paid any attention whatsoever to aesthetics in that branch of engineering. The subject is such an important one that to its consideration Chapter IV. will be specially devoted.

Principle XI.

For the sake of uniformity, and to conform to the unwritten laws of fitness, it is often necessary in bridge-designing to employ metal which is not really needed for either strength or rigidity.

The designer who recognizes this fact will usually produce structures of finer appearance than the designer who ignores it because of false notions of economy.