of this book complete specifications for the designing of highway bridges of all kinds; consequently this treatise may be said to replace the pamphlet that is now out of print.

There is considerable difference, though, between the old specifications and the new, due to two reasons, viz., first, there have been great advances made in bridge-building in the last eight years; and, second, the author has concluded to abandon the attempt to conciliate those who desire to build cheap structures, so has cut out Class D from his specifications, and has strengthened up and improved the other "classes" in several particulars, notably by raising the minimum thickness of metal from one quarter to five sixteenths of an inch.

The weights of bridges designed according to the new specifications will be somewhat greater than those designed according to the old ones, but the structures will be correspondingly better. Moreover, the new specifications will be found to be more rational, scientific, and generally satisfactory than the old ones, especially in the feature of impact allowance. It must be remembered that the author does not claim that the formula which he specifies for impact will provide exactly for the greatest possible impact on all parts of all highway bridges; but he does think that it will always be great enough, and be knows that any structure in the design of which it is used, and which is proportioned by the specifications of this treatise, will be well and properly designed in every part.