The utilization of railroads for purposes of war, has increased the importance of this kind of engineering, if it has not created an entirely new branch of the science. Bridges are the most vulnerable part of railroads, and the army operating against an enterprising enemy, and drawing its supplies over such a road, must be prepared, if it would maintain its communications, to rebuild, at a moment's notice, any bridge upon the road. This necessity during our war of 1861 — 65 produced a special corps for the con struction and Operation of Military Railroads, the chief of which for a time Gen eral Hermann Haupt is the author of the treatise from which the substance of this paper has been drawn.