the matter, he himself undertook the task and accomplished it, at least to his own satisfaction, in four years of hard work.

The following is a list of the said economic questions arranged in the order of their final solution:

1. Economics of Steel Arch-Bridges.

2. Comparative Economics of Cantilever and Suspension Bridges.

3. Economic Span-Lengths for Simple-Truss Bridges on Various

Types of Foundation.

4. Possibilities and Economics of the Transbordeur.

5. Comparative Economics of Continuous and Non-Continuous Trusses.

6. Comparative Economics of Wire Cables and High-Alloy-Steel

Eye-bar-Cables for Long-Span Suspension-Bridges.

7. Economics of Reinforced-Concrete, Steam-Railway Bridges.

8. De l'Emploi Économique des Alliages d'Acier dans la

construction des Ponts.

9. Bridge Versus Tunnel for the Proposed Hudson River Crossing

at New York City.

10. Economics of Movable Spans.

As fast as these economic investigations were finished, they were separately incorporated in chapters of this book, occasionally in toto but generally with slight modifications or omissions.

In the preparation of a few portions of the manuscript, the author has received valuable assistance from the following engineers, to whom he herewith tenders his hearty thanks: Messrs. Thomas E. Brown, Leon L. Clarke, Watson Vredenburgh, Thomas Earle, C. M. Canady, Harry K. Seltzer, Joseph J. Yates, Frank W. Skinner, Charles F. Loweth, Carl S. Heritage, J. G. Chalfant, Vernon R. Covell, Edward A. Byrne, J. B. W. Gardiner, L. H. Beach, P. S. Bond, A. H. Sabin, and Shortridge Hardesty.

The special portions of the book whereon these gentlemen have aided are indicated throughout the text, excepting in the case of the author's assistant engineer, Mr. Hardesty, who helped with certain of the calculations and gave the entire treatise a general check.

It may be of interest to some readers to learn how Chapter XLIV on "Economics of Military Bridging" happened to be included in the book; for it is a subject concerning which, before the said chapter was written, the author knew practically nothing. Incidentally it came to the attention of Major R. W. Lewis-a young engineer who served with distinction in the American Army on the battle fields of France, and a son of Col. I. N. Lewis, the famous inventor of the machine gun which bears his name, and which so greatly aided the Allies in winning the war-that this book was in course of preparation. He thereupon called on the author and asked if he would consider favorably a suggestion to insert in the treatise a chapter prepared officially by the Engineer Corps of the Army on the subject of