In spite of the numerous data on the theory, practice, apparatus, and tests of welding contained in the trade journals and metallurgical books, no previous attempt has been made to present this data in sequence under one cover. But in the last fifteen years the subject has begun to be of interest and importance. The electric, thermit, and hot-flame processes are welding all of the metals and are doing repeat and repair work that has never before been attempted. New brazing methods have also been successfully tried out and the range of good solders greatly increased.

I have given separate chapters to the commercial metals. Few of the metallurgies give much space to the working properties of the metals, especially the welding property, which is often merely mentioned.

Test and cost data must be taken with a grain of salt. The test data have been compiled from various sources. Those tests given for iron are standard and cannot be questioned. Those tests given for the special processes are more recent and in most cases have been made by interested parties. They are no doubt accurate, but at present the special processes cannot be so well represented by test data, as by the actual work they turn out. The same may be said of cost data. The prospective purchaser of welding machinery must figure the cost of his apparatus plus the cost of labor and the depreciation. But above all, he must satisfy himself that the apparatus he chooses is the best for his kind of welding.

I wish to express my thanks for assistance received from the different welding companies mentioned herein; also to James H. DeLong, for special analyses; Dr. Edward Hart, Dr. Joseph W. Richards, Prof. Oliver P. Watts, Otis Allen Kenyon, E. A. Colby, of Baker & Co., and to many others.

R. N. Hart.

Los Angeles, Cal., October, 1910.