The bibliographical aids at the close of the several chap ters are not meant to be inclusive, but are chosen because of their value to students and others as collateral reading. A discussion of certain of them along with the text treat ment, and in the light of the laboratory problems assigned, has proved helpful in the author's classes. I am indebted to Professor Willard E. Hotchkiss, for merly Dean of the Northwestern University School of Com merce, and to Professor John F. Hayford, Dean of the Northwestern University College of Engineering for read, ing parts of the manuscript and for offering many helpful suggestions for its improvement. Most of all I am indebted to my wife who has materially lightened the burden of proof reading, and who, at all stages in the preparation Of the vol ume, has been a constant source of encouragement.