In the present edition numerous changes from the first will appear, and considerable new matter has been inserted. The substance of the book, however, save for some minor details, remains practically untouched. It has seemed best not to recast Part I, as to have done so would have meant writing a new book; the working ezer cises are altogether as they were in the former edition, except that quite a little useless verbiage and space have been obviated by condensation.