It will be noted in connection with the diagrams presented in this book, that the authors have indicated lines by a single letter. This is for conciseness; and no confusion need arise if the general scheme of the parallelogram be borne in mind, which in every case has for its base line A, its left-hand side B, the upper line C, and the right-hand side D. In the more complex figures arrows are used to designate the direction of the lines.

The publishers are permitted to announce that worked models described in this book may be obtained by addressing Miss Louise M. Heller, 293 South Oakley Avenue, Chicago, Illinois.