Although the subject matter of this volume is, in large measure, identical with that of many treatises on descriptive geometry, the author has called it Theory of Engineering Drawing, believing that this title indicates better than could any other, the ultimate purpose of the book. That texts on descriptive geometry appear with some degree of frequency, with but few, if any, additions to the theory, indicates that teachers are aware of certain weaknesses in existing methods of presenting the subject. It is precisely these weakness that the present work aims to correct.