Throughout this book, I have used simple objects as illustrations upon which to base the instructions. Knowing that the weakness of many a writer of educational texts, lies in his failure to make known the things that are known to him as very simple, though in reality very hard to the reader, I have tried to men tion such simple facts in my explanation as would prove to be a stumbling block if not understood by the student. I have endeavored to make the subjects interesting and appealing, so as to leave no doubt in the reader's mind of every step that would lead to a complete grasp of reading a blue print drawing.