This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
When sketching a plan, if any one of the first few lines drawn is found to be the proper length, then this line can be made into a scale by the geometric rule for dividing a given line into equal parts.
Ill: A Scale can be Made for Use on Any Sized Sketch or Drawing
Suppose, for example, the line AB, which is to represent 12 ft., is found to be 1 ft. long. Draw a line, AC, at any angle from the point A and step off on it 12 equal parts, beginning at A. The last point, or the one at C, is connected to the end B, then eleven other lines are drawn parallel with CB. Thus AB will make a scale of 1 in. to each 1 ft. - Contributed by James M. Kane, Doylestown, Pa.