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.
Sometimes it is necessary to enlarge or reduce A plot to a different scale. This can be easily and quickly accomplished without resorting to the slow process of protracting the angles and scaling the individual lines.
Ill: Enlarging and Reducing Plots by Radial Lines from a Common Point Located Properly
Take any point, P, and from it draw light pencil lines through each of the corners of the plot. On any one of these lines, as AP, lay off with dividers AC equal to CP. Place a triangle on the line AB and with a straightedge, or another triangle, laid on the line AP, slide the former to the point C, then draw line CD parallel with AB until it intersects the radial line PB. In the same manner draw line DE parallel with BF, and so on, all about the plot. A test of accuracy will be in striking the point C with the last line. If the original plot has a scale of 40 ft. to the inch the reduced plot would be 80 ft. to the inch. If it is required to enlarge the plot to 20 ft. to the inch, make AG equal to AP, and proceed as in the first case, using G as the starting point.
The location of the point P is arbitrary and may be outside of the boundary of the plot or figure to be enlarged or reduced, but should be so located, if possible, that the radial line to any corner does not parallel either of the plot lines to that corner. If the point cannot be so located for all the lines, it may be necessary to scale the lines. A little practice in picking out the best location for the point will give gratifying results. - Contributed by Junius D. McCabe, Pittsburgh, Pa.