Mark a square of any unknown surface; divide one side into 20 equal
parts, each part of which is a unit, or one number of the scale; use said
20 parts as a scale, representing the half breast circumference, and
2½ inches; with this scale make a diagram or draft of a vest, or coat,
according to this work—all of which is done before the size of the square
is known. When done, measure the square, or 20 parts of the scale, by
inches.
Now, suppose the square turns out to be 20 inches; the size of the
garment is 35 breast, or 17½ inches and 2½ inches. If the square turns
out to be 22½ inches, the garment will be size 40, and so on, always 2½
sizes less than the square. If, however, the size of the square is too
small to make a full size, multiply its units by any number whereby you
can find a certain desired size. For instance: If the square contains 5
inches, each ¼ inch is a unit, or one part of the scale. Multiply the 5
inches by 4 and you have 20 inches. Now, take 4 units of the original,
which represents here 1 inch, and your new scale will represent 20 inches
divided into 20 parts, and will also cut size 35. If you multiply the
5-inch square by 3½, you will produce 18 inches, and if you take 3½ units
as 1 unit, each unit will represent ¹⁄₂₀ of 18 inches, and the size of
the garment will be 30½. If you multiply the 5 inches with 4½, you will
obtain 22 inches, and by taking 4½ units as 1 unit for the scale, each
unit will represent ¹⁄₂₀ part of 22 inches, and the size will be 39.