How upon a line a c an equilateral triangle can be made.

b - a c

Here is shown how upon a given line a d a pentagon can be made.

h - b e - p g - a S d - c r f - m

I find that the sides of the pentagon stand away from the circle by 1/5 of those sides.

If you wished to make a pentagon upon a given line, do as figured above. Let the given line be a d. Set the compass point on point a, and with the other make the line b c; then move that compass point, setting it on d, and with the other make the line e f; then, to find the middle of the line a d, draw the line h m through the intersections p r. This done, divide the half of the line a d into 4 equal parts, as appears at a s; and these parts may be made by opening the compasses at random, provided only the parts are equal; and set one of these parts at the top of the line p s, as appears at p g. Then draw a line from g to a, and where it cuts the line p s, there will be the centre of the circle, which will have within it the capacity to receive along its circumference five sides equal to the first given line a d.

If you wish to make an equilateral triangle upon a given line, let us say the given line is a c. Set the fixed point of the compass on point a and with the other point make the line b c; then put the point on point c and make the line a b; then put the point on point b and make the line a c; then draw straight lines from each of the 3 points, and you will have an equilateral triangle.