“Draw a large circle upon paper; divide it into three, four, five, six, seven, or eight equal parts; which being done, we may draw in every one of the divisions a figure, at our pleasure, either for garden-plats or fortifications; as, for example, in Fig. 55, we see a circle divided into six parts, and upon the division marked =F= is drawn part of a design for a garden. Now, to see that design entire, which is yet confused, we must place our glasses upon the paper, and open them to the sixth part of the circle, _i.e._, one of them must stand upon the line _b_, to the centre, and the other must be opened exactly to the point _c_; so shall we discover an entire garden-plot in a circular form (if we look into the glasses), divided into six parts, with as many walks leading to the centre, where we shall find a basin of an hexagonal figure. “The line =A=, where the glasses join, stands immediately over the centre of the circle, the glass =B= stands upon the line drawn from the centre to the point =C=, and the glass =D= stands upon the line leading from the centre to the point =E=: the glasses being thus placed, cannot fail to produce the complete figure we look for; and so whatever equal part of a circle you mark out, let the line =A= stand always upon the centre, and open your glasses to the division you have made with your compasses. If, instead of a circle, you would have the figure of a hexagon, draw a straight line with a pen from the point _c_ to the point _b_, and, by placing the glasses as before, you will have the figure desired. “So likewise a pentagon may be perfectly represented, by finding the fifth part of a circle, and placing the glasses upon the outlines of it; and the fourth part of a circle will likewise produce a square, by means of the glasses, or, by the same rule, will give us any figure of equal sides. I easily suppose that a curious person, by a little practice with these glasses, may make many improvements with them, which, perhaps, I may not have yet discovered, or have, for brevity sake, omitted to describe. “It next follows that I explain how, by these glasses, we may, from the figure of a circle, drawn upon paper, make an oval; and also, by the same rule, represent a long square from a perfect square. To do this, open the glasses, and fix them to an exact square; place them over a circle, and move them to and fro till you see the representation of the oval figure you like best; and so, having the glasses fixed, in like manner move them over a square piece of work till you find the figure you desire of a long square. In these trials you will meet with many varieties of designs. As for instance, Fig. 56, although it seems to contain but a confused representation, may be varied into above two hundred different representations, by moving the glasses over it, which are opened and fixed to an exact square. In a word, from the most trifling designs, we may, by this means, produce some thousands of good draughts.