1782. The ground-compasses (fig. 425.) are generally made of hard wood, such as oak, shod with iron, and with an iron gauge or segment (a); their length may be six feet; they are used chiefly for laying out parterres in the ancient manner; since, by a previous preparation of the soil, the curvilinear parts of such parterres can be described by them with perfect accuracy. The stationary foot is placed on a slip of board a few inches square, with a pin beneath to retain it in its place, and a lead cap above for the point of the foot.