1784. Of levels (figs. 427. and 428.) there are a variety of sorts; but the most convenient is half a square, with an iron index in the angle marked with ninety divisions or degrees. The use of these degrees is to facilitate the laying out of slopes: at a perfect level the plummet will hang at 45ï¾¦; and for a slope it may be any lesser number in ascending, or any greater number in descending from a fixed point. This level may also be used as a square to set off right angles, or indeed angles of any description.