The worst form it can have is that of a vertical plane or upright flat, and next to this the worst form is an inclined plane.
Some of these devices are, the wedge, the screw, the pulley and the inclined plane.
In Fig. 248 we treat a flight of steps as if it were an inclined plane.
It also applies to those on an inclined plane, as in Rule 8.
They made a little trench so that ...