Now the lines described by the same line travelling in a straight direction are straight, and so the cylinder would travel straight along them; for it makes no difference whether you drag the cylinder over the plane surface at the line where it fir touched the plane surface, or whether you roll it over it; for the result will always be that an equal and similar line made up of points on the cylinder will touch the plane surface, both when the cylinder is dragged and when it is rolled along.