Given a point A on a line
scribe two arcs of the same radius centered at A intersecting the line at two points on either side of A.
Call these two points B and C. Open the compass up a little more than it already is and scribe arcs centered at B and C with the same radius.
Call the point where the two arcs meet D. Draw AD.
AD is the line through A perpendiuclar to BC.
