Draw the straight line A C about 3 1/4 inches long, and assume the point P about 1 1/2 inches above the line. With P as a center and any convenient radius - about 2 inches - describe an arc cutting A C at E and F. The radius of this arc must always be such that it will cut A C in two points; the nearer the points E and F are to A and C, the greater will be the accuracy of the work.
Now with E and F as centers and any convenient radius - about 2 1/4 inches - draw the arcs intersecting below A C at T. A line through the points P and T will be perpendicular to AC. In case there is not room below A C to draw the arcs, they may be drawn intersecting above the line as shown at N. Whenever convenient draw the arcs below A C for greater accuracy.
Since P and T are both equally distant from E and F, the line P T is perpendicular to A C.