This section is from the book "Modern Shop Practice", by Howard Monroe Raymond. Also available from Amazon: Modern Shop Practice.

Draw the horizontal straight line A C about 3 inches long. With the extremity A as a center and any convenient radius - about 2 inches - describe arcs above and below the line A C. With the other extremity C as a center and with the same radius draw similar arcs intersecting the first arcs at D and E. The radius of these arcs must be greater than one-half the length of the line in order that they may intersect. Now draw the straight line D E passing through the intersections D and E. This line will cut A C at its middle point F. Therefore AF= FC.

Since the points D and E are equally distant from A and C a straight line drawn through them is perpendicular to A C at its middle point F.

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