This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The hurdle consists of two standards, a reach, and a swing. The swing is first made in the shape of a rectangle of four pieces of wood, about 7/8 in. thick and 1 1/2 in. wide, of which two are 36 in. and the others 18 in. long. These pieces are nailed together in the manner shown.
Each standard is made of three pieces of wood, 7/8 in.thick, 3 in. wide, and 18 in. long. Nail the pieces firmly together, as shown, and connect their bases with another piece of the same material, 36 in. long. When this is finished, connect the swing to the standards with long nails, A, at the ends slightly off center. Before inserting the nails, make the holes in each standard to receive them large enough to permit the nails to turn freely without allowing the heads to pass through. Thus the frame will swing freely at the slightest touch of the jumper's foot, and right itself immediately. - Contributed by C. C. Fraser, Saginaw, Mich.
Ill: The Swing of the Hurdle will Turn When Slightly Touched and Right Itself Again