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 supporting arm of the hinged shelf is constructed of a piece of gas pipe and a length of iron rod which slides snugly into the pipe. A spring catch is set in the pipe at the proper height to engage the end of the iron rod when the shelf is up. This spring must be of good size, as it holds the entire weight of the shelf. A large clock spring is suitable. One end of the spring is bent outward and upward to form a releasing handle. The other end is drilled for the two machine screws which hold it to the pipe. The spring works in a rectangular slot, cut lengthwise of the pipe. The pipe must extend 8 or 10 in. beyond the spring. The ends of the rod and of the pipe are pivoted with screws or rivets on angle pieces screwed to the shelf and wall. - Contributed by Donald A. Price, Wilmington, Del.
Ill: When the Shelf is Lowered the Support Telescopes and is Out of the Way