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.
Films develop better if the tray holding the solution is kept in motion or rocked. This is inconvenient and tiresome where a great many films are to be developed. The trouble may be overcome by the use of the rocking device shown in the sketch. It may be made of any light wood, the right size to suit the photographer's needs.
The tray holder A is pivoted on the uprights C with pins EE. The uprights are fastened to a base, B. Two braces, D, one on each side of the upright C, limits the tip of the tray holder A. The weight F works as a pendulum, which automatically rocks the tray when set in motion. - Contributed by Abner B. Shaw, No. Dartmouth, Mass.