Harry Thomason's heat storage scheme. Hot water from a trickle-down solar water heater  is stored in a steel tank buried in a bin of stones. The stones serve to increase the heat transfer area of the tank when heating cool air blown through the stones. From William A. Shurcliff, "New inventions in low-cost solar heating--100 daring schemes tried and untried", p. 184, Brick House Publishing Company, Andover, Mass.