A reference catalog of 40 devices that harvest energy from sources the world either forgot, dismissed, or has not yet learned to reach. Thermal gradients too faint to measure. The charge in falling water. The vibration of a bridge the wind destroyed. The cold of outer space, accessed through a gap in the atmosphere. The voltage in river mud that Volta measured in 1776 and never understood.

Each entry is a machine, an inventor, and a question: why did it take so long?