“Atmos” is a type of mechanical clock, manufactured in several models since 1935 by Jaeger-LeCoultre in Switzerland. The Atmos needs no batteries, no electric power, and never needs to be wound. All it requires to run indefinitely is an ambient temperature change of at least 1 degree Celsius in the range between 15 and 30 C each day. The clock stores mechanical energy from the thermal expansion and contraction of an hermetically sealed capsule containing a mixture of gaseous and liquid compounds formulated to provide maximum volumetric changes with temperature. The mechanism is illustrated here. Atmos clocks are laboriously handmade and very expensive, and are commonly gifted by the Swiss government upon visiting heads of state.