The clock's mechanism is driven by variations in daily temperature and, to a lesser extent, in atmospheric pressure. Either causes the air in a one-cubic-foot (28-litre) airtight box to expand or contract, which pushes on a diaphragm. A temperature variation of 6 °F (3.3 °C) over the course of each day creates approximately enough pressure to raise a one- pound weight by one inch (equivalent to 13 mJ or 3.6 μWh), which drives the clock mechanism.