The secret lies in small changes in air temperature. When the temperature rises, gases inside of a small chamber expand, which compresses a spiral spring. When the temperature drops again (as it is bound to do), the gas condenses and the spring relaxes. The resulting reciprocating motion, though small, is enough to winds clock's mainspring.
Here is a where you can learn more about the Perpetual ATMOS Clock.
[ Found via the MAKE magazine blog ]