Horse-powered millstone. Marble, panel of a Roman sarcophagus, 3rd century CE. From the Vigna Tre Madonne, oustide the Porta San Giovanni in Rome, 1826.
CC BY 3.0
<a href="//commons.wikimedia.org/wiki/User:Jastrow" title="User:Jastrow">Marie-Lan Nguyen</a> (2009)