 One of the most prominent writers on mechanics in antiquity, Hero of Alexandria (1st c. CE), is sometimes regarded, following Hermann Diels, as an atomist in the Democritean tradition. In the introduction to his Pneumatica, Hero describes matter as made up of particles with spaces between them. However, Hero’s account of pneumatic effects involving compression of air seems to depend on the deformation of elastic particles which can be compressed artificially but will spring back to their original shape quite vehemently. If so, his account denies a fundamental tenet of classical atomism, that atoms do not change in their intrinsic properties like shape (Berryman 2011). Pneumatics, because its effects were thought to depend on forces acting against a void, provided stimulus to discussions of the possibility of void and why its supposed impossibility would cause water to move against its natural direction. In late antiquity, John Philoponus theorized that void must be at least theoretically possible for these effects to occur (Sedley 1987).