Texts have survived from the Hellenistic period concerning a so-called pneumatikē technē. I shall refer to the field as ‘pneumatics’, although – as scholars have noted – the English transliteration does not exactly capture the sense of the subject matter, which concerns devices worked by flowing water, compressed air or steam. As Landels aptly suggests, the German title Druckwerke best captures the sense: although there are some anomalies, the field is largely about what might be called pressure effects.

The characterization of pneumatics as delimited by pressure-driven devices is, admittedly, a simplification of a vaguely defined field, and it works better for Hero's collection than for Philo's. The latter – especially in the more extensive Arabic version – includes a number of water wheels, which do not depend on pressure differentials so much as the simple weight of water. Both Pappus and Philoponus describe a branch of mechanics as being concerned with devices that raise water, perhaps by analogy parallel to the category of devices used to lift weights. However, the vast majority of devices in the pneumatica collections use pressure differentials to regulate the flow of water in different ways.

I have previously discussed the classical philosophical theories offered to account for ‘rarefaction effects’. Some pneumatic devices of the Hellenistic era work by the reverse procedure, forcing air into a smaller place than it otherwise occupies. These ‘compression effects’, as I call them, are credited to Ctesibius.