In half a dozen chapters mechanical birds are made to sing by driving air through a pipe by the pressure of flowing water. In other chapters a dragon is made to hiss and a thyrsus to whistle by similar methods. By the force of compressed air water is made to spurt forth and automatons to sound trumpets. The heat of the sun’s rays is used to warm air which expands and causes water to trickle out. In a number of cases as long as a fire burns on an altar the expansion of enclosed air caused thereby opens temple doors by the aid of pulleys, or causes statues to pour libations, dancing figures to revolve, and a serpent to hiss. The force of steam is used to support a ball in mid-air, revolve a sphere, and make a bird sing or a statue blow a horn. Inexhaustible lamps are described as well as inexhaustible goblets, and a self-trimmed lamp in which a float resting on the oil turns a cog-wheel which pushes up the wick as it and the oil are consumed. Floats and cog-wheels are also used in some of the tricks already mentioned. In another the flow of a liquid from a vessel is regulated by a float and a lever. Cog-wheels are also employed in constructing the neck of an automaton so that it can be cut completely through with a knife and yet the head not be severed from the body. A cupping glass, a syringe, a fire engine pump with valves and pistons, a hydraulic organ and one worked by wind pretty much exhaust the contents of the _Pneumatics_. In its introduction Hero alludes to his treatise in four books on water-clocks, but this is not extant. Hero’s water-organ is regarded as more primitive than that described by Vitruvius.[893] [Sidenote: Magic mirrors.] If magic jugs and marvelous automatons make up most of the contents of the _Pneumatics_ and _Automatic Theater_, comic and magic mirrors play a prominent part in the _Catoptrics_. The spectator sees himself upside down, with three eyes, two noses, or an otherwise distorted countenance. By means of two rectangular mirrors which open and close on a common axis Pallas is made to spring from the head of Zeus. Instructions are given how to place mirrors so that the person approaching will see no reflection of himself but only whatever apparition you select for him to see. Thus a divinity can be made suddenly to appear in a temple. Clocks are also described where figures appear to announce the hours. [Sidenote: Astrology and occult virtue.]