His pupil, Hero, improved it by giving it two cylinders.

There, too, the first steam-engine worked.

This also was the invention of Hero, and was a reaction engine, on the principle of the eolipile.

The silence of the halls of Serapis was broken by the water-clocks of Ctesibius and Apollonius, which drop by drop measured time.

When the Roman calendar had fallen into such confusion that it had become absolutely necessary to rectify it, Julius Caesar brought Sosigenes the astronomer from Alexandria.