This characterization by Vitruvius of the writings of Ctesibius also applies with astonishing fitness to some of the works current under the name of Hero of Alexandria,[887] who is indeed in a Vienna manuscript of the _Belopoiika_ spoken of as the disciple or follower of Ctesibius.[888] Hero, however, is not mentioned either by Vitruvius or Pliny, and it is now generally agreed as a result of recent studies that he belongs to the second century of our era.[889] His writings are objective and impersonal and tell us much less about himself than Vitruvius’s introductions to the ten books of _De architectura_. The similarity in content of his writings to those of the much earlier Ctesibius as well as the character of his terminology suggest that he stands at the end of a long development. He speaks of his own discoveries, but perhaps in the main simply continues and works over the previous principles and mechanisms of men like Ctesibius. As things stand, however, his works constitute our most important, and often our only, source for the history of exact science and of technology in antiquity.[890] [Sidenote: Medieval working over of the texts.] Not only does Hero seem to have been in large measure a compiler and continuer of previous science, his works also have evidently been worked over and added to in subsequent periods and bear marks of the Byzantine, Arabian, and medieval Latin periods as well as of the Hellenistic and Roman. Indeed Heiberg regards the _Geometry_ and _De stereometricis_ and _De mensuris_ as later Byzantine collections which have perhaps made some use of the works of Hero, while the _De geodaesia_ is an epitome of, or extract from, a pseudo-Heronic collection. The _Catoptrica_ is known only from the Latin translation of 1269, probably by William of Moerbeke, and long known as _Ptolemy on Mirrors_. It appears, however, to be directly translated from the Greek and not from the Arabic. The _Mechanics_, on the other hand, is known only from the Arabic translation by Costa ben Luca. Of the _Pneumatics_ we have Greek, Arabic, and Latin versions. It was apparently known to the author of the thirteenth century _Summa philosophiae_ ascribed to Robert Grosseteste, since he speaks of the investigations of vacuums made by “Hero, that eminent philosopher, with the aid of water-clocks, siphons, and other instruments.”[891] Scholars are of the opinion that the Arabic adaptation, which is of popular character and limited to the entertaining side, comes closer to the original Greek version of Hero’s time than does the Latin version which devotes more attention to experimental physics. The _Automatic Theater_, for which there is the same chief manuscript as for the _Pneumatics_, also seems to have been worked over and added to a great deal. [Sidenote: Hero’s thaumaturgy.]