502 ADDITIONS. 



The remaining writings of Hero of Alexandria have been the subject 

 of a special, careful, and learned examination by M. Th. H. Martin 

 (Paris, 1854), in which the works of this writer, Hero the Ancient, as 

 he is sometimes called, are distinguished from those of another writer 

 of the same name of later date. 



Hero of Alexandria wrote also, as it appears, a treatise on Pneu- 

 matics, in which he described machines, either useful or amusing, 

 moved by the force of air and vapor. 



He also wrote a work called Catoptrics, which contained proofs of 

 properties of the rays of reflected light. 



And a treatise On the Dioplra ; which subject, however, must be 

 carefully distinguished from the subject entitled Dioptrics by the mod- 

 erns. This latter subject treats of the properties of refracted light ; a 

 subject on which the ancients had little exact knowledge till a later 

 period, as I have shown in the History. The Dioptra, as understood 

 by Hero, was an instrument for taking angles so as to measure the po- 

 sition, and hence to determine the distance of inaccessible objects ; as 

 is done by the Theodolite in our times. 



M. Martin is of opinion that Hero of Alexandria lived a v . a later 

 oeriod than is generally supposed ; namely, after B. c. 81. 



