306 
HERON OF ALEXANDRIA 
the surface. He then inquires-what is the reason why the 
diver is not oppressed though he has an unlimited weight of 
water on his back. He accepts, therefore, the view of Ptolemy 
as to the fact, however strange this may seem. But he is not 
satisfied with the explanation given : * Some say ’, he goes on, 
‘it is because water in itself is uniformly heavy {lao/Bapes avrb 
ko.6’ avro) ’—this seems to be equivalent to Ptolemy’s dictum 
that water in water has no weight—‘ but they give no ex 
planation whatever why divers . . He himself attempts an 
explanation based on Archimedes. It is suggested, therefore, 
that Heron’s criticism is directed specifically against Ptolemy 
and no one else. (4) It is suggested that the Dionysius to whom 
Heron dedicated his Definitions is a certain Dionysius who 
was praefectus urbi at Rome in a.d. 301. The grounds are 
these (a) Heron addresses Dionysius as Aiovvcne XapLirporare, 
where Xa/xnporaTos obviously corresponds to the Latin clarissi- 
mus, a title which in the third century and under Diocletian 
was not yet in common use. Further, this Dionysius was 
curato'r aquarum and curator operum puhlicorum, so that he 
was the sort of person who would have to do with the 
engineers, architects and craftsmen for whom Heron wrote. 
Lastly, he is mentioned in an inscription commemorating an 
improvement of water supply and dedicated ‘ to Tiberinus, 
father of all waters, and to the ancient inventors of marvel 
lous constructions ’ (repertoribus admirabilium fabricarum 
prucis viris), an expression which is not found in any other 
inscription, but which recalls the sort of tribute that Heron 
frequently pays to his predecessors. This identification of the 
two persons named Dionysius is an ingenious conjecture, but 
the evidence is not such as to make it anything more. 1 
The result of the whole investigation just summarized is to 
place Heron in the third century A.D., and perhaps little, if 
anything, earlier than Pappus. Heiberg accepts this conclu 
sion, 2 which may therefore, I suppose, be said to hold the field 
for the present. 
1 Dionysius was of course a very common name. Diophantus dedicated 
his Arithmetica to a person of this name (rifuwTare pot Aioiwi«), whom he 
praised for his ambition to learn the solutions of arithmetical problems. 
This Dionysius must have lived in the second half of the third century 
A. D., and if Heron also belonged to this time, is it not possible that 
Heron’s Dionysius was the same person ? 
2 Heron, vol. v, p. ix.