e man who would coolly appropriate some discoveries of others under
cloak of a mere prefatorial reference was perhaps an expounder rather
than an innovator, and had, it is shrewdly suspected, not much of his
own to offer. Meanwhile, it is tolerably certain that Ctesibius was the
discoverer of the principle of the siphon, of the forcing-pump, and of a
pneumatic organ. An examination of Hero's book will show that these are
really the chief principles involved in most of the various interesting
mechanisms which he describes. We are constrained, then, to believe that
the inventive genius who was really responsible for the mechanisms we
are about to describe was Ctesibius, the master. Yet we owe a debt of
gratitude to Hero, the pupil, for having given wider vogue to these
discoveries, and in particular for the discussion of the principles of
hydrostatics and pneumatics contained in the introduction to his
book. This discussion furnishes us almost our only knowledge as to the
progress of Greek philosophers in the field of mechanics since the time
of Archimedes.

The main purpose of Hero in his preliminary thesis has to do with the
nature of matter, and recalls, therefore, the studies of Anaxagoras and
Democritus. Hero, however, approaches his subject from a purely material
or practical stand-point. He is an explicit champion of what we nowadays
call the molecular theory of matter. "Every body," he tells us, "is
composed of minute particles, between which are empty spaces less than
these particles of the body. It is, therefore, erroneous to say that
there is no vacuum except by the application of force, and that every
space is full either of air or water or some other substance. But in
proportion as any one of these particles recedes, some other follows
it and fills the vacant space; therefore there is no continuous vacuum,
except by the application of some force (like suction)--that is to
say, an absolute vacuum is never found, except as it is produced
artificially." Hero brings forward some thoroughly convincing proofs of
the thesis he is maintaining. "If there were no void places between the
particles of water," he says, "the rays of light could not penetrate the
water; moreover, another liquid, such as wine, could not spread itself
through the water, as it is observed to do, were the particles of water
absolutely continuous." The latter illustration is one the validity of
which appeals as forcibly to the physicists of to-da