our lawns, the revolving arms being attached
to a plane on which several pairs of statuettes representing dancers
are placed, An even more interesting application of this principle of
setting a wheel in motion is furnished in a mechanism which must be
considered the earliest of steam-engines. Here, as the name implies, the
gas supplying the motive power is actually steam. The apparatus made
to revolve is a globe connected with the steam-retort by a tube which
serves as one of its axes, the steam escaping from the globe through two
bent tubes placed at either end of an equatorial diameter. It does
not appear that Hero had any thought of making practical use of this
steam-engine. It was merely a curious toy--nothing more. Yet had not the
age that succeeded that of Hero been one in which inventive genius
was dormant, some one must soon have hit upon the idea that this
steam-engine might be improved and made to serve a useful purpose. As
the case stands, however, there was no advance made upon the steam motor
of Hero for almost two thousand years. And, indeed, when the practical
application of steam was made, towards the close of the eighteenth
century, it was made probably quite without reference to the experiment
of Hero, though knowledge of his toy may perhaps have given a clew to
Watt or his predecessors.

{illustration caption = THE SLOT-MACHINE OF HERO (The coin introduced at
A falls on the lever R, and by its weight opens the valve S, permitting
the liquid to escape through the invisible tube LM. As the lever tips,
the coin slides off and the valve closes. The liquid in tank must of
course be kept above F.)}

In recent times there has been a tendency to give to this steam-engine
of Hero something more than full meed of appreciation. To be sure, it
marked a most important principle in the conception that steam might
be used as a motive power, but, except in the demonstration of this
principle, the mechanism of Hero was much too primitive to be of any
importance. But there is one mechanism described by Hero which was a
most explicit anticipation of a device, which presumably soon went out
of use, and which was not reinvented until towards the close of the
nineteenth century. This was a device which has become familiar in
recent times as the penny-in-the-slot machine. When towards the close of
the nineteenth century some inventive craftsman hit upon the idea of an
automatic machine to supply candy, a box of cigarett