accomplished through the expansive power of heated air. For
example, the doors of a temple are made to swing open
automatically when a fire is lighted on a distant altar, closing
again when the fire dies out--effects which must have filled the
minds of the pious observers with bewilderment and wonder,
serving a most useful purpose for the priests, who alone, we may
assume, were in the secret. There were two methods by which this
apparatus was worked. In one the heated air pressed on the water
in a close retort connected with the altar, forcing water out of
the retort into a bucket, which by its weight applied a force
through pulleys and ropes that turned the standards on which the
temple doors revolved. When the fire died down the air
contracted, the water was siphoned back from the bucket, which,
being thus lightened, let the doors close again through the
action of an ordinary weight. The other method was a slight
modification, in which the retort of water was dispensed with and
a leather sack like a large football substitued. The ropes
and pulleys were connected with this sack, which exerted a pull
when the hot air expanded, and which collapsed and thus relaxed
its strain when the air cooled. A glance at the illustrations
taken from Hero's book will make the details clear.

Other mechanisms utilized a somewhat different combination of
weights, pulleys, and siphons, operated by the expansive power of
air, unheated but under pressure, such pressure being applied
with a force- pump, or by the weight of water running into a
closed receptacle. One such mechanism gives us a constant jet of
water or perpetual fountain. Another curious application of the
principle furnishes us with an elaborate toy, consisting of a
group of birds which alternately whistle or are silent, while an
owl seated on a neighboring perch turns towards the birds when
their song begins and away from them when it ends. The "singing"
of the birds, it must be explained, is produced by the expulsion
of air through tiny tubes passing up through their throats from a
tank below. The owl is made to turn by a mechanism similar to
that which manipulates the temple doors. The pressure is supplied
merely by a stream of running water, and the periodical silence
of the birds is due to the fact that this pressure is relieved
through the automatic siphoning off of the water when it reaches
a certain height. The action of the siphon, it may be added, is
correctly explained by Hero as due to the greater weight of the
water in the longer arm of the bent tube. As before mentioned,
the siphon is repeatedly used in these mechanisms of Hero. The
diagram will make clear the exact application of it in the
present most ingenious mechanism. We may add that the principle
of the whistle was a favorite one of Hero. By the aid of a
similar mechanism he brought about the blowing of trumpets when
the temple doors were opened, a phenomenon which must greatly
have enhanced the mystification. It is possible that this
principle was utilized also in connection with statues to produce
seemingly supernatural effects. This may be the explanation of
the tradition of the speaking statue in the temple of Ammon at
Thebes.

{illustration caption = DEVICE FOR CAUSING THE DOORS OF THE
TEMPLE TO OPEN WHEN THE FIRE ON THE ALTAR IS LIGHTED (Air heated
in the altar F drives water from the closed receptacle H through
the tube KL into the bucket M, which descends through gravity,
thus opening the doors. When the altar cools, the air contracts,
the water is sucked from the bucket, and the weight and pulley
close the doors.)}

{illustration caption = THE STEAM-ENGINE OF HERO (The steam
generated in the receptacle AB passes through the tube EF into
the globe, and escapes through the bent tubes H and K, causing
the globe to rotate on the axis LG.)}


The utilization of the properties of compressed air was not
confined, however, exclusively to mere toys, or to produce
miraculous effects. The same principle was applied to a practical
fire-engine, worked by levers and force-pumps; an apparatus, in
short, altogether similar to that still in use in rural
districts. A slightly different application of the motive power
of expanding air is furnished in a very curious toy called "the
dancing figures." In this, air heated in a retort like a
miniature altar is allowed to escape through the sides of two
pairs of revolving arms precisely like those of the ordinary
revolving fountain with which we are accustomed to water 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


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