anywhere before this work of Hero of a clear
understanding that the expansive properties of the air when compressed,
or when heated, may be made available as a motor power. Hero, however,
has the clearest notions on the subject and puts them to the practical
test of experiment. Thus he constructs numerous mechanisms in which the
expansive power of air under pressure is made to do work, and others in
which the same end is 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 throug