[Illustration: THE SUPPOSED HERO ENGINE.]

Hero appears to the popular imagination as the greatest inventor of the
past. Every school boy knows him. Archimedes, the Greek, was the
greater, and a hundred and fifty years the earlier, and was the author
of the significance of the word "Eureka," as we use it now. But Hero was
the pioneer in steam. He made the first steam-engine, and is immortal
through a toy.

The first _practical_ device in which expansion was used seems to
have been for the exploiting of an ecclesiastical trick intended to
impress the populace. There is a saying by an antique wit that no two
priests or augurs could ever meet and look at each other without a
knowing wink of recognition. Hero is said to have been the author of
this contrivance also. The temple doors would open by themselves when
the fire burned on the altar, and would close again when that fire was
extinguished, and the worshippers would think it a miracle. It is
interesting because it contained the principle upon which was afterwards
attempted to be made the first working low-pressure or atmospheric
steam-engine. Yet it was not steam, but air, that was used. A hollow
altar containing air was heated by the fire being kindled upon it. The
air expanded and passed through a pipe into a vessel below containing
water. It pressed the water out through another pipe into a bucket
which, being thereby made heavier, pulled open the temple doors. When
the fire went out again there was a partial vacuum in the vessel that
had held the water at first, and the water was sucked back through the
pipe out of the bucket. That became lighter again and allowed the doors
to close with a counter-weight. All that was then necessary to convince
the populace of the genuineness of the seeming miracle was to keep them
from understanding it. The machinery was under the floor. There have