been thousands of miracles since then performed by natural agencies, and
there have passed many ages since Hero's machine during which not to
understand a thing was to believe it to be supernatural.

[Illustration: THE TEMPLE-DOOR TRICK.]

From the time of Hero until the seventeenth century there is no record
of any attempt being made to utilize steam-pressure for a practical
purpose. The fact seems strange only because steam-power is so prominent
a fact with ourselves. The ages that intervened were, as a whole, times
of the densest superstition. The human mind was active, but it was
entirely occupied with miracle and semi-miracle; in astrology, magic and
alchemy; in trying to find the key to the supernatural. Every thinker,
every educated man, every man who knew more than the rest, was bent upon
finding this key for himself, so that he might use it for his own
advantage. During all those ages there was no idea of the natural
sciences. The key they lacked, and never found, that would have opened
all, is the fact that in the realm of science and experiment there is no
supernatural, and only eternal law; that cause produces its effect
invariably. Even Kepler, the discoverer of the three great laws that
stand as the foundation of the Copernican system of the universe, was in
his investigations under the influence of astrological and cabalistic
superstitions. [Footnote: Kepler, a German, lived between 1571 and 1630.
His life was full of vicissitudes, in the midst of which he performed an
astonishing Even the science of amount of intellectual labor, with
lasting results. He was the personal friend of Galileo and Tycho Brahe,
and his life may be said to have been spent in finding the abstract
intelligible reason for the actual disposition of the solar system, in
which physical cause should take the place of arbitrary hypothesis. He
did this.] medicine was, during those ages, a magical art, and the idea