no knowledge that they devised a way of using the light-flashes for
any but the simplest prearranged messages. The mirrors of the Pharaohs
were probably used to flash light for signal purposes. We know that
the Persians applied them to signaling in time of war. It is reported
that flashes from the shields were used to convey news at the battle
of Marathon. These seem to be the forerunners of the heliograph. But
the heliograph using the dot-and-dash system of the Morse code can
be used to transmit any message whatever. The ancients had evolved
systems by which any word could be spelled, but they did not seem to
be able to apply them practically to their primitive heliographs.

An application of sound-signaling was worked out for Alexander
the Great, which was considered one of the scientific wonders of
antiquity. This was called a stentorophonic tube, and seems to have
been a sort of gigantic megaphone or speaking-trumpet. It is recorded
that it sent the voice for a dozen miles. A drawing of this strange
instrument is preserved in the Vatican.

Another queer signaling device, built and operated upon a novel
principle, was an even greater wonder among the early peoples. This
was known as a clepsydra. Fancy a tall glass tube with an opening at
the bottom in which a sort of faucet was fixed. At varying heights
sentences were inscribed about the tube. The tube, being filled with
water, with, a float at the top, all was ready for signaling any
of the messages inscribed on the tube to a station within sight and
similarly equipped. The other station could be located as far away
as a light could be seen. The station desiring to send a message to
another exhibited its light. When the receiving station showed its
light in answer, the tap was opened at the bottom of the tube in each
station. When the float dropped until it was opposite the sentence