Fog-horns, whistles, etc., may be similarly employed during fogs or
amid thick smoke. A short blast represents a dot, and a long one a
dash.

The heliograph, which established communication by means of short and
long light-flashes, is another important means of signaling to which
the Morse code has been applied. This instrument catches the rays of
the sun upon a mirror, and thence casts them to a distant receiving
station. A small key which throws the mirror out of alignment serves
to obscure the flashes for a space at the will of the sender, and so
produces short or long flashes.

The British army has made wide use of the heliograph in India and
Africa. During the British-Boer War It formed the sole means of
communication between besieged garrisons and the relief forces.
Where no mountain ranges intervene and a bright sun is available,
heliographic messages may be read at a distance of one hundred and
fifty miles.

While the British navy used flashing lights for night signals, the
United States and most other navies adopted a system of fixed colored
lights. The system in use in the United States Navy is known as the
Ardois system. In this system the messages are sent by four lights,
usually electric, which are suspended from a mast or yard-arm. The
lights are manipulated by a keyboard situated at a convenient point on
the deck. A red lamp is flashed to indicate a dot in the Morse code,
while a white lamp indicates a dash. The Ardois system is also used by
the Army. The perfection of wireless telegraphy has caused the Ardois
and other signal systems depending upon sight or sound to be discarded
in all but exceptional cases. The wig-wag and similar systems will