tric waves carrying the messages may be sent out at widely varying
lengths. Marconi found that it was possible to adjust a receiving
station so that it would receive only waves of a certain length.
Thus stations which desired to communicate could select a certain
wave-length, and they could send and receive messages without
interfering with others using different wave-lengths, or without the
receiving station being confused by messages coming in from
other stations using different wave-lengths. You know that when a
tuning-fork is set in vibration another of the same pitch near it will
vibrate with it, but others of different pitch will not be affected.
The operation of wireless stations in tune with each other is similar.

[Illustration: A REMARKABLE PHOTOGRAPH TAKEN OUTSIDE OF THE CLIFDEN
STATION WHILE MESSAGES WERE BEING SENT ACROSS TO CAPE RACE

The camera was exposed for two hours, and the white bars show the
sparks leaving the wires for their journey through the air for
seventeen hundred miles.]

[Illustration: MARCONI STATION AT CLIFDEN, IRELAND

These dynamos send a message straight across the ocean.]

An example of the value of tuning is afforded by the manner in which
press reports are sent from the great Marconi station at Poldhu. Each
night at a certain hour this station sends out news reports of the
events of the day, using a certain set wave-length. Each ship on the
Atlantic and every land station within range which is to receive the
reports at that hour adjusts its receiving set to receive waves of
that length. In this way they hear nothing but the Poldhu news reports
which they desire to receive, and are not troubled by messages from
other stations within range.

Secrecy is also attained by the use of tuning. It is possible that
another station may discover the wave-length being used for a secret
message and "listen in," but there are so many possible wave-lengths
that this is difficult. Secrecy may also be secured by the use of code
messages.

Many of the advantages of tuning were lost by the international
agreement which provided that but two wave-lengths should be used for
commercial work. This, however, enables ships to get in touch with
other ships in time of need. With his telephone receivers the operator
can hear the passage of the waves as they are brought to him by his
aerial and the dots and dashes sound as buzzes of greater or less
length. Out of the confusion of currents passing throug