The signal bridge was the center for visual signals to other ships steaming in company with the Slater. Signals originated with the Captain or Officer of the Deck and were relayed by signalmen to other ships. Lookouts were constantly on the alert for other ships that might be signaling the Slater. These signals were relayed in one of three ways.
In semaphore, a signalman holding two flags stood on a high platform and extended his arms to different positions that represented letters of the alphabet. This was the most rapid way of communication, well suited to plain language, but usable only in daylight. (see http://www.anbg.gov.au/flags/semaphore.html)
Another daylight signaling system was flag hoists, colored flags raised to the yardarm on the ship's halyards, or pulley lines. Each flag represented a letter, number, or tactical term. Code books were used to encipher messages because the Naval ships had special codes that were secret. The International Code of Signals was known to all nations and used when communicating with merchant ships. (see http://www.anbg.gov.au/flags/signal-meaning.html)
Flags were hung alphabetically in the large canvas covered "flagbags" on fingers, that allowed air to circulate and keep them from mildewing. When making up a message, the signalman would clip the required flags together to make a message, then hoist the signal. Surrounding ships would hoist the answering pennant when the message was read and understood.
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