The "word of hand" mentioned by Michael McGuffin was a message written by
Captain Smith and carried by quartermaster Olliver directly to Chief Engineer Bell. White Star Line provided special pads of paper for this purpose. Although he carried the folded paper, Olliver denied looking at the contents. With the deaths of both Smith and Bell all human knowledge was lost. Still, a bit of digging through the IMM/White Star Line rule book gives us a probable answer. At the time of that message it was known that Titanic had struck an iceberg and was taking on water. In that condition it would likely become necessary for the Chief Engineer to fill, pump, or transfer ballast water in the tanks of the double bottom. The rule book was specific about the Commander's role in work of this sort:
"25. Ballast Tanks.--The ballast tanks are never to be filled or pumped out at sea or in port except by the express instructions or permission in writing of the Commander..."
Shifting ballast was and remains a first-line measure of damage control. Even so, under company regulations Chief Bell required written permission from Captain Smith to do so. The Captain could not have used the ship's interior telephone to give oral permission. He had to write it down and send it on paper to Bell. "Word of hand" seems quaint to us today, but in 1912 many ships still lacked internal telephones. The importance of Rule 25 grows out of maintaining the stability of a damaged vessel. Shifting, discharging, or adding ballast water improperly could cause a damaged vessel to capsize.
Of course nobody knows for certain what the Captain wrote. More likely the message containd the required written permission to shift Titanic's ballast water. A less likely but still possible message would have been an instruction to send as many stokers and trimmers on deck as possible.
Social messages like "happy birthday" can probably be ruled out.
-- David G. Brown