Semaphore is a signalling system using hand-held flags or just arms. The position of the flags or arms indicates a letter or number, the letter is determined when the arms are held in a fixed position. Semaphore date from the early 1800s and mechanical semaphore stations were an early means of long distance communication.
Morse with dots and dashes was invented by Samuel Morse in the 1840s, and was widely used for telegraph and radio. Flags can also be used.
In Ransome's world
Semaphore is introduced in Winter Holiday where the Ds see the Swallows and Amazons practicing signalling to each other. Nancy draws stick figures for each letter in Dick's pocket book so that he can learn the positions. Later in Winter Holiday the mumps stricken Nancy uses drawings of stick figures with their arms in semaphore positions to write a secret message to the other explorers, as she also does in Secret Water (SW1
In Missee Lee, Captain Flint uses the same technique to send a secret message from his cage to the Swallows and Amazons being held by Miss Lee (ML14). But a Brownie troop found eight errors in the original drawing of the message (eg L for M), and Arthur Ransome arranged for a new block to be made (see The Life of Arthur Ransome page 386).
Missee Lee made a signal code for Three Island men using whistles and taught them English letters as she had learnt Morse as a Girl Guide in England. She tapped with her fingers on the table, giving the call up sign in Morse (ML13).
When Dick asks about signals without any flapping at all, Nancy says that it is Scarecrow signalling ... Semaphore. Tons better in some ways, but more people know Morse .... You can’t mix them up because one is dots and dashes and moving about all the time, and in the other each letter stays still while you make it, or at least you stay still while you’re making a letter (WH5).
In Pigeon Post the scouts signal the approach of Squashy Hat, signalling Like windmills, for hours and hours, but it wasn’t any good because you never looked at us. Nancy replies I suppose we can’t have done rather sheepishly for her (PP18). At night Roger flashes back with a torch Y .. O .. H .. O using Morse (PP22). Later the returning moles signal by Semaphore: Titty put her left hand behind her back, and held her right hand out and slanting downwards. Then she held her left hand slanting up and her right slanting down. She did it again, and then whirled both hands to show that he had finished a word ... Roger reads A .. L .. L. But Nancy replies in Morse going at an awful lick ... flicking a handkerchief PP25
In Secret Water Nancy sends a message with a picture of dancing savages and Commander Walker says We ought to have that young woman to teach signalling to naval cadets (SW1). Later Roger asks the Mastadon if the Eels use Morse ... and semaphore, with flags. He replies Savages don’t ... (though) its all right for explorers … not for savages. We’ve got our own way. He has said they use rockets (SW9). When Daisy says Do show us how you do your messages Nancy replies You have to know semaphore .... and draws a page of the semaphore ABC: Its the arms that matter. You can maker the legs do anything you like. Nancy also sees the Eels secret looking-glass message .... with the words all inside out (SW22).
In The Big Six young Bob waves wildly to the Death and Glories who are being towed by the Cachalot. Pete says Semaphore .... that stuff Dick and Dorothea show us at Easter .... (but) I've forgot it (BS8).