Ciphers, however, have almost always been resorted to when secrecy was desired. This sounds like a contradiction. But people who are not experts use them because they think they are more secret, since they look so. And experts use them when they are concerned only with _temporary_ secrecy. They use them, then, because cipher messages can be written and translated (by one’s correspondent) without any equipment, like a code book, and much more rapidly than code. Thus, if a general in the field wishes to send a message ordering a colonel to advance in two hours, he sends it in cipher, because it would take the enemy more than two hours to decipher the message even if he intercepted it immediately, and because after the two hours have elapsed the information in the message would be of no value to him. A _cipher_ is the substitution of some symbol for a letter of the alphabet. The substituted symbol may be another letter--as writing _e_ when you mean _a_. Or it may be a figure--as using 42 when you mean _m_. Or it may be an arbitrary sign--as * to mean _c_. In cipher, then, every word is spelled out, but the word _Washington_ might be spelled x=‖½?!^:°B if you had agreed that w = x a = = s = ‖ h = ½ i = ? n = ! g = ^ t = : o = ° n = B That is called a _substitution_ cipher, because some other letter or symbol is arbitrarily substituted for every letter. But another kind is called a _transposition_ cipher, because in this the letters of the alphabet are simply transposed by agreement--the simplest and most obvious example being to reverse the alphabet, so that _z_ stands for _a_ and _y_ for _b_, etc. Such a transposition cipher would read: Alphabet of plain text abcdefghijklmnopqrstuvwxyz Alphabet of cipher zyxwvutsrqponmlkjihgfedcba and _Washington_ would be spelled _dzhsrmtglm_. [Illustration: A letter from John Devoy, an Irish-American, exposing his hand in a plot with the Germans to foment revolution in Ireland] Perhaps the cleverest transposition cipher ever devised--it is so good that the British Army uses it in the field and, moreover, has published text books about it--is the very simple “Playfair” cipher. First a square is drawn, divided into fifths each way. This arrangement gives twenty-five spaces, to contain the letters of the alphabet--_I_ and _J_ being put in one square because there would never be any plain sentence in which it would not be quite obvious which one of them is needed to complete a word of which the other letters are known. [Illustration] Next a “key word” is chosen--and herein lie the cleverness and the simplicity of this cipher, because every time the key word is changed, the whole pattern of the alphabet is changed. Suppose the key word is _Gardenia_. It is now spelled out in the squares: The second A is left out, as there must not, of course, be duplicates on the keyboard. Now the rest of the alphabet is written into the squares in their regular sequence: [Illustration]