3. _Cipher correspondence_.--There are a great number of systems of cipher-writing; one of the simplest is described in the following letter of Marshal Soult to General Neil, dated September 26, 1806, in which Soult writes: "His Majesty advises me to agree upon a cipher with you, which you will have to use in future in your correspondence with the Chief of Staff of the Army. In the execution of this you can, in my opinion, best make use of a pamphlet (of which the title is given in the letter). The first figure will give the number of the page; the second figure the line, reckoned from above, not counting the heading. The third number will serve as indication of the required word or letter, and give its position in the line indicated by the second figure: if the figure means a whole word, you will underline it; if it only means a letter, you will not do so. Commas must be put between the indicating figures." The disadvantages of this system consisted in the slowness of construction and deciphering of the message, since it would almost always be necessary to indicate not whole words, but each letter separately, for which three figures are necessary each time. To avoid the latter disadvantage a dictionary can be employed for the execution of the cipher, whereby in most cases one could succeed in giving the whole word with two figures. A certain officer proposed to set up for this purpose a special military dictionary, similar to the marine signal-books existing in all countries. In this dictionary one could enter in alphabetical order all figures, letters, constantly occurring syllables, and complete military expressions. The officer in question was of the opinion that 3,000 ciphers would be quite enough to form a complete code. Thus all figures, letters, words, etc, would be numbered in the book in succession from beginning to end. In the preparation of reports one could then indicate by one cipher a figure, a letter, a syllable, and sometimes a whole word or even a whole phrase. The disadvantage of such a system consists in that its foundation is always the same, and that the printed dictionary could easily be secretly obtained from all neighbouring States. In order to obviate this, a few alterations were subsequently proposed. One alteration was to the effect that one could take a given number to serve as a key; in preparing reports the key-number would always be added to the number under which is designated the required word in the dictionary. It may be assumed that the word "regiment" is required to be coded, and this word is number 500 in the dictionary; the key-number is 25; so in the report the word is indicated by 525. It is of course obvious that one could subtract, instead of adding, the key-number.