IV. The musical cipher. The construction of this cipher, is similar to that of No II. The circle EFGH (fig. 3.) is to be divided into twenty-six equal parts, in each part there must be wrote one of the letters of the alphabet: and on the anterior circle ILMN, moveable round the center O, there is to be the same number of divisions: the circumference of the inner circle must be ruled in the manner of a music paper; and in each division there is to be placed a note, differing either in figure or position. Lastly, within the musical lines place the three keys, and on the outer circle, the figures that are commonly used to denote the time.
Then provide yourself with a ruled paper, and place one of the keys, as suppose that of ge re sol, against the time two-fourths at the beginning of the paper, which will inform your correspondent how to fix his circle. You then copy the notes that answer to the several letters of the words you intend to write, in the manner expressed at fig. 5.
A cypher of this sort may be made more difficult to discover by frequently changing the key, and that will not in the least embarrass the reader. You may likewise add the mark # or b to the note that begins a word, which will make it more easy to read, and at the same time give the music a more natural aspect. This cipher is preferable to that of No II. above, as it may be inclosed in a letter about common affairs, and pass unsuspected.