The nearest approach to a _corresponding seal_ that occurs prior to the sixteenth century, is that described in a decree of Cardinal Otto, who was papal legate in 1237, by which the bishops were to bear on their seals their title, office, dignity, and even their proper names. About this period _mottos_ were likewise generally introduced, but none of those before the publication of the noble author's work were at all adapted for secret correspondence; or, in fact, had they any mode of combining moveable characters in the matrix for the purpose of varying the impression. The principle upon which those described by the Marquis must have been formed is simply this: a frame similar to those in which seals are generally mounted having been first prepared, a number of moveable circles may be made to slide within each other on one common centre. If three are employed, they should be engraved with the numerals, the alphabet, and, if intended for secret writing, the third circle may be furnished with any arbitrary signs that may suggest themselves. These, by means of a key, of which both the corresponding parties must possess a duplicate, may be combined to form the day of the week, month, year, &c. It would be found very useful in preventing and detecting the mistakes which so frequently occur in the delivery of letters, if the seals in common use were provided with at least two of these revolving circles, with the day of the month and hour of the day engraved on their face, parallel to the stone. A particular part of the arms or cipher being used as an index hand, it would then show the precise hour the letter was sent, without the trouble of dating, &c.