"The instrument in question consists of a plate, usually made of metal, pierced by a number of holes of different sizes and irregularly spaced. When the writer sets out to prepare his message he lays the grid on the paper, and marks in the letters making up the words of his despatch through the apertures. Then the screen is removed, and the blank spaces are filled up with writing which has nothing to do with the real subject matter, the process being repeated until the entire message has been coded. The recipient is provided with a precisely similar grid. By applying it to the communication he is then able to read, through the holes, the text of the secret message. The ancient Romans used a variation of this method, somewhat as follows. A long strip of paper was wound spirally about a cylinder or cone; the writing was then done parallel with the axis of the metal form. When unrolled, the communication seemed to be made up of arbitrary signs really parts of letters which were entirely unintelligible. The recipient, however, by rewinding the strip on a precisely similar form, would be able to read the message. "Of course we may rule out the mechanical device. In this case we have a long communication of several hundred words, and the grille would be impracticable--too wasteful of space." "That disposes of No. 2," said Betty hopefully. "What next?" "In class 3 the coded message consists of numbers, or even of pure symbols--stars and daggers or what not. The latter variation is generally pure substitution, and may be called kindergarten cryptology. No one but a rank amateur would employ such a system. "In the numeral code each correspondent is supplied with a dictionary, the same edition of course. Each word of the original message is represented by a group of five numbers, two designating the location of the required word on the page, and the remaining three denoting the number of the page itself. The process, both of coding and of uncoding, is very laborious, and hardly pays for the trouble involved. Another way to use the two dictionaries is to interpret the words of the code message by substituting other words removed a certain definite distance up or down the column. Suppose it is agreed that 'fifteen down' shall be the key, and that the despatch, as received, reads: _Bull Collier_. The recipient takes his copy of the dictionary, looks up the word _Bull_, and counts down fifteen, getting the word _Buy_. Similarly, _Collier_ gives him _Copper_, and the decoded message will mean: 'Buy copper.' Finally, we may use a predetermined series of numbers as a key formula. We then divide the message to be coded into the same number of letter groups, and work out an intricate transposition, reversing the process in order to decode." "Rather makes your head ache," remarked Betty plaintively. "Besides, this cypher doesn't use numbers at all."