III. The corresponding spaces. Take two pieces of pasteboard or stiff paper, through which you must cut long squares, at different distances, as you will see in the following example. One of these pieces you keep yourself, and the other you give to your correspondent. When you would send him any secret intelligence, you lay the pasteboard upon a paper of the same size, and in the spaces cut out, you write what you would have understood by him only, and then fill up the intermediate spaces with somewhat that makes with those words a different sense.
I shall be much obliged to you, as reading alone engages my attention at present, if you will lend me any one of the eight volumes of the Spectator. I hope you will excuse this freedom, but for a winter's evening I don't know a better entertainment. If I fail to return it soon, never trust me for the time to come.
A paper of this sort may be placed four different ways, either by putting the bottom at the top, or by turning it over, and by those means the superfluous words may be the more easily adapted to the sense of the others.
This is a very eligible cipher, as it is free from suspicion, but it will do only for short messages: for if the spaces be frequent, it will be very difficult to make the concealed and obvious meanings agree together: and if the sense be not clear, the writing will be liable to suspicion.