This section is from the book "A Library Of Wonders And Curiosities Found In Nature And Art, Science And Literature", by I. Platt. Also available from Amazon: A library of wonders and curiosities.
Take two pieces of pasteboard, or stiff paper, out of which cut a number of oblong figures, at different distances from each other, as in the following example. Keep one of these pieces for yourself, and give one to your correspondent; and when you are desirous of sending him any secret intelligence, lay the pasteboard upon a sheet of paper of the same size, and in the spaces which are cut out, write what you would have him only to understand, and fill up the intermediate parts of the paper with something which makes with these words a different sense. Then, when your correspondent receives this letter, by applying it to his pasteboard, he will be able to comprehend your meaning.
1 shall be | much obliged to you, as reading alone I engages my attention | at present, if you will send me any 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.