"Here is a pencil for you to write your letters to me with.

It will last about ten years, and at the expiration of that time you can have another.

Write with it on any paper, and the marks will be like those of an ordinary drawing-pencil; but as fast as they are written they disappear.

Trouble not about this circumstance--write all you have to say, and when you have finished your letter your closely covered pages shall seem blank.

Therefore, were the eye of a stranger to look at them, nothing could be learned therefrom.