General Clinton, concerned about what General Howe was planning and doing, made use of a mask to write a secret message in a letter to General Burgoyne. Before writing the letter, Clinton had placed an hourglass-shaped mask on a piece of paper and then had formed the secret message within that shape.

The unmasked letter had enough false information in it to fool any American who happened to see it. But when Burgoyne viewed the letter with the mask, he read Clinton's view of the real situation: Howe has made a bad move; I don't have enough men to do anything about it.

The author’s main purpose in this excerpt is to

describe a form of code writing using an example.
explain the mistakes of General Howe during the war.
entertain the reader with a humorous story of miscommunication.
argue that using a mask is the best method for sending secret messages.

The author’s main purpose in this excerpt is to describe a form of code writing using an example.

In the text in question, the author describes how General Clinton inserted a secret message in a text apparently unrelated to the information that he wanted to convey. That is, through a real situation, he exemplifies how a secret message could be introduced that changed the course of history.

one thousand and one nights, also known as arabian nights is a collection of stories whose basic structure is that of embedded narrative. this technique refers to a main story which works as the common frame - or framing narrative - for the other stories, and then this series of stories which are connected to the main one. this technique can also be referred to as a story within a story. the stories stem from the original tale, some begin and end on their own, some are connected to the previous or following one, by means of questions that are answered in the subsequent tale, or characters which reappear.