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.