Science Talk

Invisible Ink and More: The Science of Spying in the Revolutionary War

John Nagy, author of Invisible Ink: Spycraft of the American Revolution, discusses the codes, ciphers, chemistry and psychology of spying in the American Revolution, in a talk recorded by podcast host Steve Mirsky at the historic Fraunces Tavern in New York City. Plus, we'll test your knowledge of some recent science in the news. Web sites related to this episode include

John Nagy, author of Invisible Ink: Spycraft of the American Revolution, discusses the codes, ciphers, chemistry and psychology of spying in the American Revolution, in a talk recorded by podcast host Steve Mirsky (pictured) at the historic Fraunces Tavern in New York City. Plus, we'll test your knowledge of some recent science in the news. Web sites related to this episode include

Podcast Transcription

Welcome to Science Talk the more or less weekly podcast of Scientific American posted on April 20th, 2010. I'm Steve Mirsky. This week on the podcast:

Voice: You would write with one chemical and then have to apply a second chemical which then would make the message appear.

Steve:          That's John Nagy, author of the new book Invisible Ink: Spycraft of the American Revolution. He spoke last month at the famous Revolutionary War hangout at the Fraunces Tavern in Lower Manhattan and we'll hear an edited version of his talk plus we'll test your knowledge of some recent science in the news. John Nagy is a founding father himself, founder of the American Revolution Round Table of Philadelphia. He has written numerous books about the war and since spying involves some technology and science I thought it [would] be fun to hear about it in our venue. Nagy begins with a specific case about a captured document written in ciphers and symbols then he talks in more general terms about codes and ciphers, the chemistry employed to send messages in invisible ink, and some of the psychology involved in fooling your enemy during war.

Nagy:          The American Revolution begins up in Boston after the battles of Concord and Lexington; the American army surrounds the British who are under a virtual siege. The British have no one on staff who really is an expert in ciphers and codes and they have no system actually in place to send coded messages to their operations in Canada or even here in New York. American General George Washington takes command of the troops at Cambridge, and again on the American side there was nobody in charge of cryptology. Now what we do have a situation where merchants are not totally ignorant of codes and ciphers. When they deal with their agents in London, they had a tendency to use a very cryptic cipher system to tell their agents what price to sell at and so on, and so if their messages [were] intercepted, their mail, that's, in the 18th century you had no privacy in the mails. You put in the mail, it was public information and you could be pretty much be assured that somebody was going to read [it] along the way. Now, one of the situations, you have Dr. Benjamin Church who is the Surgeon General of the Continental Army. The only problem is, Dr. Church is a British spy. He has been on the British payroll since at least 1772, and so while he is running the American hospitals, he is sending in information to British generals in Boston. He wasn't able to send his mail directly in[to]  Boston, [it] had to go down to Newport. So what he did is he sent his mistress who was a prostitute in Boston down to Newport to one of her former clients to get the message to the British captain of the vessel. She delivers the message to him, he promises to take it onboard ship, sees [a] message that's all in ciphers and symbols and so he decides not to do it, and takes the document goes to the Governor of Rhode Island who then sends him up to American General Greene who, being a merchant, knows that it's in a coded message. The American puts two teams to decipher the message and they both, using frequency analysis—in other words, which letter[s] appear the most common you then back it out—the most common letter in English is the letter E, so you start with figuring out what appears the most times and that's the letter E, and then you work down from there. They come up with the exact translation, both of them and that's the code that they wrote down that he had used. Now, the difference between a cipher and a code; everybody has a tendency to misuse the terms. A cipher is [when] a character or letter represents another character or letter, [such] as A is equal to 1 or ‚àÜ (delta), if you're using a symbol, is equal to letter K or something similar. [A] code [is when] a character represents an entire word, and usually you're going to need a code book to identify what the codes are because, you're going to be using so many, generally, you're not going to be able to keep track of them in your head. One of the things we need to realize is the alphabet in the 18th century is not our alphabet; it is not identical. In the 18th century the letters I and J are the exact same letter; there is no difference between the two. The letters U and V are the same letter. So if I give you the letters IVLY today it would look strange, and in [the] 18th century they would automatically make the change and adjust it and immediately know that what I meant was July. And shorthand writing is another form of codes and ciphers, and the earliest book that I [have] found on shorthand dates to 1586, so it was well known. Joseph Stansbury's cipher is very simple it's A [is] equal to Z, B is equal A;  [it's a]one-letter shift. Joseph Stansbury is the person in Philadelphia who received the messages from Benedict Arnold, wrote them into cipher and codes and they were brought across New Jersey by two different methods, brought here to New York, they were given to the Reverend Jonathan Odell who then decoded the messages and then turned them into British Headquarters right here at One Broadway. They also used book codes. It is very important that two people sending an[d] coding and decoding the messages are using the same edition, otherwise you'll find out you put down the word "balloon" and they are reading it as "balogna". The first number is usually the page, the second number is the line, and the third number is the word. So 45-9-8 would mean to go to page 45, go down to line 9 and go over to word 8 and that would be your word; obviously you've got to have the same book.  Dictionary codes—very popular used by all sides. A dictionary, the most common one used was Entick's New Spelling Dictionary. It has a list of words, alphabetical order, two columns and you've [got] just about every word that you could possibly want. What they would do is, they'[d] put a dot over the number to indicate whether it was the first column or the second column. They had a tendency when it was the first column to just ignore the dot. They would also do things like add 20 to the page or 7 or what have you. There is also an instance where they'd page the book backwards to try and keep it [hidden]. Pigpen cipher—we would call it like a tic-tac-toe board—they call that a pigpen. You would place the letters in each one of the quadrants, and the two sides, the sender and the receiver had to agree which letters are going where, okay. So as long as you understood the positioning of the letters in the different slots, you're able to transcribe a message. So if you see in the first upper-left quadrant the letters ABC, if you drew just the upper left quadrant without putting anything in it, it would indicate the letter A. If you drew the quadrant and put one dot, it would indicate the letter B. The upper left quadrant with two dots would be the letter C and so on. Now all sides [wound up using]—the Americans, the British, the French, there's even a Hessian diary that's partially written in a pigpen cipher. The pigpen cipher by the way is used up through the American Civil War. The next thing is called the Cardan grille; we would call [it] more of a mask; the sections where it squiggles, those would be cutouts in the page; so how you would use this is you would put the mask on the paper, put your secret message in the cut out holes, take the mask off, and then write the rest of the message around it. They also use mask code with the hidden center. We would call it an hourglass mask; in the 18th century, they called it a dumbbell mask. Here's a letter written with a hidden center. Here's the actual hidden message, okay, and what the message says is that Sir William Howe has gone to the Chesapeake. It was sent by General Henry Clinton up to General Burgoyne telling him that Howe was not coming up to Hudson to help him; he has gone to attack Philadelphia. Invisible ink: Anything that's mildly acidic will work, whether it be milk, lemon juice, grapefruit juice, or during World War II they actually used urine; anything that'll weaken the fibers of the paper. What happens is that the fibers once weakened, when you bring it next to heat, the weakened fibers will darken first. The only thing you have to remember is just to take it away from the heat or the whole document goes brown. But anything that'll weaken the fibers will work in doing it. I am sure some of you, when you [were] younger, tried writing with milk or lemon juice and you put it next to a lightbulb or what have you and it does work. They also had in the 18th century three different methods that tells you of chemical reactions, where you would write with one chemical and then have to apply a second chemical, which then would make the message appear. Applying heat to that chemical would have no effect and would not make it visible. Washington's deceptions—now the one thing that I do have to say about Washington for somebody who never told a lie, he's certainly [stretched] the truth an awful lot. He also did a thing called a troop multiplication, at Morristown after the battles of Trenton and Princeton. The American army goes up and encamps in Morris Town and while there, normally you would put most of your troops, cluster them in houses try and keep as many together as you could; Washington [went] the exact opposite way. He would put one or two soldiers in a house so that the area that his troops had to occupy was much greater than the amount of troops he had. So the spies are reporting back to British headquarters here in New York that Washington's army extends over such a great area, so they're reporting back that he has three to four times more soldiers than he actually has. To me one of the best one[s] occurs by General Putnam at Princeton, New Jersey, again after the battles of Trenton and Princeton. Putnam is there with 50 men, you have the British army in New Brunswick, and the rest of the American army is at Morristown. So the bulk of the British army could come down and squash him in instant if they wanted to. There is a British officer who was wounded very badly at the battle of Princeton. He was not expected to live; asked for permission to have a British officer come out of New Brunswick, take his last will and testament. Putnam agreed but insisted [it] had to done at night. What Putnam did is in all the empty houses he put candles to make them look like they're occupied. He then had his 50 soldiers march past the house where the last will and testament was being taken sometimes one or two a[t a] time, sometimes six at a time, sometimes a dozen and sometimes all 50. When the British officer goes back to [New] Brunswick he reports that Putnam is at Princeton with 4,000 soldiers. [Fake reports for spies—]Washington liked to make up fake reports and send them into British headquarters here in New York. He did so well that after the Battle of Brandywine in Pennsylvania the British army captured an American report but was absolutely convinced it was fake, because they were getting so many fake reports that they refuse to believe it. Now another one is, what Washington's deception was that he needed to steal a march in 1781 to move the American army and the French army from north Jersey and basically Westchester and Putnam counties past the British across the Delaware River and eventually down to Yorktown to hook up with Lafayette down there and attack Cornwallis. So [what] Washington uses is called the deception battle plan, and the deception battle plan is also used in World War II for landing at Normandy, it's also used in Desert Storm by General Schwartzkopf to do an end-around on the Republican Guard. They use[d] Washington's plan. First thing you need is a clear objective, and the clear objective was [he] needed to steal a march across New Jersey south without being attacked by the British, who were located in New York and Staten Island. You have to know the enemy's assumptions. Washington originally was planning to attack New York with the French; the British in New York believed that, and so what you have to do is that once you know what the enemy believes, you then have to reinforce their belief that that is what you're going to do. So the next thing would be method selection. The options, one of the things that he did is since they were using the French army and the French army, a bulk of their diet is bread; he had brick ovens being built and they were built at Chatham, New Jersey. He issued orders [for] the preparation of building ovens at the Highlands right near Sandy Hook, and he also was issuing orders for supplies to be brought to the French ovens at the Highlands, once the French troops arrive but since the French troops were never going there he could right as many orders [as] he wants because none of these contracts would ever take place. The other thing that he did is he had troops assigned to go to [Perth Amboy] and they went down by the [water's] edge, he wanted the British to observe them collecting bricks. As the ovens [in Chatham] were being built, Washington was able to steal a march. But what he does is he has 30 boats put on carriages that are brought to Springfield, New Jersey, for the anticipated attack on Staten Island and there would be no other reason to bring the boats there because they weren't going to Virginia. So he has gone and convinced the British that there's going to be an attack on Staten Island. The eventual result is the fact that his army and the French army were able to move across New Jersey without being attacked, and as most of you are aware they make it down to Virginia and Cornwallis surrenders at Yorktown. That gives you pretty much of a run through on the spycraft that was used during the American Revolution. There's many more codes and ciphers that are in the book. At this point I would like to [open] up to some questions and hopefully I have some answers, and we have one back there.

Speaker 1: Are any of the ciphers still used today by the government, and if not what other sorts of technology have, like, taken over you know hiding secret[s] and things like that.

Nagy:          Computers.

Speaker 1:          Computers.

Nagy:          Actually you can use your computer and encode messages far superior than what were used in the American Revolution. Computers today design the codes, and where I talked about a one letter shift that's one transaction, they'll then take the one letter shift which, actually they would use different combinations, and then make multiple shifts and if you get into reading about the Enigma machine and the Japanese purple codes you'll see the thing goes to multiple layers of transcriptions and the only way to decode this stuff is using a computer. You really need to be a mathematician today to be doing codes and ciphers at that level. There's also one other thing that I did not mention that we actually used here in New York, that was used at One Broadway, was the thing called the Language of Flowers. There was a young girl who was at Putnam's headquarters up in the top of One Broadway was a [widow's] watch and she would go up there and observe the American troops and then come down and draw paintings, paintings of flowers. The last real book written on this subject is about 1835, but by using flowers you could give a description as to how many troops were there, where they were going to be just amassed on the border, whether they were going to attack. There is one individual who claims that in a boutique of flowers he could put the equivalent of eight pages of text. By the way, you hang spies, you normally don't shoot them. Only a gentleman is shot in the 18th century; a spy is not a gentleman. So a spy is not shot, which is why Andre was hung. He asked to be shot, which would indicate he was a gentleman, and if he was a gentleman he could not have been a spy, and they shouldn't be execute him so they had to hang him, and they could not shoot him. It's 18th century etiquette.

Steve:          Do you have any specifics on the chemistry that was used in some of the hidden messages that's more sophisticated than the vinegar, lemon juice and add heat, and would they actually then have something else on the page that they could wash off?

Nagy:          One of the formulas used is [UNCLEAR] which is not chemically available today; the Americans, their original supply came from Sir James Jay in London [who] claims [to] have invented [it], although it was, the formula, was known by the British for 100 years before, so he probably may have taken a liberal license [in] claiming that he invented it or he may have tweaked it a bit. The Americans later on set up, [James Jay's brother], John Jay, sets up a laboratory around Peekskill, New York, to manufacture the agent and reagent that are used, so once the laboratory is actually established the Americans have a plentiful supply. The [okall] at the time was available to any medical supply, so any place [that] there was a surgery, they would have had it.

Steve:          The Jay Chemical compound is also known as sympathetic stain or white ink and you would write with it between the lines of another document or in the margins or in the backs of pages that included other writing. John Jay was one of the authors of a series of documents not written in invisible ink, the Federalists Papers, he then became the first Chief Justice of the United States. Now you just heard my edit of John Nagy's talk, which included just the parts that I thought were most relevant to the scientifically interested listener. Nagy also spent considerable time discussing things like dead drops which is how Morgan Freeman knows how to find Tim Robbins at the end of Shawshank by the way, and messages hidden in buttons and balls of yarn, things like that. C-span was also there when Nagy spoke and you can access the full 52-minute talk in their archives. I created a shortened url for you,

Now it's time to play TOTALL……. Y BOGUS. Here are four science stories, but only three are true. See if you know if story is TOTALL……. Y BOGUS.

Story number 1: The legislature of Wisconsin has declared the official state colors to be black and white in homage to dairy cattle.

Story number 2: Physicist [Roy] Glauber had his Nobel Prize stolen from his home.

Story number 3: Insurance companies have almost 2 billion dollars invested in fast-food companies.

And story number 4: Putting up a sign in a building [extolling] virtues of taking the stairs and an arrow showing where the staircase is increased stair use by 34 percent.

Times up.

Story number 4 is true. Stair use went up 34 percent after the appearance of the sign showing where the stairs where and noting that climbing the stairs burned lots more calories than taking the elevator. The study appeared in the Journal of Physical Activity and Health.

Story number 3 is true. Health and life insurance companies have 1.88 billion dollars invested in the top five publicly traded fast food chains. [That's] according to a study in the American Journal of Public Health and insurance companies have more than twice that much money invested in… tobacco companies. For more see Katherine Harmon's April 15th article on our Web site called "Health Insurers Make Big Bucks from Big Macs".

And story number 2 is true. Glauber's Nobel Prize was stolen and has not yet been found, although the suspect is in custody, because while he was in Glauber's house the alleged thief left behind a supermarket receipt that included his food stamp number, which the cops used to track him down. As the local police chief said, clearly the victim and the alleged perpetrator in this case are on opposite ends of the IQ spectrum.

All of which means that story number 1, about Wisconsin making black and white the official state colors to honor Dairy cows is totally TOTALL……. Y BOGUS. But what is true is that the Wisconsin legislature has announced the designation of an official state microbe. And the winner is Lactococcus lactis, because you can make cedar cheese without it. The representative who came up with the bill noted that "this microbe is really a very hard worker." Lactococcus also helps in the production of Colby and Monterey Jack.

(music plays)

Well that's it for this episode. Get your science news at where you can read Larry Greenemeier's Observations blog item called "Who needs high-speed broadband?". And follow us on Twitter where you'll get a tweet every time a new article hits the Web site. Our Twitter name is @SciAm. For Science Talk the podcast of Scientific American I'm Steve Mirsky. Thanks for clicking on us.

Share this Article:


You must sign in or register as a member to submit a comment.

Give a Gift &
Get a Gift - Free!

Give a 1 year subscription
as low as $14.99

Subscribe Now! >


Email this Article