Mountflorence to the Secretary of State, Paris, 18 March 1801

01 The letter

James Cole Mountflorence had been in Paris since the 1790s as chancellor of the American consulate and go-between for the American envoys. On 18 March 1801 he wrote to the Secretary of State, still John Marshall as far as he knew. The letter reached the department on 12 June under Madison and was docketed by Jacob Wagner. Its first paragraph is in code. The rest of the page is clear: the Council of Prizes would try no American ships until the convention of 1800 was ratified, and six American ships were in quarantine at Malaga.

The Madison Papers abstract (Secretary of State series, vol. 1, p. 27) says “Partly in code; key not found. Coded section apparently discusses Louisiana cession.” Founders Online added a full transcription in 2015 with the numbers left as they stand. On 10 April 1801 David Humphreys, the minister at Madrid, told Madison that he could not decode a message from Mountflorence with the key the department had sent him. So the code was not the one Humphreys held.

02 The decrypted text

Code groups are given with the text; spelled names in capitals; [?] marks a word fixed only by its place in the alphabet and the sense.

I have it from very good Authority, 357.723.640.544.529.684 DUPONT,
that 679 Spain has ceded 719 to, 434 France, not 432 Florida, 249 but[?], 688 the[?], 521 Mississippi[?],
with all its Dependencies.
This was done by some of the 666 secret Articles 228 at[?],
475.723.529.377.723.460.475.475.377 LUNEVILLE,
as Compensation for 684.723.664.251.207.529.756 TUSCANY,
given 250 by, 434 France, 719 to, 640.207.660.500.207 PARMA.

I have it from very good Authority, Dupont, that Spain has ceded to France, not Florida, but the Mississippi, with all its Dependencies. This was done by some of the secret Articles at Lunéville, as Compensation for Tuscany, given by France to Parma.

The report is accurate in substance. By the secret treaty of San Ildefonso (1 October 1800) Spain had agreed to return Louisiana to France in exchange for a kingdom in Italy for the Duke of Parma’s son. The treaty of Lunéville (9 February 1801) gave Tuscany to the house of Parma, and the convention of Aranjuez settled the exchange on 21 March 1801, three days after this letter. Spain kept the Floridas. Mountflorence, or his informant, places the cession in secret articles at Lunéville. “The Mississippi with all its Dependencies” is how he names Louisiana.

03 The code

All 25 groups that occur, in numerical order. S = fixed by a spelled name and checked between names; I = inferred from the slot between known values and the sense.

04 Method

The punctuation gives the structure away. Groups joined by full stops form four runs of 6, 9, 7 and 5 groups, and each run is one word spelled letter by letter. The groups between commas are whole words.

1. The nine-group run after “Articles” repeats in the pattern ABCDBEAAD. LUNEVILLE fits it once U and V are one letter, and the treaty of Lunéville, five weeks old, was the treaty that disposed of Tuscany.
2. The last run, ABCDB, is PARMA. The run after “Compensation for” shares A and N with these and reads TUSCANY. The name after “Authority” then reads D-U-P-O-N-T from letters already known, plus two new ones.
3. The fifteen letter values rise with the alphabet (A 207 … Y 756), so the code is one-part. Each word group must then lie between the letters around it: 679 between S 664 and T 684 is Spain (“679 has ceded”), 434 between E and I is France, 666 between S and Spain is secret, 432 just before France is Florida. 521 between M and N is Mississippi, 688 and 719 between T and U are the and to, and 228, 249 and 250 between A and C are at, but and by.
4. The decipherment was first made from the Founders text, where 250 had to be both C (in TUSCANY) and “by”. That predicted a slip: the C ought to be 251. The National Archives microfilm, located at frame 0097 of roll T1-1, shows 251. The other 38 groups agree with Founders.

No plaintext of the letter and no existing key were used. The probable words came from the history of the 1800–1801 treaties. The statistical check (docs/_validate.py) returns “too short” at 39 groups. What holds the reading is the alphabetical order of all 25 values, the cross-checks between the four names and the confirmed prediction.

05 What remains uncertain

• All 39 groups have a value. The 27 letter tokens are fixed by spelling (grade S here, cross-checked). The 12 word tokens are grade I.
• Spain, France, secret and Florida are narrow: the slot and the sense agree. “To” is fixed by its two uses (“ceded to France”, “to Parma”); “by” is fixed by its slot between but and C.
• Weaker: 521 Mississippi (any word from Mi- to My- fits the slot; the sense asks for Louisiana by another name), 249 but, 228 at and 688 the, common words chosen from the slot and the grammar.
• “Dupont” is not identified. Pierre-Samuel du Pont de Nemours was in America at the time.

Full texts

The transcription and the decryption of every document on this page, so that the reading can be used and checked. Short files are shown here; all are in the repository.

Decipherment of the two coded sentences

Mountflorence reports, on Dupont's authority, that Spain has ceded to France not Florida but the Mississippi (Louisiana) with its dependencies, by secret articles at Luneville, in compensation for Tuscany given by France to Parma.

# Mountflorence to Madison (addressed to Marshall), Paris, 18 March 1801
# NARA RG 59, Despatches from US Consuls in Paris (T1), roll 1, frame 0097 (catalog NAID 211302912;
# https://catalog.archives.gov/medialz/dc-metro/rg-059/302031/T1/T1-01/T1-01-0097.jpg), 1 p.
# Transcribed from the microfilm frame, 26 Oct 2026, and checked against Founders Online Madison/02-91-02-0009.
# One difference: the fourth group of the name after "Compensation for" is 251 on the frame; Founders prints 250.
# One token per code group, in text order. Clear words in [brackets]; separators as written ('.' inside a run,
# ',' between words). Manuscript lines: l.1 "I have it ... 357.723.640.544." / l.2 "529.684, ... with all its" /
# l.3 "Dependencies ... 475.723.529." / l.4 "377.723.460 ... 756," / l.5 "given ... 207."
[I have it from very good Authority,] 357. 723. 640. 544. 529. 684, [that] 679 [has ceded] 719, 434, [not] 432, 249, 688, 521, [with all its Dependencies.]
[This was done by some of the] 666 [Articles] 228, 475. 723. 529. 377. 723. 460. 475. 475. 377., [as Compensation for] 684. 723. 664. 251. 207. 529. 756, [given] 250, 434, 719, 640. 207. 660. 500. 207.

I have it from very good Authority, Dupont, that Spain has ceded to France, not Florida, but[?] the[?] Mississippi[?], with all its Dependencies. This was done by some of the secret Articles at[?] Luneville, as Compensation for Tuscany, given by France to Parma.

# Mountflorence to Madison (addressed to Marshall), Paris, 18 March 1801: the coded sentences, group by group.
# Key: key.tsv (rebuilt here from the ciphertext; grades in NOTES.md). Code groups decoded: 39 of 39.
# Spelled names in capitals; words read from their alphabetical slot and the context marked [?] where only the slot fixes them.

l.1-2  357. 723. 640. 544. 529. 684    D U P O N T                    DUPONT
l.2    679                              Spain
l.2    719, 434                         to France
l.2    432, 249, 688, 521               Florida, but[?] the[?] Mississippi[?]
l.3    666                              secret
l.3    228                              at[?]
l.3-4  475. 723. 529. 377. 723. 460. 475. 475. 377   L U N E V I L L E   LUNEVILLE
l.4    684. 723. 664. 251. 207. 529. 756   T U S C A N Y               TUSCANY
l.5    250, 434, 719                    by France to
l.5    640. 207. 660. 500. 207          P A R M A                       PARMA

Reading:

I have it from very good Authority, DUPONT, that Spain has ceded to France, not Florida, but[?] the[?] Mississippi[?],
with all its Dependencies. This was done by some of the secret Articles at[?] LUNEVILLE, as Compensation for TUSCANY,
given by France to PARMA.

Key: key.tsv

06 Sources

• National Archives, RG 59, Despatches from United States Consuls in Paris, 1790–1906 (microfilm T1), roll 1, frame 0097: catalog NAID 211302912, frame image.
• Founders Online, Mountflorence to Madison, 18 March 1801 (full transcription) and the abstract, The Papers of James Madison, Secretary of State Series, vol. 1 (1986), p. 27.
• David Humphreys to Madison, 10 April 1801 (abstract in the same volume): he could not decode a message from Mountflorence with the department’s key.

Transcription, key, decoder and notes: targets/mountflorence1801/.