Fifty Thousand Letters, One Pigeon

The Public Domain Review's essay on early microphotography, "Dancing Naked on the Head of a Pin", ends in the Siege of Paris. There René Dagron shrank letters onto film and pigeons flew them into the city. It gives his total as about 2.5 million dispatches. Wikipedia gives two other totals. Dagron's own account of 1871, La poste par pigeons voyageurs, explains all three.

What Dagron wrote

Chaque pellicule était la reproduction de douze ou seize pages in-folio d'imprimerie, contenant en moyenne, suivant le type employé, trois mille dépêches. La légèreté de ces pellicules a permis à l'administration d'en mettre sur un seul pigeon jusqu'à dix-huit exemplaires donnant un total de plus de cinquante mille dépêches pesant ensemble moins d'un gramme. Each film reproduced twelve or sixteen printed folio pages, holding on average, depending on the type used, three thousand dispatches. They were so light that the administration could put up to eighteen of them on one pigeon, more than fifty thousand dispatches weighing together less than a gram.

Toute la série des dépêches officielles et privées que nous avons faites pendant l'investissement de Paris, au nombre d'environ cent quinze mille, pesaient en tout deux grammes. … Si on veut maintenant multiplier le nombre des dépêches par le nombre d'exemplaires fournis, on trouve un résultat de plus de deux millions cinq cent mille dépêches… The whole series of official and private dispatches we made during the siege, about one hundred and fifteen thousand of them, weighed two grams in all. … If you multiply the number of dispatches by the number of copies supplied, you get more than two million five hundred thousand dispatches… (my translations)

One count, three numbers

• 115,000 is the number of different messages, by Dagron's count. Wikipedia's article on Dagron gives the same figure.
• 2,500,000 is that number multiplied by the copies made. Each film went out again and again on different birds until Paris confirmed it had arrived, so the average message was copied about 22 times. The PDR's "he later estimated that he sent about 2.5 million dispatches" is right if you read "sent" as copies.
• 1,150,000 is Wikipedia's "Pigeon post" article: "150,000 official and 1 million private communications were carried into Paris". That is exactly ten times Dagron's 115,000. It may count something Dagron didn't, such as the paper photographs used before his films. But a tenfold gap ending in the same digits looks more like a slipped zero than a second count.

By coincidence the outbound balloons carried about the same number: Godard's table in Glaisher's Travels in the Air (1871) says the Post Office balloons took out "about 2,500,000 letters". Two and a half million went out by air and two and a half million copies came back by bird.

Brewster's pinhead

The essay quotes Sir David Brewster in the 1857 Britannica: "A family group of seven complete portraits occupies a space the size of the head of a pin; so that ten thousand single portraits could be included in a square inch." Both halves can be true only if the pin is small. Ten thousand to the square inch gives each portrait 1/10,000 of a square inch. Seven of them fill a circle 0.76 mm across. That is Brewster's pinhead.

John Benjamin Dancer's 1853 microphotograph of the Sturgeon memorial put nineteen lines and 680 letters into one-sixteenth of an inch, 1.6 mm. That is a line every 0.08 mm and, at about 36 letters a line, letters some 0.04 mm wide, if the image was about as wide as it was tall.

Smaller checks

What stands on what