Supplementary materials for
The First (Known) Statistical Graph:
Michael Florent van Langren and the "Secret" of Longitude
Michael Friendly, Pedro Valero-Mora and Joaquíin Ibáñez Ulargui

The 1644 graph

Other images and data

La Verdadera

Van Langren's cipher

In La Verdadara, van Langren says (p. 6)
Van Langren began the study of the Longitude for sea and land by means of the Moon in the year 1621. In 1625 he informed the infanta Isabel about this method to calculate the Longitude, as well as a second method that Langren had discovered (that it is written later in dark letters), as it is known from a letter that infanta Isabel wrote to his Majesty in the same year of 1625.
and includes the following ciphered text at the end of p. 8.
There are several open questions that arise from this:

For those interested in pursuing these questions further, we provide the following resources:

Note that it was not unusual for scientists in that time to publish declarations of discoveries in code, often using anagrams, to be able to claim priority, while concealing their method. See: