A golden Sun sits at the end of a long blue pointer. Beneath it is a blue disk scattered with stars. Around the disk, red, green, and blue compass points reach toward a ring of Roman numerals.
This is the tide volvelle in Jacques Devaulx’s manuscript of 1583: a calculating instrument made with rotating disks. The colours are splendid. The instruction I keep returning to is less showy.
Before moving the Sun, you have to set the harbour.
The transcription by Marie-Thérèse Castanet-Leroy begins the procedure with a requirement: il convyent premyerement scavoir quelles y sont les marez — you must first know what the tides are there. In the rule used here, the necessary local information is the hour of high water at new moon.
You put the lunar pointer against that hour and hold it there. Then you move the solar pointer to the Moon’s age on the inner scale. The solar pointer now also indicates an approximate high-water time on the outer scale.
The disks carry a reusable rule. The harbour supplies a setting the rule cannot obtain from lunar age alone.
In Véronique Hauguel’s explanation of the instrument, each day of lunar age adds forty-eight minutes. Her example begins with a port setting of six o’clock: one day gives 6:48, two give 7:36, three give 8:24. Rotation does the addition. This is a historical approximation, not a method to use for navigation today.
A few decades earlier, Guillaume Brouscon drew another way to look up the local information. In his 1548 tide-index chart, fine lines branch across the blank water between a compass rose and the ports. They connect places to compass directions encoding tidal information. The sea looks threaded.
The BnF’s description suggests that this graphical system could have served mariners regardless of their language. That is a possibility, not evidence that a diagram needs no learning. The reader still has to know what kind of connection a line represents. What looks like ornament to me can be a lookup procedure to someone who knows how to use the page.
Neither the chart nor the dial contains everything a pilot needs. Hauguel’s account explicitly returns to currents, the seabed, wind, and the ship’s loading. A modern NOAA explanation likewise describes how shoreline shape, bays, river flows, wind, and atmospheric pressure affect local water levels. A lunar rule is not the whole sea.
I came looking for a little paper machine. I stayed with the instruction that precedes its movement: first know the tides there. Then hold the lunar pointer steady, and turn the Sun.