The end product of ninety years of brass was a pocket booklet: the tide table. Every number in it descends from a machine-drawn curve like the ones I teach you to decode in Reading a Prediction Roll. This guide teaches the table itself — the document sailors actually carried.
From roll to booklet
After the setup day and the run, clerks read each high and low water off the roll — time and height — and entered them in ledgers. A second clerk checked every entry against rhythm (see the difference-check routine). The checked figures were typeset into daily pages: date, morning high (time + height), morning low, evening high, evening low, sometimes moon phase alongside. One booklet per port per year; the great offices issued hundreds.
Reading a page
Pick any date line and read across: the times are local (note which meridian — old tables mix port time, zone time, and daylight saving with cheerful inconsistency), the heights measured from chart datum (see glossary). Spring ranges show as tall numbers early and late in the lunar fortnight; neaps huddle mid-page. Compare against the spring–neap mechanism and the page breathes before your eyes.
Using a historic table today
Planning a heritage sail or checking family lore (“grandfather launched at high water, March 1938”)? Find the year’s table in a library, confirm the time meridian in the front matter, and apply the port’s modern datum offset if given. Expect agreement with modern software within minutes at well-analysed ports — the machines earned their keep, as Machine No. 2’s record shows.
Anatomy of a page, line by line
Take a magnifier to any date line and here is what each element is doing. The weekday sits left — not decoration: navigators cross-check the day against their own reckoning, and a wrong weekday flags a misprint the way a wrong total flags a ledger. Next come the morning high (time, then height), the morning low, and the evening pair. Heights are in feet and tenths (or metres, in later editions), measured from chart datum — the low reference explained in the glossary. Small type at the foot gives the time meridian and any daylight-saving rule; this footnote, dull as it looks, is where most modern misreadings are born. Moon-phase symbols in the margin let a practiced eye see the spring–neap breathing without doing a single sum.
Three traps that catch everyone
The meridian trap. Port time, zone time, railway time, summer time — a 1943 table and a 2024 app can disagree by a round hour with both being right. Always read the foot of the page first.
The datum trap. Harbours revise chart datum as surveys improve. A height of “14.2” in 1938 and “14.2” today may sit on different zeroes — fine for nostalgia, fatal for engineering.
The flat-trough trap. In shallow-water ports the low water sprawls (see Why 37?), and clerks sometimes flagged the time “doubtful” rather than guessing — the decoding judgment I teach in Reading a Prediction Roll. Respect the flag; don’t average it away.
Things worth clarifying
Why do old tables disagree with my app by minutes? Time meridians, datum revisions, harbour works since, and better modern constants. Minutes-level drift across a century is normal — and small.
What is the commonest reading mistake? A.M./P.M. confusion and daylight-saving traps. Clerks flagged doubtful entries rather than guessing; extend them the same charity.
Where do I find old tables? Hydrographic archives, maritime museums, and secondhand nautical booksellers. The museums guide points at the right reading rooms.
A date to try
Pick a date that matters to you — a grandparent’s sailing, a historic landing, your own harbour visit last summer — and look up its tide in the year’s table. Read the time, apply the meridian note, picture the water. Then check the same date in a modern app and compare. The minutes between them are a century of harbour works, datum revisions, and better constants — and the fact that they nearly agree is the machines’ quiet memorial. For wilder waters, read Tides at War; for the instruments behind the numbers, Staff vs. Gauge vs. Predictor.