The machine’s output was not a table — it was a long paper roll covered in a wavy ink line. Turning that line into the tide tables sailors bought required human eyes, rulers, and judgment. This guide teaches the decoding, so you can read a surviving roll like a 1910 clerk.
What is on the roll
- The curve. Pen height = predicted water height; paper advance = time. Standard scales (e.g. one inch per hour, one foot of water per inch) were printed on the margin.
- Time ticks. A second pen or punch marked each hour and emphasised midnight — essential, because paper stretched and motors wavered.
- Annotations. Operators pencilled the port name, constituent set, date span, and any stoppages in the margins. Never trust an unannotated roll.
The clerk’s routine
1. True the time scale. Measure the actual spacing of hour ticks with dividers; if the paper stretched, prorate. A 1% stretch over a fortnight moves high water by minutes — enough to matter.
2. Find the turning points. Slide a straightedge along the curve and mark where it kisses each crest and trough. Shallow-water distortion (see Why 37?) makes some troughs flat and ambiguous — clerks flagged these “doubtful” rather than guessing.
3. Read heights. Against the datum lines printed on the roll, read each high and low to the required precision (tenths of a foot, typically). Note any pen skips: a dry pen draws a gap, not slack water.
4. Tabulate and check. Enter times and heights in ledger form, then difference-check: consecutive highs ~12.4 h apart (or ~24 h where diurnals dominate — see M2/S2/K1/O1), ranges swelling toward springs. Anything violating the rhythm got re-read.
Try it yourself
Many archives publish roll scans; print one at full resolution and work a week with dividers and a magnifier. Compare your readings against the printed table for the same port and year (libraries hold them) — matching a clerk’s numbers to the minute is a genuine thrill, and the best possible lesson in what “accuracy” meant in 1910. Setup context: How Operators Set Up a Run.
Details people trip on
Why didn’t the machine print numbers directly? Printing counters existed on paper but the pen-and-roll was cheaper, more reliable, and gave a visual check: a wrong setup showed instantly as a nonsense curve.
What is the commonest beginner mistake? Mistaking the flat top of a shallow-water tide for two separate highs. When in doubt, count cycles per day against the port’s known character.
Where can I find rolls to practise on? Hydrographic office archives and observatory collections; some are digitised. Ask archivists for “marigrams and prediction rolls” together — comparing predicted rolls against actual gauge records teaches the most.
Your first roll
Here is the assignment I give every newcomer: print one archived roll scan at full resolution, work a single week with dividers, and compare your high waters against the printed table for that port and year. If you land within a minute on three days running, you have matched a 1910 clerk — and you will never look at a tide app the same way. The rolls those clerks served came off giants like Machine No. 2; the whole pipeline sits in one picture in the beginner’s guide.