Museum labels and old manuals assume you already speak the language. This glossary is the key I wish I had started with: forty terms, each decoded the way an operator would explain it at the bench. Bookmark it and open it beside every other article.
The waves
- Constituent — one regular wave in the harmonic sum, e.g. M2. Full cast list: Why 37?.
- M2 — principal lunar semidiurnal, 12.42 h. The backbone of most tides.
- S2 — principal solar semidiurnal, 12.00 h. Beats with M2 to make springs and neaps.
- K1 / O1 — the big diurnal (once-daily) waves, 23.93 h and 25.82 h.
- N2 / K2 / P1 / Q1 — smaller companions from orbital ellipticity and declination.
- Mf / Mm / Ssa — long-period constituents (fortnightly, monthly, semiannual) that shift the baseline.
- Overtide — a wave born in shallow water, not the sky: M4, MS4, M6. The estuary distorting M2 into faster children.
- Compound tide — a shallow-water wave from two parents interacting, e.g. MS4 from M2 + S2.
- Doodson number — six-digit code identifying a constituent by its astronomical recipe (e.g. 255.555 for M2). Doodson’s catalogue system, still used in software. Origin story: Bidston.
The numbers
- Amplitude (A) — a constituent’s size at your port, in metres or feet. Set on the machine by sliding the crank pin.
- Phase / epoch (φ, κ, g) — where a constituent’s peaks fall at your port. Set by rotating its shaft against the zero mark. Three notations (κ, g, G) differ only in reference meridian — old manuals mix them freely, which confuses everyone once.
- Harmonic constants — the amplitude + phase pairs per constituent: the port’s fingerprint, extracted by analysis.
- Mean sea level (Z0) — the baseline everything adds onto.
- Datum — the reference level heights are measured from (chart datum sits near lowest astronomical tide).
- Age of tide — the delay (a day or two) between new/full moon and the biggest springs; the ocean’s inertia made visible.
The analysis
- Harmonic analysis — extracting constants from a gauge record. Methods: Kelvin to Doodson.
- Least squares — Doodson’s rigorous fitting method: find the constants minimising total squared error.
- ** nodal corrections (f, u)** — slow 18.6-year wobbles of the lunar orbit, applied as fiddly adjustments to amplitude and phase. Operators hated them; analysts insisted.
- Marigram — the ink trace from a recording tide gauge: the raw material of analysis. Contrast with machine output in staff vs. gauge vs. predictor.
- Prediction roll — the machine’s inked output. Decoded in my roll guide.
The machine
- Predictor — the machine that sums constituents into future tides (this archive’s subject). Distinct from the rarer analyser, which worked backwards from records to constants.
- Crank pin / amplitude pin — the sliding pin setting one constituent’s amplitude.
- Layshaft — the main driven shaft distributing motion to every constituent train.
- Worm gear / setting worm — fine-adjustment gearing for phases; also the drive into slow shafts.
- Pulley cascade / adder — the floating-pulley system summing all motions onto one wire. Drawn in the pulley article.
- Pen carriage — the moving holder carrying the pen across the paper.
- Time-scale gearing — gearing setting how fast paper advances (e.g. one roll-width per fortnight).
- Backlash — lost motion in gear meshes, visible as pen wobble. The enemy of small constituents.
- Still water / slack water — the moment of high or low tide, when the stream turns. What the tables promise and navigators pay for.
The products
- Tide table — the printed book of daily high/low waters clerks compiled from rolls.
- Springs / neaps — the fortnightly breathing of the range: biggest near new/full moon, smallest at quarters. Mechanism: M2 + S2 beating.
- Diurnal inequality — morning high differing from evening high; extreme where K1/O1 dominate.
- Storm surge — weather-driven water, never predictable by harmonic machines. The limit everyone understood — stressed in the beginner’s guide.
How to keep this page
Don’t memorise forty terms — nobody does. Instead: open this glossary in a second tab beside Kelvin’s prototype or Machine No. 2 and look up each word the moment it puzzles you. Terms learned at the point of confusion stick; terms memorised in isolation evaporate. And when a museum label contradicts my definition, believe the label and tell me — living language outranks any glossary, including this one.