Archived report · 3 min read · Set in Kessin Reach ·
The Survey of the Second Tide
Instruments spaced along a surveyed reach record a second daily tide that appears on no chart, and the instruments agree with each other.
- filed: "after the equinox"
- reference: "KR/ST/08"
1. This is the report of the survey of Kessin Reach made by the port survey party in the season after the equinox, covering the reach from the Middle Beacon to the head of the sands. The party worked from the surveying skiff, laying and lifting moorings on the spring tides. Instruments in use: three tide staffs, at the Ross Head stair, the Middle Beacon, and the Old Customs slip, read by observers at fixed hours; a self-registering float gauge at the Middle Beacon; a bottom pressure recorder laid off the drying bank; and a current meter on the station mooring.
2. The finding is stated once. On eighteen days of clean record, between one noon and the next noon, two complete high waters are registered and one low water. The first high water agrees with the printed predictions for the reach. The water then falls normally, holds at a level it is not predicted to hold at, rises from that standing level to a second high water at 03:33 local without exception, and falls away to the day's predicted low water. The interval from the first high water to the second was seven hours and forty-four minutes on all eighteen days. Two high waters and one low water between two noons is an arrangement the arithmetic of tides does not provide.
3. The observations agree with each other. Staff, float gauge, pressure recorder and current meter return the same curve to within a tenth at every cross-check and at every hour. It is the printed predictions and the charts that disagree, and they disagree with all four instruments at once.
4. The checks were made. The staffs were read for rank and settlement. The pressure recorder was lifted and calibrated against the basin gauge, and found correct. The float well was sounded and cleaned. The office constants were recomputed and the harmonic analysis was run twice from scratch. Both runs return one constituent the printed tables do not carry. It is labelled K-X in this report. Its period is half a lunar day, its phase is fixed at 03:33, and the branch has no name for it.
5. The observation this survey cannot explain away stands in the current record. Through the first fall the meter runs the ebb as it should. Through the second rise from the standing level the meter reads slack throughout: no stream enters the reach and none leaves it, and the level rises one point one metres with the meter saying that nothing moved. A rise without a stream is outside instruction, mine and the branch's alike. The water for the second tide appears in no record of going out, and the day closes on the predicted level, the account balanced without any column for the balancing.
6. Action taken. The charts for the reach carry a pencil annotation at the Middle Beacon: second high water, 03:33, unconfirmed. The chart branch ruled the annotation out once and it was ruled back in, and the matter was noted for attention after the equinox, which is the season this survey was made in. The party has been stood down for the winter in the ordinary way and the moorings left on their buoys.
7. The survey files open. Every instrument involved returned honest curves. They agree with one another to the tenth and with no chart in the office. The file is left standing on the question the record itself poses at 03:33 on every recorded day: what rises in Kessin Reach on the hour the moon does not draw, and where does it find the water to rise a second time off a level it never fell to?