page fifty-one

The Sounding

Fifty rooms, and in every one of them you could see what you were working on. Today's number lays a harbour under the sea where nobody can look at it: a bar across the bay, one gap the ebb has cut through it, and a rock or two the old chart never showed. You have a boat, a hand lead and the hours until half past three. Tomorrow a ship drawing more water than the bar comes in by whatever track you lay.

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The chart

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Soundings go on the chart in fathoms with the odd feet small beside them, reduced to low water springs. The shading guesses between your casts: red where they say there is less than she needs, faint green where they say there is enough, nothing where you have not been. An R is where the arming came up scored: rock.

The tide pole
The sounding book

The leadsman calls the water as it is. Take off what the boy reads at the pole and you have the depth at low water springs, which is what a chart shows, so that anyone can add tomorrow's tide back on.

Harbour
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The arrow keys walk to other days, and Z takes up the last mark of the track. Every day is a different harbour, tide and ship.

What is real here and what is not

The lead line, its calls, the reduction to a datum and the way a ship's track was judged are real. The harbours, the ships and the seabed are the day's number.

  1. 01

    Marks and deeps

    A hand lead line was marked with leather, calico, serge and bunting at two, three, five, seven, ten, thirteen, fifteen, seventeen and twenty fathoms. The fathoms between were the deeps, and the leadsman called them by feel: by the mark five, by the deep six, and a half four, a quarter less seven. The hollow in the bottom of the lead was armed with tallow, and what stuck to it went on the chart as well.

  2. 02

    Reduction

    The water over a bank changes by the hour, so a raw cast means nothing a day later. Surveyors kept a tide pole going all day and took its height off every sounding, reducing them all to one level, here low water springs, so that whoever reads the chart adds the tide of their own hour back on.

  3. 03

    Bars and gaps

    Where a river or a tidal basin empties into the sea, the ebb carries sand out and drops it where the current slackens, building a bar across the mouth. The same ebb keeps one gap scoured through it, and the gap wanders. Finding it, and the rock nobody had marked, was the reason the boat went out.

  4. 04

    What the page simplifies

    The tide here is one clean semi-diurnal curve, the same all over the bay, with no stream to set the boat or the ship off her line. Positions are perfect, where a real surveyor fixed every cast with a sextant on two shore marks. The ship's passage is quick enough that the tide does not change under her, and the Hydrographer's track is a shortest path on a grid, not a seaman's judgement.

Fifty-one rooms, and one number doing all the work. Today it lays a seabed nobody can see, cuts a gap in the bar and keeps it to itself until the lead finds it.