Archived report · 3 min read · Set in Skerry Head ·
The Magnetic Agreement of the Head
The annual variation determination for Skerry Head returns one degree of swing in step at every observing station, including the ridge hut that was closed and stripped of instruments thirty years ago.
- filed: "with the hydrographic year"
- reference: "SH/MN/21"
1. This report is made with the closing of the hydrographic year, by the undersigned compiling officer, on the annual determination of compass variation for Skerry Head. The determination rests on three observing stations, the Head light, the pilot station at the quay, and the old coastguard observation hut on the ridge, and the figures of all three are set down in a single tabulation before they are published to the charts.
2. The method is long settled. Each observing station takes the sun's azimuth against a standard compass on four days in the year, reduces the readings, enters the variation in its observation book to the nearest minute, and the books come to the office under seal at the year's end. The tabulation carries a column for each station and the figures are compared across it before any one of them is moved up to the published value.
3. The agreement of the columns is the matter of this report. For fourteen years now, at every station and on every observation day, the variation at the Head has stood one degree less than it stood the year before, and the same one degree falls on the pilot station's figures in the same twelve months. No station runs minutes ahead of another or behind it, though the instruments differ in make, in age and in error, and a coincidence of that shape, held year after year, is not a thing the rules allow to pass unentered.
4. The third column is the one the rules do not reach. The ridge hut was withdrawn from the watch in 1961; its standard compass was taken off to stores under receipt, its keys returned to the office, and the last entry in its observation book is in the hand of Mr J Pretty, the officer then withdrawn, and stops at that date. The tabulation has never stopped with it. A figure for the hut is carried in the third column every year, and the source column against it gives the same three words each time: per the year book.
5. Checks were made under the rules. The light's standard compass was swung by the examining officer in the spring, and his report allows no error large enough to hide a degree in, nor small enough to make one. The pilot station's azimuth instrument was compared against it and found in agreement. The hut's own compass is in the stores, still heavy with the grease it was put away in, and no office receipt shows it moved since 1961.
6. The office cannot account for the third column's arithmetic, and I have set down what the papers do allow. The hand that carries the hut's figures since 1961 is square on the ruled lines and not the hand of Mr Pretty, and not the hand of any clerk now serving; it is the same hand from year to year; and where the two live columns have once moved by a minute out of turn, the third has moved with them, so that the agreement of the three holds exactly. The register of this Archive holds no writ against the ridge, or against any observer of it, which is the only comfort the tabulation leaves me.
7. The published value goes out as the mean of the three columns, so the ships in the channel are steering by a figure the closed hut stands in for as much as the two live stations do. The mean is the same whether it is struck from two figures or three, which the tabulation proves every year and the rules cannot explain.
8. The hydrographic year is therefore closed with the columns agreeing, one degree turned, all square, all signed. The question I am obliged to leave standing at the foot of this report is the one the third column asks without meaning to: whether the headland turns its degree for the ships that pass it, or the hut on the ridge is still observed by something that was never taken off to stores, and which of the two has been keeping the needle right for the pilots in the dark of the year.