Archived report · 4 min read · Set in Stanhope Road ·
The Standardisation of Shadows Measured at the Calibration Wall, Stanhope Road
The Board of Standards reports that every shadow cast on the calibration wall runs longer than the certified sources permit, by one fixed margin of eleven millimetres, and the margin answers no inquiry put to it.
- filed: "at the quarterly check"
- reference: "SR/SS/31"
1. This report is filed at the quarterly check by the Board of Standards office at Stanhope Road, and concerns the shadows taken on the calibration wall of the north yard, which the office measures four times in every year against the certified sources.
2. The calibration wall is a smoke-brick wall thirty feet from base to coping, faced level and scribed with gauges off the yard line at one-foot intervals. It has stood trued square to the yard since 1951.
3. The procedure is settled. On one set day of each quarter, at ten of the morning and again at two of the afternoon, the certified rods, being the six-inch, the foot, and the yard, are stood upright on the scribed baseline, and their shadows are measured along the wall's face with the certified tape. The measured lengths go onto the worksheet in ink.
4. Before the measuring, the office computes the permitted lengths from the certified heights against the almanac altitude, entered in red on the same worksheet.
5. From the quarter following the Misprint, the measured column has run longer than the permitted column by eleven millimetres. Every rod, both hours, every quarter. The margin has not moved in four years.
6. The margin is fixed and not proportional. The six-inch rod and the yard rod come out eleven millimetres long alike, which no geometry of similar triangles will produce, when the longer rod should carry the larger excess if the fault lay in the light or the wall. The excess attaches to the measurement and puts itself out to no rod.
7. The possible errors were taken in turn and retested. The wall was resurveyed to the water level by Mr. Albert Prew of this office, and the gauges stand true to the scribed line. The tapes were recertified against the brass standards in the strong room by Mr. Eric Sandwell of the weights office, and no error was found in them. The altitudes were checked by Miss Joan Talboys against the almanac and against the angle-piece readings taken at the wall on the day, and the two agree to the minute.
8. The office then changed the source. For one worksheet the sun was put out and the certified lamp stood in at a fixed radius, and the lamp's shadow came off eleven millimetres long. The lamp was put out and the standard candle stood in, and the candle's shadow came off eleven millimetres long. A control was taken with the stand shaded from every source, and the control read nothing at all.
9. It follows, and the office has minuted it so, that the margin does not reply to the source. It is not a scale factor and not an angle mistaken. It behaves as a constant carried forward, the way a rounding is carried when nobody has asked the office to state it.
10. The permitted column keeps true. It is longest at midwinter and shortest at midsummer, moves with the almanac within a hair, and the measured column moves with it, eleven millimetres over, at every point of the year. The margin neither grows nor shrinks, and it has outlasted two printings of the almanac.
11. Under the standing instruction of 1979 the yard is not entered between three and the half hour's end, and the office therefore holds no reading inside the Thin Hour. The worksheet carries a ruled space for that interval, and the eleven millimetres are carried across the space untested. The office does not propose to test them there.
12. The quarterly check has referred the margin to the standing committee on departures from standard, which sat last in June and adjourned without a finding, and sits again with the January check. The committee has before it the worksheets, the resurvey, the recertified tapes, and the candle. It has nothing between the arithmetic and the light to weigh against either.
13. In the meantime the margin is carried on every worksheet as a standing deduction of eleven millimetres, and the published shadow tables for Murrow rule accordingly, so that the trade is not kept waiting on what the committee has not finished.
14. The report ends where the quarter ends, and carries the committee's question to its January minutes, the same question it has carried every quarter since the margin first stood: whether the eleven millimetres are an error our instruments have agreed among themselves to make, or a length something is adding to our shadows that we have not certificated, and what it is being added for.