The Gillingham projectTwo miles to Rochester, one to Chatham?
Measure the three towns, compare with Adams, and work out what kind of error it is.
The learner looks up the three towns in the Ordnance Survey's gazetteer of place names, which gives a single reference point for each, and measures the distances between them on the Earth's surface with the geographiclib package. Gillingham to Rochester comes out at 5,697 metres, which is 3.54 statute miles. Gillingham to Chatham is 4,085 metres, or 2.54 miles. Chatham to Rochester is 1,618 metres, 1.01 miles. Adams said 2 and 1. So both of his figures are short by a mile and a half. The obvious guess is that the "English mile" of 1611 was a different length from ours, and that guess is exactly what the program is built to test.
Adams's distances against the modern gazetteer points, our Python run, 28 September 2026| Pair | Adams (English miles) | Measured (statute miles) | Measured minus Adams |
|---|
| Gillingham to Rochester | 2 | 3.54 | 1.54 |
| Gillingham to Chatham | 1 | 2.54 | 1.54 |
| Chatham to Rochester | 1 (implied, if in a line) | 1.01 | 0.01 |
| Ratio of the first two | 2.0 | 1.39 | Does not match |
| Straightness: GC + CR minus GR | Zero if in a line | 6 metres | Almost perfectly in line |
If the problem were the unit, every distance would be off by the same factor, so the ratio of the two distances would survive. It does not: Adams's ratio is 2.0 and today's is 1.39. Working backwards, his "mile" would have to be 2.85 kilometres long from one pair and 4.08 kilometres from the other, which is impossible for a single unit. Now try the other explanation, a shifted starting point. The differences are 1.54 and 1.54 miles, identical to two decimal places. One constant offset explains both numbers perfectly.
The geometry shows why. Adding Gillingham to Chatham and Chatham to Rochester gives only 6 metres more than the direct Gillingham to Rochester distance, so the three reference points lie almost exactly on one line with Chatham in the middle. Adams's own numbers make the same shape: 2 miles minus 1 mile leaves 1 mile from Chatham to Rochester, and today's figure is 1.01. His description fits the modern map beautifully, provided his Gillingham stood about a mile and a half nearer Chatham than the point the gazetteer uses today. The learner concludes that the evidence points to a reference-point difference, not a faulty mile, and writes down what would be needed to go further: where the town centre lay in 1611.
Ages 8 to 11
Put three coins in a line, measure between them with a ruler, then move one and see what changes.
Ages 11 to 15
Compute the three distances in Python and compare differences and ratios.
Ages 15 and up
Test scale against offset properly and check how straight the line really is.
Adams's words, our measurements
The quotations come from the Project Gutenberg edition of the Diary of Richard Cocks, edited by Edward Maunde Thompson. Town positions are Ordnance Survey Open Names points, read through postcodes.io. All distances and comparisons are our own calculations.
