The Rochdale projectRanking canals by locks per mile
Find every lock count in an old book, turn words into numbers, then compare fairly.
The learner downloads Pratt's book from Project Gutenberg and writes a pattern for sentences like "92 locks in 32 miles". It finds zero matches. Reading the text shows why: Pratt writes "ninety-two", "seventy-four" and even "two hundred and eighty-two". So the next job is a small function that turns number words into integers, handling hyphens, "hundred" and the "and" in the middle. With that in place, the pattern "X locks in Y miles" finds 15 figures. It still misses others, such as "ninety in 100 miles" on the Grand Junction Canal, where the word locks is left out, so the learner lists what the program missed and decides whether to widen the pattern or add them by hand.
Locks per mile from Pratt's figures, our Python run, 27 September 2026| Stretch (Pratt) | Locks | Miles | Locks per mile |
|---|
| Huddersfield Narrow, Huddersfield to Ashton | 74 | 20 | 3.70 |
| Worcester and Birmingham, Worcester to Tardebigge | 58 | 16 | 3.63 |
| Rochdale Canal, Manchester to Sowerby Bridge | 92 | 32 | 2.88 |
| Birmingham Canal, Birmingham to Aldersley | 43 | 17 | 2.53 |
| Grand Junction, Paddington to Braunston | 90 | 100 | 0.90 |
| Pratt's average for Britain | 1 per 1ΒΌ miles | | 0.80 |
| Hamburg to Berlin | 3 | 230 | 0.013 |
Two more parsing traps appear. Pratt writes two and a half as "2-1/2", which a careless parser reads as 2 minus a half, or as a range. And his national figure is written the other way up, one lock for every 1-1/4 miles, so the rate is 1 divided by 1.25, which is 0.8 locks per mile, not 1.25. With everything on one scale, the Rochdale Canal has almost 2.9 locks for every mile, well over three times the British average he quotes, and more than 200 times the German stretch.
The last lesson is about comparing like with like. Pratt also describes short flights: 29 locks in 2-1/2 miles at Devizes and 30 at Tardebigge in "3 miles or so". Their rates, above 10 locks per mile, would top the table, but a flight is the steepest part of a canal, not a whole canal, and the Tardebigge flight sits inside the Worcester stretch already listed. So flights go in a separate table. Pratt also says the Rochdale Canal crosses "an elevation 600 feet above sea level". Dividing 600 by 92 is tempting, but it is a height, not a climb, and boats go up one side and down the other, so the learner declines to compute a lift per lock for Rochdale.
Ages 8 to 11
Write numbers as words and back again, then share out locks along a drawn canal.
Ages 11 to 15
Build the words-to-number function in Python and compute locks per mile.
Ages 15 and up
Measure what the pattern missed, separate flights, and chart the ranking.
Pratt's figures, our arithmetic
Every lock count and distance is quoted from Edwin A. Pratt's British Canals as published on Project Gutenberg. The parsing, the rates and the rankings are ours.
