London fixed the right problem for the wrong reason

A city, a smell, and a sewer built on a mistake.

In the summer of 1858, London got hot. Temperatures climbed past 30 degrees Celsius, well into the 90s Fahrenheit, and stayed there for weeks.

The Thames had been taking in the city's sewage for years. In that heat, it started to bake. The smell got so bad it earned a name before the summer was over. The Great Stink.

Parliament had a front-row seat. The new Houses of Parliament sat right on the river. Staff soaked the curtains in chloride of lime to cover the smell. It didn't work.

Lawmakers who had put off paying for a real sewer system for years suddenly found the money. Benjamin Disraeli brought in a bill to fund it, and it became law in 18 days.

The man who got the job was an engineer named Joseph Bazalgette. Over the next couple of decades, his crews built a network of tunnels that caught the city's waste before it reached the river and carried it far downstream, east of the city.

Here's the part that matters.

Most of the people paying for it believed disease spread through bad air. Miasma. If you could smell the rot, the thinking went, you were breathing in the sickness. Cholera, which had killed tens of thousands of Londoners in waves since the 1830s, was supposed to work the same way.

So in a real sense, the sewers were a very expensive project to get rid of a smell.

The smell wasn't the problem.

In 1854, a doctor named John Snow had traced a cholera outbreak in Soho to a single water pump on Broad Street. Cholera spread through contaminated water. Snow had the evidence. Most of the establishment wasn't convinced. He died in June 1858, right as the stink was building.

And then the miasma believers built the thing that proved him right.

By moving sewage out of the river, Bazalgette's system also moved it out of the drinking water. When cholera hit London again in 1866, it hit hardest in a part of the East End that wasn't yet connected to the new system. It was the city's last major cholera outbreak.

The tunnels are still down there. Bazalgette's brick sewers still form the backbone of how London moves its waste today, more than 150 years after a bad smell finally got Parliament to act.

The right fix. Built for the wrong reason. It worked anyway.

That's a little uncomfortable, if you sit with it. People like to think good results come from good understanding. Usually they do. But sometimes you get the outcome right while getting the explanation completely wrong, and nothing on the surface tells you which happened.

That's the trap. When something works, it feels like proof that your reasoning was sound. London's engineers could easily have taken the end of cholera as evidence that miasma theory was correct. For a while, plenty of people did.

A good result can protect a bad idea for a long time. So it's worth asking why something worked every once in a while, especially when it did.

The next problem might not be so forgiving.

"It is a capital mistake to theorize before one has data."

Sherlock Holmes, in Arthur Conan Doyle's "A Scandal in Bohemia" (1891)