In 1919, as American industry was being transformed by precision instruments — pyrometers that could read steel temperatures to the degree, Brinell and Rockwell testers that could quantify hardness, and a whole new vocabulary of "scientific management" — a construction engineer named John Lord Bacon and his collaborator Edward R. Markham slipped a quiet warning into the preface of the third edition of a small technical manual.

The introduction of heat measuring and hardness testing instruments, together with various other modern appliances, and up to date systems of doing work have made necessary a broader knowledge of heat-treating methods than was formerly the case: for after all the most important factor is the man doing the work.

Read that last clause again. In a book celebrating the arrival of measurement, the authors insist — almost defensively — that the instruments don't displace the craftsman. They make him more necessary, not less.

Bacon was an instructor in Forge Practice and Machine Design at Lewis Institute in Chicago (later absorbed into the Illinois Institute of Technology), and Markham described himself as a "Consulting Engineer Specializing in Heat Treatment of Steel." These were not romantic anti-modernists. They were engineers who wrote a textbook published by John Wiley & Sons, the same press that helped lay the foundation of American industrial education. They wanted their book on the desks of factories that were buying pyrometers.

And yet they understood something that the next century of industry would repeatedly forget and rediscover: a tool that returns a number does not return a decision. A pyrometer can tell you the steel is at 1450°F. It cannot tell you whether this particular billet, hammered this way, for this part, is ready to quench. That judgment lives in a person who has burned, cracked, and over-tempered enough pieces to feel the difference.

The earlier preface, written by Bacon alone, made the same argument about pedagogy:

The author believes that the text book should be used to explain principles and give examples, not to give minute explicit directions for making a set of exercises.

In other words: don't teach the recipe. Teach the principle, and let the student earn the recipe by ruining a few pieces of steel.

What's striking is how directly this maps onto modern debates about automation and AI. Every generation hears the same promise — that the new instrument will deskill the work, that judgment will become an interface, that the craftsman is obsolete. And every generation discovers, around the time the instruments are fully deployed, that someone still has to interpret the readout, recognize the edge case, and decide when the standard procedure doesn't apply.

Bacon and Markham wrote that line in 1919, watching forge shops fill up with thermocouples. The diagnosis travels remarkably well to a 2026 reader watching codebases fill up with copilots.