In the latest Age of Invention newsletter, Anton Howes digs deeper into the question of why it took so long for the steam engine to be invented:

As I hinted in Part II, there were still more wonders to issue from [Cornelis] Drebbel’s workshop — many of them building on the same principles as his perpetual motion machine.

So it’s worth a very brief recap of how that device worked. Drebbel had improved upon an ancient experiment involving an inverted flask in water: that is, to heat the base of a long-necked glass flask and place it mouth-first into a bucket of water. The heated air trapped inside the flask would bubble out, and as the remaining air cooled, the water of the bucket would rise up into the flask.

The inverted flask experiment. The air bubbles out on the left. As the remaining trapped air cools, on the right, the water rises up the flask (in fact pushed up by the pressure of the atmosphere).

Drebbel’s big breakthrough was to notice that once the water was already sucked into the flask, it would continue to rise and fall even when it wasn’t being heated or cooled on purpose — movements that were the result of natural changes to atmospheric pressure and temperature.

From this continued movement — to his mind, a harnessing of the perpetual movement of the universe itself — Drebbel then constructed a machine that seemed to show the ebb and flow of the tides, as the liquid inside it rose and fell between the cold of night and the heat of day. He also exploited that same rise and fall of the liquid in order to rewind clockwork that continually showed the time, day, months, and years, along with the cycle of the zodiac and the phases of the moon.

Few have now heard of Drebbel’s perpetual motion machine, but noticing that same rise and fall of the liquid in response to changes in the weather would also serve as the basis for the invention of the thermometer and barometer. Or, more accurately, to the reinterpretation of the ancient inverted flask experiment as a device capable of measuring both temperature and atmospheric pressure. (The two different applications were not disentangled and isolated until later, as we’ll see below, and so in modern terminology the initial device is often referred to as an air thermoscope.)

[…]

For alchemists like Drebbel, being able to control the temperature of furnaces and ovens was a valuable prize, because so much of their skill in manipulating metals and minerals depended upon it. The alchemist’s art — the intangible, tacit skill built up over years of experience — was one of sensitivity to heat, being able to judge, by feel and by look, the varying intensities of flame, and then to manipulate it so as to keep it at a constant level. The art was known as pyronomia, or as regimen ignis — the governing of fire.

At some point before 1624, Drebbel worked out that he could exploit the inverted flask experiment to radically improve furnaces. He did this in two ways. One was simply to affix a mercury thermometer to the furnace, to indicate its heat (mercury, with a higher boiling point, would be less liable than water to simply evaporate away). But the other, and more ingenious way, was to create a feedback mechanism to control the oven’s heat automatically. Drebbel placed a cork to float atop the mercury in yet another thermometer, which as it rose or fell would then cover or uncover the furnace’s air supply. He could thus choose a desired heat, and then let the oven do the rest. If it grew too hot, the air supply would be restricted. If it grew too cold, it would be increased. Drebbel had invented the thermostat, and perhaps one of the first widely-applied practical feedback control mechanisms.

Drebbelian self-regulating ovens, or Philosopher’s Stoves, spread beyond England, to be adopted in France, the Netherlands, Germany, and even across the ocean in New England — they were a major source of business for the husbands of Drebbel’s daughters, the brothers Abraham and Johannes Sibertus Kuffler, to whom he passed many of his secrets. Drebbel even applied its thermostatic principles to artificially incubating eggs, for which maintaining a constant temperature was essential. To give an idea of how big a deal this was, Francis Bacon filled his techno-utopian vision of a New Atlantis with “furnaces of great diversities, and that keep great diversity of heats; fierce and quick; strong and constant; soft and mild; blown, quiet; dry, moist; and the like”, some of which, like the incubator, were able to provide even the gentle heat of animal bodies. Drebbel, in Bacon’s lifetime, was thus producing the stuff of science fiction. He was, as one admirer termed him, a true Mysteriarch.

And the Mysteriarch did not stop there. Just before his death in 1633 he was working on improving the stoves, making them more efficient, reducing the need for people to attend the fire, and reducing their smoke. They could thus be applied to drying hops, malt, fruit, spices, and gunpowder, heating rooms in houses, and distilling fresh water from sea water. His heirs, the Kufflers, even made the stoves portable enough to be used for baking the bread for armies — they were allegedly used by Frederick Henry, Prince of Orange, in his various successful campaigns against Spain. Both the portable ovens and the seawater distilling machines were apparently used in the 1650s aboard ships headed for the Indian Ocean.

By the 1620s, then, many of the key elements for a steam engine were already coming into fairly widespread use. Scientists across Europe, inspired by Drebbel’s perpetual motion and Santorio’s thermometer, were eagerly pursuing the possibilities from expanding and contracting gases. And Drebbel had invented a widely-used thermostatic feedback system — a general concept that would later prove extremely useful in making steam engines practicable. Feedback systems and safety valves would come to regulate the movements of engines and reduce the risks of them overheating and exploding.