The last technology mentioned in The Baroque Cycle is described on pages 883 ff of The System of the World. Daniel Waterhouse is visiting the steam-powered pump, at a mine in Cornwall, in the winter of 1714-15. He sees and contemplates several features of the installation, which must be compared to the true nature of an engine working on the Newcomen cycle.
“Plenty of steam leaks out around it [the seal at the edge of the piston], but most stays where it belongs.” “This platform is dripping wet, and yet it’s warm, for the used steam exhaled by the Engine drifts round it and condenses on the planks.” “The level ground below the Engine is pocked all around, with wreckage of Newcomen’s boilers.” “He wonders if these Cornish men have the faintest idea that they are sitting around an explosive device.” “..., the seams and rivet lines joining one curved plate to the next radiate from the top center just like meridians of Longitude spreading from the North Pole.” “Below is a raging fire, and within is steam at a pressure that would blow Daniel to Kingdom Come (just like Drake) if a rivet were to give way.” “The steam is piped off to raise water, ...”
My old Encyclopaedia Britannica includes a good diagram of a Newcomen engine, but there is no adequate explanation of its operation. The following description is based mainly on what I remember, from the course where I learned about this cycle, in 1948.
A Newcomen engine does not exhale used steam. The steam enters the cylinder during the return stroke, as the piston rises and the pump rod goes down. All of that steam is supposed to be condensed inside the cylinder during the power stroke, by a spray of cold water into the cylinder. That condensation leaves a partial vacuum in the cylinder. The pressure of the ambient air on the top of the piston pushes it down, so the pump rod goes up, doing useful work by lifting a quantity of ground water. The water in the cylinder, consisting of the condensed steam and the sprayed-in water, is released during the next return stroke. The only steam which comes out of a Newcomen engine is leakage.
Because the main function of the steam is to keep air out of the cylinder, it need not be at high pressure. As I recall, a gauge pressure of 3 or 5 pounds per square inch (psi), i.e., 1/5 or 1/3 of atmospheric pressure, would be plenty. This may or may not qualify the engine as a dangerously explosive device.
The description of a succession of failed boilers suggests to me that Stephenson may have presented Newcomen as engaged in a program of increasing the boiler pressure. The Newcomen cycle is so incredibly inefficient, that the first-order effect of an increased pressure is a reduction of the efficiency. One must burn more coal to increase the temperature and pressure of the steam, but all of that added heat energy is thrown away during the condensation in the cylinder. The concept of efficiency was poorly understood at that time, and an experimenter may have felt that a possible increase in the speed of the engine were a good thing. (I can use the subjunctive mood, too.)
One cannot see the top center of the boiler on a typical Newcomen engine, because the vertical cylinder is immediately above the boiler, with a valve between. There is no pipe, to carry the steam away from the boiler.
The combination of features, use of steam at low pressure and condensation of all the steam inside the apparatus, also was employed in the engine developed by James Watt after about 1769. (See Wikipedia or Encyclopaedia Britannica for this history.) It was far more efficient than Newcomen’s engine, because the condensation took place in an external condenser, thereby allowing the cylinder to stay hot.
Altogether, Stephenson’s description comes closest to that for an 'expansion' engine, in which the full boiler pressure is admitted to the cylinder for only a fraction of the power stroke. Such an engine is more efficient if it has an external condenser, to allow expansion to below atmospheric pressure, and to recycle the feed water. However, it can be built to operate in an open cycle, in which case it does exhale used steam, after the steam has expanded to drive the piston during the remainder of the power stroke. Such an engine does operate at high pressure, perhaps hundreds of psi. Increasing the boiler pressure does directly increase the efficiency. However, that type of engine was not developed until after 1800, when Watt’s patents expired. The most familiar example (with quite different mechanical arrangements), was probably the steam locomotive, as seen in old movies.