Anybody who has boiled water knows that a positive pressure builds up when steam is produced. Indeed the first conceptual design of a steam engine (the Aeolipile) was a "positive pressure" engine. The fact that condensing steam produces a vacuum, however, is much less intuitive. Why, then, were the first industrial steam engines (Newcomen engines) atmospheric engines that produced work as the result of the vacuum created by condensing steam? Why did they not use the more "obvious" solution of steam pushing a piston? Were it technical concerns (high pressure leading to boiler explosions)?

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    $\begingroup$ I don't have refs, hence the comment: But my understanding was that that it was technological: The ability to create steam-tight seals (less critical with Newcomen's design) on the valves and cylinder. Note that it took Watt a lot of work and experiments to get his system to work. He had to bring in the help of Boulton for help with machining and fabrication technology/techniques. $\endgroup$
    – winwaed
    Commented Aug 5, 2016 at 12:45

2 Answers 2


The best answer to that question is the following book:

The steam engine of Thomas Newcomen by L. T. C. Rolt, J. S. Allen 1977 Moorland Pub. Co. ; New York : Science History Publications,

To summarize what I learned from this book.

  1. The boilers did not generate enough steam pressure to move pistons. You needed to use the weight of the atmosphere for the power stroke.

  2. Even if the boiler could generate high steam pressures, the art of machining pistons and cylinders was still in its infancy

Kevin Olsen


Thomas Savery used steam pressure in his engines to force water to a height beyond the 10.3m limit afforded by a vacuum but from what i've read he only managed a pressure of around 45psi and he had several incidents busting his boilers. So i guess the materials at the time just weren't strong enough.

  • $\begingroup$ Could you provide reliable sources rather than just an "i guess"? $\endgroup$
    – tox123
    Commented Sep 2, 2017 at 16:40

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