Mark Denny

I'm a science writer, and my books explain how things work.

. (1) Ingenium: Five machines that changed the world

This book is based upon five papers that I wrote for the European Journal of Physics, explaining how five historically important machined worked. Turns out they also provide interesting examples for university physics students. The five machines are: the longbow (chapter 1), the waterwheel and the windmill (taken together, in chapter 2), the trebuchet (a medieval siege engine, chapter 3), the anchor escapement (it's what makes mechanical clocks and watches tick, chapter 4), and the flyball governor (the whirling balls that control steam engines, chapter 5).  

Medieval mangonel and trebuchet siege engines, reconstructed by a Danish museum. Image courtesy of The Medieval Centre, Nykoebing F, Denmark


"The book excellent teaching aid for physics teachers and students (the invention of such creative aids being one of Denny's stated aims). The sections on machinery are superb...Ingenium inspires us by its sheer conviction that we can understand the mechanics; if we want, we can even build and improve upon the machinery." Ilia Stambler, Bar-Ilan University, in European Legacy (spring 2008).

"The subject matter is extremely well described...and the author provides good insight into mankind's struggle and intellectual quest to apply the very stuff we call science." Brian Gee in School Science Review (May 2008).

For more reviews see the publisher's website:

and the website:

Additional material:

 This old drawing of an early Watt steam engine does not show the flyball governor, but does show two other key innovations that made Boulton and Watt's steam engines such a great commercial success. Inside the flywheel you can see two gearwheels--the so-called 'sun and planet' gear mechanism. Watt could not use the more familiar (to modern engineers) connecting rod and crank because, in his day, this mechanism was under patent to a competitor. So Watt adopted the sun and planet system, invented by an employee, William Murdoch. The sun gear (centered on the flywheel axle) is orbited by the planet gear. One orbit of the planet generates two turns of the flywheel. The second innovation is shown top right, hanging beneath the beam end. This assembly of connecting rods is Watt's 'parallel motion mechanism'. This device was invented by James Watt and it is the innovation of which he was most proud. Its purpose is to keep the crankshaft (connecting beam and piston) vertical, whatever the beam angle. Why? To increase efficiency--energy is wasted if the vertical power stroke of the piston is dissipated in horizontal motion. The geometry of the parallel motion mechanism reduces the horizontal component of motion to one part in 4,000. Eighty years later a Frenchman, Peaucellier, invented a linkage mechanism that produces a perfect straight line from circular motion (of the beam).

One of Renaud Beffeyte's full-size trebuchet reconstructions. (Registered pattern --Renaud Beffeyte.)