Sunday, October 14, 2007


kw: book reviews, nonfiction, technology, history, physics

No doubt many little boys have done what I did, to try to make a bow out of a green stick and a piece of postal twine, and tinkertoy arrows. We played endless games of "cowboy and indian". While we were warned by our parents not to use the bows in hot pursuit, we did try them out for target practice against the side of the house...if they would shoot that far.

But who, without ever seeing a bow, first thought to make one, and turn it into a hunting or warmaking weapon. Archery is so old, it may be pre-Cro-magnon. At least it predates written history by a few millennia. In the closing chapter of Ingenium: Five Machines that Changed the World, Mark Denny speculates on just this point, but the note is mainly romantic. Whether, for example, the fire-starting bow came first or not is a chicken-and-egg sort of speculation.

In the first chapter of the book, Dr. Denny outlines the history of arrow- and bow-making from the middle ages to the near-present. His interest in the book is to show first, the huge effect the bow and four other technologies had on the development of human civilization, and then to outline the physics of each mechanism to show how it can be a teaching tool with physics students. His explanations are clear enough that those without the math background are not left uninformed, but brought to at least a qualitative understanding of the physical and mechanical principles.

Mills powered by both water and wind are his second subject. Denny outlines the development of water power, beginning with undershot mills, then the more efficient overshot (as shown here), but then shows how a better analysis led to undershot mills with better vane design that are as efficient as overshot mills, and how both led to the turbine. Here and in two other chapters he gives us brief biographies of those responsible for key developments.

Because of the need for at least a little elevation difference, water mills led to the development of dams, sluices, gates, and other technologies, long prior to their need for water-retention purposes.

Windmills employ the same physics principles as water mills, with the proviso that the driving fluid is more compressible. The more fickle nature of wind adds requirements for steerability and variable-efficiency designs.

I know of mills that used both wind and water power, and could switch between them as the seasons dictated (you can use wind when the river is frozen, for example). But I couldn't find a usable picture of one to show here.

Interestingly, we all made little whirligigs and other wind-powered devices as children, but I don't know of anyone that made a working toy water mill. We did, however, dam up a lot of creeks, mainly to see how much water we could impound. We used the same "rule" used by beavers: add stuff where the trickling sound is the loudest.

I've made several small trebuchets. Firstly, to figure out how they work in the first place, later in collaboration with my son and a friend for Science Olympiad competitions. We always came in third in the state. We concluded we are good designers, but not great ones. Great fun, though. It gave the neighbors a start when we tested a machine smaller than a camp chair, that could throw a golf ball 25-30 meters.

I learned that "trebuchet" refers just to the counterweighted catapults that have a hinged counterweight. A similar device with a weight fixed to the throwing arm, whether on wheels or not, is "mangonel". I learned, first from earlier reading, and here in more detail from Denny, that the hinge or wheels are needed to increase efficiency by allowing the counterweight to fall in a nearly straight path; also that the sling greatly increases efficiency.

The word "Ingenium" was first applied to these siege engines. In English, it morphed in to "Engine", and the attendant "Ingeniers" into "Engineers."

At age nine or ten, I was given a defunct wind-up alarm clock by my father. It was one of these old monsters about six inches in diameter, with two springs so the alarm didn't run off the time spring. It didn't take me long to get it apart. Somehow, I had the foresight to take note of the way the gears and plates fit together, because I got it back together, and it ran!

Antique clocks became a minor obsession for Dad and us boys. By the time we all moved out, he had about thirty running antique clocks, many of them small mantel clocks, that sat in a special bookcase he'd built into the den, with extra-deep pockets so a clock sat in front of each shelf of books. It sounded amazingly wonderful at noon or midnight. He gave a clock or two to each of us, and some of us collected others. I have several, but only keep one running. I am the only Westminster Chime enthusiast in the house at present!

The secret to the steady running of all pre-electronic clocks is the Anchor escapement. It has been adapted both to pendulum clocks and to balance wheel "carriage" clocks and pocket watches, and of course to the navitational clocks that the British government commissioned from the 17th to the early 19th Centuries. It and its older sibling, the crown and verge escapement, are speed-governing devices that probably led to the high-powered deviced discussed last.

The principle behind the flyball governor is far from obvious, when you first see one. At first, it seems that the rising weights absorb some power, and so regulate engine speed. This would work only for the very weakest of steam engines.

What it really does is this: the rising balls pull a sleeve up the shaft, and the sleeve is attached to a valve that opens as the balls fall, and closes as the balls rise. It is the quintessential negative feedback device.

The physics discussion showed that the device is very nonlinear, and that this characteristic causes great difficulties in the control of more powerful engines that must respond quickly to changes in load. The key finding is that there must be a certain amount of friction in the governor assembly. This causes it to lag its feedback just a little, then overshoot, so it allows a bit of a jerk in speed, then damps it quickly and helps it settle quickly. Too little friction, and there it too little damping to get engine speed to settle down quickly. That delight of physics students, a friction-free world (which makes the math much, much simpler) is very hard to control!

Get several historians and scientists together, and you'll get any number of lists of the inventions that changed the world most. Prior to the 20th Century, at least, these five get my vote (and while I share the thought that Agriculture had the greatest influence of all, it isn't a device).

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