Vogel: Cat's Paws and Catapults

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Steven Vogel, Cat's Paws and Catapults : Mechanical Worlds of Nature and People. New York : W.W. Norton & Company, 1998. 382 pages, with notes, references, and index. Illustrated with graphs, diagrams, drawings, and photographs.

I was a bit distrustful of this book, judging entirely by its cover. The title seemed almost frivolous to me, and the subtitle promised things I've been promised before by books that failed to deliver.

I couldn't have been more wrong: Steven Vogel delivers. Some with a more accounting bent would love to categorize this book as "popular biomechanics" and move on, but just calling it "biomechanics" is too easy. Vogel looks at natural engineering in biological forms as wrought by natural selection, and at human engineering in technological products, and then weaves the two together into a whole cloth that is delightfully greater than the sum of its parts.

Vogel asserts that he is but a poor biologist trying to understand the engineering and learn a few lessons, but his modesty is misguided. His appreciation for and understanding of the engineering analogies of the biological systems, and the biological analogies of the engineered systems, is profound, and he lays it all before the reader with direct and precise language.

Here at Ars Hermeneutica we look favorably upon any book in any subject that exhibits an analytical approach to its subject, so that it may serve as an example. Cat's Paws and Catapults has it in profusion. Barely a sentence goes by without some perceptive analysis. Here, for instance, is a comparative discussion of stiffening plates, in natural and human technology:

When it comes to stiffening flat plates with a minimum of material, nothing touches insect wings. insects commonly invest only about 1 percent of body mass in their wings. Yet the wings move at several meters per second through the air, and many reverse their movement several hundred times each second. To get sufficient stiffness for this demanding application, they combine curvature, veins, and lengthwise pleats. Curvature may be an aerodynamic necessity in all but the very small insects, but it presents a special problem for beating wings: The direction of curvature that's right for a downstroke is wrong for an upstroke. Many insects have a fine solution: Their wings are built so they're curved by the force of the wind that strikes them. Since the wind on the wings reverses between upstroke and downstroke, their curvature reverses too.

Thin, flat surfaces deflect with even small loads. The rounded shapes of our automobiles seem made for minimizing drag and wind noise and for maximizing sales appeal—the latter perhaps by tacit allusion to well-curved human bodies. In fact, the roundness of cars serves primarily to stiffen them, if more attractively than folds or ripples. Pressing a piece of metal into a curved shape is much simpler and uses less material than spot-welding a lot of stiffening strips to a plate. Sometimes we use a cruder fix; heating ducts are most often rectangular in cross section, so they form huge arrays of thin, flat, metal sheets. Minor changes in pressure or even temperature all too easily bow these sheets sideways and produce disconcerting noises. We minimize the problem (but don't fully fix it, at least in my house) by putting slight diagonal creases in the flat surfaces. [p. 62]

The implication is that Vogel writes with a great economy of words and expression, with a consequent increase in the density of ideas in the text. There's a lot "why?", and a lot of answers in this book. But the reader's response is obvious: read a bit more slowly and savor the ideas.

Sometimes little gems just slip out without effort; the following example is from a figure caption. The metric unit of force is the kilogram-meter-per-second-squared, a unit named the Newton. Newton, of course, is noted for the story of the falling apple and his theory of gravity. But this is the first time anyone has given me this easy picture to grasp and remember just how big a Newton is:

A Newton is a force about equal to the weight of an apple. [p. 83]

Everywhere throughout the book is this weaving in and out of engineering and biology, biology and engineering, and what each says about the other. It's a fun and eye-opening path to take. After talking about cooking and thermal conductivity, Vogel moves on to serving the food.

When the food reaches the table, though, high thermal conductivity becomes a nuisance. The coldness of metal and the warmth of wood—celebrated in aphorisms and figures of speech ("cold steel," etc.)—is the perceptual signal of the difference in thermal conductivity. A piece of aluminum feels colder than a piece of pottery because the aluminum more rapidly conducts heat away from your hand. Similarly, a silver or aluminum dish conducts the heat away from the food, increasing the area over which the heat is shifted to the surrounding air and moving heat to the place where you're holding the dish. We ate cold food from metal mess kits on scout trips but blamed it entirely on the weather. Tea or coffee cooks more rapidly in a metal pot, especially in aluminum, copper, or silver, which have the highest conductivities. According to at least one guide, the governor of colonial Williamsburg, in Virginia, ate off silver dishes—the worst of all possible materials. Since cooking was done in an outbuilding, one suspects that no governor every enjoyed a nice hot meal at home. Conversely, metal handles often get uncomfortably war. Stirring a cup of boiling liquid with a silver spoon shows the downside of high thermal conductivity. The upside is good value as radiators for cooling internal-combustion engines and for steam heating in homes.

The high thermal conductivity of metals would be useful to living things in a number of situation. When large and moderate-size animals do heavy physical activity, they generate waste heat that they have to transfer to their surroundings. High conductivity would help get the heat from muscles to skin, but human conductivity is low, about that of water. So we resort to convection and evaporation instead of conduction. In convection, heat is moved from one place to another by physically moving some heat4d material rather than just shifting the heat from one bit of material to the next. The usual materials moved are fluids—hot blood and hot breath—and moving them requires pumps and energy. In evaporation, heat is moved by making water evaporate and then moving or discarding the water vapor. Evaporation takes energy, which the vapor contains; when the vapor drifts away, the body is left cooler. We sweat; dogs pant; we both lose water thereby. But water must be obtained and carried around, and both activities can prove troublesome. [p. 125]

Some authors shy away from using mathematics of any sort in their text, often leading them to ridiculous circumlocutions to explain ideas that could be far more easily explained with an equation or a fraction—and still not scare the average reader. I was pleased that Vogel took this approach. Here, for instance, is his clear and economical way of explaining this difficult concept with just a little math.

Flying insects use elastic storage, and in their wing hinges are pads of the best elastic polymer known to either technology, resilin, which got brief mention in Chapter 2. Resilin was discovered about 1960 by a great Danish scientist, Torkel Weis-Fogh, who measured its resilience as an astonishing 97 percent. Resilin thus loses only 3 percent of the energy put in rather than the 7 percent of our collagen. The difference in power economy, though, is probably trivial, for 97 percent is only a little better than 93 percent. What must matter more is the mischief caused by any energy that's lost: It turns into heat, and 3 percent is less than half of 7 percent, which means that the muscular flight motor can be run a lot harder without cooking itself. [p.203]

Endlessly fascinating observations, blending biology and engineering, seem to litter the narrative, a narrative that nevertheless is quite coherent and organized. There are any number of curiosities to pick up and examine, and each one has a little conceptual lesson on offer.

A person could hang from a patch of standard Velcro less than five inches in diameter, but such a patch can be peeled loose with one hand. To hold strongly but release with ease, Velcro (like adhesive tape) takes clever advantage of what's called a dimensional reduction. It holds against a force extending over a broad area, while it peels in response to a force concentrated along a line; it attaches in two dimensions but releases in one. A pair of scissors or a zipper does the same thing, except that instead of reducing an area to a line, it reduces a one-dimensional line to a nondimensional point. To bring matters full circle, dimensional reduction is important in nature as well. As they get food, lots of animals peel and tear—that is, they reduce an area to a line or a line to a point; that's how a cat uses its claws and a caterpillar cuts through a leaf. Conversely, some limpets have broad skirts that make them hard to peel off rocks. Also, grasses have lengthwise veins that help them resist crosswise tearing. [pp. 269—270]

Vogel's understanding, which he tries very hard to share so we can appreciate its sweep, is profound and not just limited to observations of details, but comprehension of the more abstract as well. And, he has drawn lines, as he explains in his conclusion:

Similarity of product need not imply similarity of process. I'm persuaded that comparing the products of the two technologies [nature's and humankind's] lends breadth to our thinking and gives insights not otherwise evident. About processes I'm more equivocal. Natural selection is a most peculiar process, and its limitation are inadequately appreciated. One often encounters analogies between the processes by which human technology changes and evolution by natural selection. I think these badly need the scrutiny of a biologist. To start with, a clear distinction ought to be made between mechanism and history—the distinction we biologists make between natural selection and natural history. The more basic problem is that an analogy doesn't explain. One judges an analogy not by whether it's true, trivial, or untrue but by whether it's useful, nonuseful, or misleading. [p. 298]

I had great, great fun reading this book, and I learned a great deal from reading it. I can recommend it to most general readers who find the topic at all appealing.

-- Notes by JNS