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Steven Vogel: Cat’s Paws and Catapults:
mechanical worlds of nature and people
(Norton: 1998)


“Life’s what biology’s about. Technology is something else altogether. Or, so I believed before I got into a kind of biology that’s about technology as well as life. More to the point, it - biomechanics - looks at the technology of life, at the mechanical world of nature. Sometimes that world resembles the mechanical world that we humans have created. But sometimes the two differ strikingly. This book compares those two technologies. It’s about the ordinary things and creatures around us; it intends, immodestly, to change the way you look at your surroundings - at least a little.... At the same time, I want to inject an element of sobriety into our romantic view of living things. The elegance of natural design seduced a lot of us into becoming biologists. Nature does what she does very well indeed. But - and here’s the rub - why should she do so in the best possible way? And, why should she provide a model for what we want to do? I want to ruffle our tendency to view nature as the gold standard for design, and as a great source of technological breakthroughs. Beyond that, I want to argue that natural design provides no honest foil for skewering human technology.”
(Vogel, pp.9-10)

One of the more glaring absences from the Humanities, as they are generally conceived today, must be the total absence of any serious attempt at taking seriously the built world we inhabit, the skills, understandings and ideas which underpin it, and the, shall we say, “engineering” mindset which has generated it. For all of the skilled and insightful analyses of writers such as Henry Petroski and James E. Gordon - perhaps the exemplars of what we might call the “history and philosophy of engineering” - they simply might as well not have written, if their impact on the Humanities was to be the sole criterion of judgement...

But this, of course, is paralleled by the very similar lacuna in relation to the life sciences...a remarkable feat, is it not, that the study of “man” can so easily dispose both of the fundamentals of nature and environment...leaving the way clear, presumably, for the usual language games. Thankfully, however, for those of us not trapped in said prisonhouse of language, we have a marvellous guide to these two worlds of praxis, in the shape of Steven Vogel...a highly literate biomechanic, with a genuine gift for explaining both of these worlds - and for helping us understand exactly how & why they differ...


“Ours is a single world.... Human technology may have become vastly more complex, but it has lost diversity.... [However,] we do have an alternative technology as a mirror in which to view our own...[and,] as systems to compare we could ask for nothing better than nature’s designs and human inventions. Nature’s technology occurs on the surface of the same planet as that of human culture, so it endures the same physical and chemical limitations and must use the same materials. But nature copes and invents in a way fundamentally different from what we do.... Just look around you. Right angles are everywhere: the edges of this page, desk corners, street corners, floor corners, shelves, doors, boxes, bricks, and on and on. Then look at field, park, or forest. Where are the right angles? Absent? No, but rare, which raises questions. Why so few right angles in nature? Why do civilizations find them so serviceable?”
(Vogel, pp.15-16)

“Natural and human technologies differ extensively and pervasively. We build dry and stiff structures; nature mostly makes hers wet and flexible. We build of metals; nature never does. Our hinges mainly slide; hers mostly bend. We do wonders with wheels and rotary motion; nature makes fully competent boats, aircraft, and terrestrial vehicles that lack them entirely. Our engines expand or spin; hers contract or slide. We fabricate large devices directly; nature’s large things are cunning proliferations of tiny components.... But, one can easily make too much of these differences. Both bicycle frames and bamboo stems take advantage of the way a tube gives better resistance to bending than a solid rod. A spider extends its legs by increasing the pressure of the fluid inside in much the same way as a mechanical cherry picker extends to prune trees or deice planes. Both technologies construct things using curved shells (skulls, eggs, domed roofs), columns (tree trunks, long bones, posts), and stones embedded in matrices (worm tubes, concrete)....[and,] both use corrugated structures to get stiffness without excessive mass - whether the shell of a scallop, one of the rare swimmers among bivalve molluscs, or the stiffening structures of doors, packing boxes, and aircraft floors, or fan-folded paper and occasional roofs.... So, is this a book about copying nature? Emphatically not. As we’ll see, on surprisingly few occasions has copying proved useful. Indeed, felicitous transfer of bits and pieces should not be expected. We’re dealing with separate contexts of mechanical design, each system uniquely integrated by its own elements of internal harmony and consistency.”
(Vogel, pp.16-19)

“Consider a cat’s ear and a door hinge. In one technology, orientation is changed by making things bend; in the other, by making them slide or roll. [But] my plastic file box has bending hinges; when my joints move, my bones slide along one another. Once again, the distinction between the two technologies is one of degree and default. And once again, such things as historical and evolutionary continuity, the availability of materials, and the modes of manufacture underlie the distinction, in short, the factors that underlie the different ways objects get designed and built. If you want a context for the present comparisons, consider whether without the contrasting world of natural design, you would have wondered about the consequences of living in structures and using devices that are built of stiff stuff. Or, would you have guessed how multifaceted is flexibility? What’s most familiar biases our thinking, and what’s most familiar is mostly what we ourselves make.”
(Vogel, p.105)

By thus contrasting the two systems, Vogel manages to genuinely illuminate the characteristic modes of action - and hence, thought - which each relies upon much more than had he simply examined biomechanics - or human engineering - in isolation. And, as his probing questions make clear, Vogel is hardly content to remain on the level of praxis alone - for it is, in fact, systems as a whole he wishes to interrogate, and the result casts a highly insightful light upon characteristic modes of thought and action unthinkingly dismissed by most so-called “Humanists” today. Moreover, it is also a relevation to, in effect, see our technology through nature’s eye’s (and visa versa)...albeit the conclusion will be hardly to the liking of the romantics amongst us:


“The dazzling diversity of the living world too easily disguises the fact that the evolutionary process faces constraints much more severe than anything impeding human designers. We biologists recognize these constraints, but we don’t often rise above our natural chauvinism and make enough public noise about them. Every organism must grow from an initially smaller to an ultimately larger size. Nature, in effect, must transmute a motorcycle into an automobile while providing continuous transportation. The need for growth without loss of function can impose severe geometrical limitations...[and] only one major group has reconciled support and growth. We vertebrates have a skeletal system that can grow and remodel itself continuously. By contrast with mollusc shell and arthropod cuticle, bone is a living tissue...[which] may be the greatest vertebrate innovation, the central item in our success as moderate-size to large creatures. [Furthermore,] organisms must reproduce, so...that’s adding a requirement that the automobile must tow behind it a factory for making automobiles, or at least making motorcycles.... [And] after growth and reproduction comes dispersal. Sometimes the three may be done by a single form of an organism, as in creatures like us that have simple life histories. In other cases, elaborate metamorphoses may separate dramatically different forms. We’re the unusual ones; metamorphoses and the resulting complex life histories characterize most plants, many insects, and almost all of the great diversity of marine invertebrates.... Such conversions must seriously constrain the range of possible designs.”
(Vogel, pp.23-5)

"Other limitations are imposed by what we might call informational constraints.... As we’ve learned from our computers, two-dimensional representations - graphics - absorb far more memory than mere text...[but] organisms, though, are three-dimensional, and details as fine as a millionth of a millimeter are important.... Thus, the shape of an organism has to be set by, relatively speaking, a very sketchy set of plans.... This shortage of information clearly underlies a lot of biological design...[such as] building large organisms out of lots of cells.... The evolutionary process has its hands tied in yet another way. Every organism is a product of its particular evolutionary history. Such a history limits design far more than ensuring that today’s disc will work in yesterday’s  computer. It’s tempting to assume that every organism is optimally attuned to its personal circumstances as a result of its lengthy evolution, but it’s profoundly wrong. Ancestry traps an organism.... As a designer, then, nature is not only glacial in speed, but lacking in versatility and erratic in performance. Fundamental innovation comes hard and, once achieved, it disseminates almost entirely within a lineage. To a remarkable extent, the dazzling diversity in nature represents superficial features of an exceedingly conservative and stereotyped character...and the bottom line is immediate profit - surer reproduction. Trial, error, patience.”
(Vogel, pp.25-31)

“How curious a process is ‘design’ in nature! For better or worse, composites are what a mindless, blundering, information-starved, and minimally-coordinated system might be expected to make. Specifically, their properties are highly sensitive to tinkering with the amounts and arrangements of their constituents on a microscopic level. In short, they’re what one ought to expect from microscopic improvisation, natures way, as opposed to macroscopic deliberation, our human mode.”
(Vogel, p.127)

“Design in human technology generates change and progress in a way that’s a lot easier on the intuition than design in nature. On the other hand, it’s a lot harder to encapsulate succinctly.... The design process clearly involves the planning, anticipation, and deliberation of which natural selection is incapable. But the really basic items of human design are so old that we know little or nothing of their beginnings. Who, after all, invented the right angle? Who first fabricated things from metals?”
(Vogel, pp.31-2)

The sheer range of relevant aspects of technology that Vogel manages to survey in this book is quite staggering, and well beyond the capacity of this review to survey. Nonetheless, mention (at least) should be made of his treatment of the physics of cracking, the different structural qualities which emerge from relying upon either tension or compression...not to mention “engines”, levers, wheels, “batteries”, and more. And, if your eyes glazed over automatically once I started listing said topics, be assured that Vogel’s explanations are never less than clear - even to the mechanically-inept, such as myself - his examples (from both technologies) never less than fascinating, and his prose invariably poised and incisive. We simply cannot better this book, as a way into the world(s) of mechanical praxis.


“Size matters, and like evolution, it will pervade all that follows. For one thing, an effective design for large things often works poorly for small things, and vice versa. For another, our two mechanical technologies span an enormous range, from a virtual macromolecule to the largest of human structures. For yet another, nature’s products are generally smaller than ours.... [Moreover,] not only are most organisms smaller than we, but in most groups smallness is the ancestral condition.... Nature starts small. Organisms are basically built up from cells rather than divided into cells; the earliest fossils are microscopic. Human technology goes the other way.”
(Vogel, pp.39-41)

The full implications of this basic difference - like the natural requirement for growth within functionality - are profound, and extend across several key phenomena. At the cellular scale (the key functional/manufacturing level in organisms) and right up to that of insects and small mammals and birds, processes tend to be dominated by viscosity, surface tension, and diffusion...whilst above that, gravity and inertia become the dominant forces to be reckoned with. Thus, the simple scale of our basic “building blocks” determines which world we live in, on a functional level - even though both of these technologies, in truth, exist on the very same planet. Try this for an example:


“Consider the moving waves made by wind blowing across a body of water. What keeps them wavy is the water’s inertia. What makes the waves flatten out are the water’s surface tension and weight. For ripples two-thirds of an inch or less between crests, surface tension is the more important...[but] for larger waves, weight - gravity - predominates, and...the shift makes big waves and small waves behave very differently. In particular, the relationship between the size of waves and the speed at which they roll along depends on whether they are big or small. For big waves, bigger is faster...[and since] an ordinary boat can’t easily exceed the speed of waves as long as its hull...large ships go faster than small ones, and even small ones go faster than ducks and muskrats. But for tiny ripples...the rule is just the opposite: Smaller is faster. The world of a minute surface boat, such as a whirligig beetle, must be something like a freeway with small, fast sports cars and large, slow vans.”
(Vogel, p.51)

Cat’s Paws and Catapults is packed with such examples, Vogel not only pointing out things which may often appear too obvious to be noteworthy, but also explaining the reasons why they are so, and drawing attention to how various factors interrelate to shape these mechanical worlds in such distinctive ways. And, in summarizing the differences between them, he produces such a lengthy listing that, in fact, it appears surprising we so readily assume that nature readily offers us useful models for emulation in this sphere. Yet, of course, that is the truism...


“The more closely we look at the technologies of natural selection and human contrivance, the less similar they appear. We might well have guessed otherwise, in the light of their common situation. Life has proliferated on our planet for several billion years, and we’ve been making things for a million or so - ample time for underlying imperatives to make themselves felt. Yet these basic differences persist:

* Nature uses fewer flat and more curved surfaces than we do.
* Ours is a far more rectilinear world while nature shows little bias in favor of right angles.
* Corners in our technology are abrupt; nature’s are more often rounded.
* Numerous mechanically separate but individually homogenous components make up our devices; nature uses fewer components whose properties vary internally.
* Nature’s designs take advantage of diffusion, surface tension, and laminar flow; gravity, thermal conductivity and turbulence matter more for ours.
* We most often design to a criterion of adequate stiffness, while nature seems more commonly concerned with ample strength.

* Partly as a consequence, our artifacts tend to be more brittle while nature’s are tougher.
* As another consequence, our things move on sliding contacts between stiff objects, whereas nature’s objects bend, twist, or stretch at predetermined places.
* As an additional result, we minimize drag with streamlined bodies of fixed shape, but nature often does so with nonrigid bodies that reconfigure in flows.
* Human technology makes enormous use of metals, while metallic materials (as opposed to materials containing metal atoms) are totally absent in nature.
* As a result, we use the ductility of metals to prevent crack propagation; nature does as well, but with foams and composites instead.
* We more commonly load materials in compression, while nature more often loads in tension.
* Concomitantly, we make greater use of shear preventatives such as nails and mortar to keep stacked objects aligned.
* Structures with tensile sheaths outside and pressurized fluid inside are both more common and more diverse in natural designs than in ours.
* For such hydrostatic and aerostatic systems, nature’s predominant fluid is water, while our structures mostly contain air or some other gas.
* We make profuse and diverse use of rolling devices based on the wheel and axle; but things rarely roll in nature, and only one true wheel and axle is known.
* Our prime movers - engines - are based on rotation or expansion; most of nature’s are based on sliding or contracting.
* Many of our engines extract mechanical energy from temperature differences, whereas all natural engines are isothermal.
* Levers in human technology most often amplify force at the expense of distance, while nature’s commonest levers amplify distance at the expense of force.
* Our devices store mechanical work as electrical, kinetic, gravitational, or elastic energy; nature mainly uses the last two and most often the last one.
* Our fluid transport devices often interchange pressure drop and volume flow, but equivalent transformers are rare in nature.
* Surface ships have long played an important role in human technology, but nature overwhelmingly prefers submarines.
*Our factories dwarf the items they produce; natures factories make products far bigger than themselves.
* We judge our devices best when they need only minimal maintenance, but nature’s devices get continuously rebuilt.
* Our technology is as dry as nature’s is wet.”
(Vogel, pp.289-91)

“Listing differences hints at interrelationships among them. If gravity dominates, then stiff materials to resist it find special utility. Stacking becomes a reasonable way to build things, with shear preventatives like nails to prevent sliding. Permitting such sliding here and there then defines joints. And so on. Using metals makes their special properties available (for instance, high thermal and electrical conductivities) and allows otherwise impractical devices (such as wires) and construction methods (like pressing and forging). Composites drop from crucial to simply useful. And so forth. Each domain, nature’s and ours, thus develops its distinctive coherence, consistency, and rationality, each a well-integrated entity in its particular context. Might we mix and match among the features of the two technologies, generating a vast number of further ones? All but a few would surely lack that degree of coherence, consistency, and rationality; the combinations of features that mark each of our two transcend historical accident. What determines the kinds of devices that a technology finds effective? For one thing, its physical situation: the size of a technology’s artifacts, whether its basic medium is air or water, whether it works at a surface or suspended in a gas or liquid, and so forth. For another, how it goes about doing things: production methods, degrees of resistance to revolutionary change, relative ease of technological diffusion and, once again, so forth. Even social interactions matter to how a technology goes about its business. Nature faces severe limits when organizing and coordinating the efforts of individuals; one might say ‘institutional limits’ to sharpen the comparison with human efforts.... [And] our facility for generating transportation systems also means that we can transcend nature’s need to make things out of locally available materials.”
(Vogel, pp.291-2)

“A list of similarities turns out to be such a scattershot mix of major matters and minor details that we gain little from the itemization. [Moreover,] most similarities between the technologies emerge from inescapable physical rules and environmental circumstances, both matters we’ve already dwelled upon...[And,] at this point, more subtle and abstract similarities hold more interest: similarities of process and historical trajectory rather than of product, similarities for which physical context provides no adequate explanation.... I’m persuaded that comparing the products of the two technologies 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 limitations 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 professional biologist.”
(Vogel, pp.292-8)

Steven Vogel’s Cat’s Paws and Catapults is exactly the sort of book specialists should produce for the more general readership. Literate, witty, comprehensive, and full of unexpected insights into both nature and our technological world, it shows how both natural and human engineering work as coherent systems...thus teaching the rest of us how engineers think from a uniquely comparative vantage-point. When Herodotus fathered history (and anthropology), with all he could find out as his starting-point, and Terrence claimed that “nothing human is alien to me”, they were setting the correct goals for the Humanities...and, it is to our shame that we have rarely tried to live up to them. The world of praxis - the hands-on world of practical creation & invention - is a central part of what it means to be human, and any Humanities worthy of the name needs to comprehend it. And, with Vogel’s work as a starting point, that task will be both thought-provoking, and entertaining to boot...


“This view of nature as a technology has provided an unusual perspective on the world around us...[and] identifying specific devices we might profitably emulate constitutes the least of what we can gain.... That each technology is a coherent entity, remarkably distinct from the other, can be either advantage or disadvantage. Perhaps the best encapsulation, if a little trite, is that nature shows what’s possible. Disparaging things were said about analogies, but real utility balances the risk analogies pose of short-circuiting proper analysis and explanation. A useful tool emerges from recognition that the technologies involve analogous time courses of development and ways of operation. One can test the logic and credibility of hypotheses about how one system operates by examining what happens in the other. We’ve seen places where the products of the two technologies coincided. And we’ve seen places where the products proved surprisingly different. Each may carry a prescient message. Coincidence between these vastly different technological contexts directs attention to constraints that neither can escape, constraints that we must try to identify. Different solutions to the same problems or different devices for the same task imply something equally interesting: the possibility of a third or fourth solution or device.... Since major innovation isn’t easy, a little help might go a long way....For the human designer, a perceptive look at nature’s technology can...provide the wide-angle view that reveals possibilities that would otherwise escape consideration.... Or, perhaps...the value of an external reference for any attempt at understanding.”
(Vogel, pp.309-11)



John Henry Calvinist