ElectroportalElectricChargerBicycles,Tricycles,EfficientLighting,andmore
British Medical Journal 1994;309:1741-1745 (24 December)

 British Medical Journal (December 24, 1994) ;309:1741-1745
 
 Hobby horses
 De inertia urbanorum
 Ronald Williams, general practitioner
 London SW1X 9SW
 
 When I first started using a bicycle for my home visits in central
 London 22 years ago I had in mind only fine sunny days. But I saved
 even more time when it was wet, for the cars then clogged the streets
 in even greater numbers. Now, 87000 cycling km (54000 miles) later,
 the disparity between the slowness of the motor car and the swiftness
 of the bicycle is so apparent that some comparison of these
 vehicles--and their effects--is called for.
 
 In the first century BC Cicero knew the difference between inertia and
 velocitas. But he can hardly have imagined, or been equipped to

 analyse, the complexities of the machines and mechanisms now in
 question, even though the remarkably quiet and highly developed power
 unit of the modern bicycle is identical in all important respects to
 that used by the ancient Romans in getting themselves on foot to the
 forum.
 
 Furthermore, the efficiency of the car engine in converting chemical
 energy into mechanical work is only 20%1 whereas that of the cyclist
 is about 25%.2 This is hardly surprising, for our fuelling, motor, and
 control systems have been refined and tested, often to destruction,
 under appallingly rigorous conditions over 500 million years.
 Fortunately, we can still use this clean and extremely complex
 equipment to advantage even in 20th century London, although not while

 we remain strapped in our cars. It is a source of continuing wonder to  
 me that healthy general practitioners will sit inert behind their
 steering wheels, queuing for half an hour to cross a bridge, when they
 could reach their destination in a fraction of the time on a bicycle.
 It is not as if all that much effort were required, for the cyclist
 uses five times less oxygen and energy--than the walker in covering a
 given distance,3 and 60 times less than the car, assuming average,
 moderate speed.4
 
 Our technology now allows us to tap the energy reserves of the earth
 to a degree which would have been unthinkable 200 years ago. The known
 reserves of oil have risen by 10 times between 1950 and 1990,5 and
 reserves of coal and natural gas have risen equally remarkably. Our

 use of these resources is rising at a global rate of 2% a year,5 and
 estimates of how long the supply will last vary from 100 to 600 years.
 World total energy consumption is increasing even faster than the
 continuing exponential growth in population--from 1 terawatt (1 x 1012
 watts) in 1890 to 3.3 in 1950 and 13.7 terawatts in 1990.6 By 1991 the
 world's commercial energy consumption had risen to around 8000 million
 tonnes of oil equivalent energy (10.7 terawatts) a year.5 Ninety five
 per cent of this energy is derived from fossil fuels and fuel for
 transport now accounts for 60% of total oil consumption.5
 
 These advances have made possible, at least in developed countries and
 particularly in cities, a level of physical inertia for which we may
 not yet be physiologically adapted. Some town dwellers, no doubt
 sensing this danger, try to raise their activity level by frequenting
 golf courses, where they can be seen ambling gently about or simply
 just standing. But for the real inertia enthusiast--if this is not too
 much a contradiction in terms--no one spoke out more honestly than
 that splendid, if slightly overweight American, Robert Benchley:
 "Whenever I feel the need for exercise, I go and lie down for half an
 hour until the feeling passes."
 
 A recent advertisement seen above a traffic jam in London announced a
 new British car which can accelerate from 0-60 mph (0-96.5 km an hour)
 in 6.2 seconds. Now I have the highest regard for this particular make
 because I have a much cheaper and less sporty model myself and am
 devoted to it, but the energy implications of such acceleration are
 worth noting. In my own car it takes me not 6.2 but 22 seconds to
 reach that speed, and applying Newton's second law of motion, f = ma
 (force = mass x acceleration), the energy--and therefore the
 fuel--required for these two acceleration rates differs by a factor of
 3 1/2, although the time saved by such acceleration in relation to a
 completed journey-- assuming that speed limits are respected--is
 small. It would be so nice if exhaust gases did not matter.
 Unfortunately, it seems that they do.
 
 In 1990 carbon dioxide emissions into the atmosphere in Britain
 totalled 160 million tonnes of carbon. 7  Of this, over 70% came from
 power stations, industry and agriculture, and households, in that
 order. Road transport accounted for 19%--30 million tonnes of
 carbon--and just over half of this derived from the private use of
 cars. As this country's contribution to the total world output of
 carbon dioxide is barely 3%,7 the global tonnage is quite considerable
 and carbon dioxide accounts for roughly half the climate changing
 impact of the greenhouse gases. 8
 
 The cyclist's output of carbon dioxide is from 1/60 to 1/100 that of
 his or her car,4 and the cyclist breathes out no oxides of sulphur or
 nitrogen and no burned or unburned hydrocarbons. Cyclists have the
 added advantage that they can calculate the moments of inertia of
 their speeding front and rear wheels with a clear head. Ask London
 motorists about their cars' moments of inertia and all they can think
 of is their local main road or the M25.
 
 Unprecedented levels
 
 According to a recent discussion document from the Department of the
 Environment, "the concentrations of greenhouse gases in the atmosphere
 are now approaching levels unprecedented for at least the last 100000
 years." 7 The particular greenhouse gases implicated are not specified
 and no reference is given for this statement, but as it comes in a
 government publication it must be true; and indeed it is, although not
 scientifically precise, for the methane and carbon dioxide
 concentrations 100000 years ago were only average. 9 It was 30000 years
 before that, at a time of high temperatures, that these two gases
 reached the peak which brought them nearer to present day levels.
 
 Such seemingly esoteric information derives from the analysis of air
 bubbles entrapped at various depths in the ice below Vostock, the
 Russian research station in central east Antarctica and one of the
 coldest places on earth. This decidedly frigid billet is at an
 altitude of 3490 metres (11450 ft), has an annual mean temperature of
 -55°C, is at latitude 78° 28'S and rests on an ice sheet 3700 metres
 (12100 ft) thick. 9 It is not at all a comfortable surface for the
 cyclist, and were it not for the motor car and the Department of the
 Environment's anxiety about pollution we would not be considering such
 an inhospitable place here.
 
 But for the record, at a depth of 2000 metres (6560 ft) the Vostok ice
 cores and their contained air bubbles are believed to date back 150000
 years, and this assumption tallies well with climatic information
 deduced from deep sea sediment studies.9 By early 1993 the drill had
 reached 2546 metres (8350 ft), which takes the measurements well back
 into the penultimate ice age.
 
 Yet the Department of the Environment's statement needs putting in
 context, and the first thing to be said about the greenhouse or
 glasshouse effect is that it is not entirely a bad thing. Without it
 our atmosphere would be 30°C colder than it now is and life as we know
 it would not exist. 10 But what about 130 000 years ago and what about
 our ancestors, for surely they deserve a thought? The hominids around
 then in this part of the world were Neanderthals, who had evolved in
 Europe some 20 000 years earlier and were not to disappear for another
 90 000 years, when they were displaced by Homo sapiens sapiens,
 Cromagnon man, who almost certainly originated in Africa. By that time
 the last ice age was well established; it did not end until about 9000
 BC.
 
 We can say two things with reasonable certainty about Neanderthal
 people. Firstly, they did not own motor cars and, secondly, if they
 gave way to inertia they perished. They were probably also, according
 to experts at the Natural History Museum, compassionate beings, for
 there is evidence that their injured could live long lives.
 
 But if there were then no internal combustion engines; no domestic,
 industrial or power station use of gas, oil, or coal; and no large
 scale cutting down and burning of forests, what was pushing up the
 green-house gases 1 30 000 years ago? Surely this is a relevant
 question, yet the Department of the Environment is carefully silent
 about that, and with some reason, for the incredibly complex factors
 involved in climatic change are still not fully worked out.
 
 The two main greenhouse gases--if we exclude the most important agent
 of all, water vapour--are carbon dioxide and methane, 11 and both have
 been around for aeons, although methane was a relatively late arrival,
 probably not reaching significant atmospheric proportions until the

 Archean era--3.8-2.5 billion years ago--when the first anaerobic
 methanogenic bacteria appeared. 12  The appearance of free oxygen
 awaited the evolution of photosynthetic bacteria and plants 3.0-2.5
 billion years ago, and oxygen was not to arrive in significant
 atmospheric concentration until even later: two billion years ago. 13
 
 The atmosphere that resulted from the planetary accretion which formed
 the earth 4.6 billion years ago was, it seems, dominated by nitrogen,
 carbon dioxide, and, as always, water vapour.12  In greenhouse terms
 methane is more potent than carbon dioxide and it also is increasing,
 at 2% a year.14  Molecule for molecule it is a 21 times more powerful
 trapper of the earth's reflected heat than carbon dioxide, although
 chlorofluorocarbons are 12 000 times more powerful in this respect8
 and in 1990 were increasing by 5% a year. 14 The fourth significant
 greenhouse gas, nitrous oxide, is 150 times more powerful than carbon
 dioxide in this sense. 11 It arises in part from the denitrification of
 soils stimulated by mineral fertilizers, 14 is possibly also produced

 by lightning, and certainly produced by the motor car. The fifth
 greenhouse gas is ozone, which is approximately 2000 times as
 effective as carbon dioxide in retaining the earth's heat, 11 although
 present in far lower atmospheric concentration.
 
 The flatulent hippopotamus
 
 About 130000 years ago, when temperatures were high in the last
 interglacial period, it is possible that the surge in methane was
 related to the increase in area of peat and wet lands following
 glacial retreat, 15 the methane arising--as it does in rice
 paddies--from microbial decomposition processes in oxygen deficient,
 richly organic soils. It has also to be said that the ruminants and
 the odd flatulent hippopotamus would have contributed their not
 inconsiderable share. When climatic warming reached its peak 125000
 years ago this large and potentially lusty mammal is known to have
 frequented the area of Trafalgar Square and even to have trundled as
 far north as Leeds. Indeed, it extended even further north and west in
 Yorkshire, for in 1838 its bones were found beneath a layer of glacial
 clay deep under more recent deposits in Victoria cave above Settle,
 together with the remains of rhinoceros, bear, reindeer, and ox, all
 regular methane, and carbon dioxide, contributors. 16 The amphibious
 hippos would no doubt have kept to their rivers and swamps, but they
 had for a neighbour an eager collector of left over bones--and a
 natural tenant of that high cave--the hyena.
 
 Over short distances a hippopotamus can move faster than a man can
 run, and in my particular part of London it could almost certainly
 have splashed around faster than today's rush hour motorist can move,
 for a male hippo in the mating season seeing off a rival from the
 neighbourhood of Hyde Park Corner would not be constrained, as we are,
 by queues at traffic lights.
 
 But in this 20th century it is not so much methane as carbon dioxide
 which is our problem. It is estimated that some 150000 commuters enter
 central London by private car during the weekday morning peak between
 7 am and 10 am, the average length of their journey being 24 km (15
 miles). 17 About 738000 people come in by train. The carbon dioxide
 emission in vehicular exhaust at 22 km per hour (14 mph)--a fair
 average speed for these motorists--is 200 g/km, 18 and this more than
 doubles when the car is crawling. So at least 1440 tonnes of carbon
 dioxide are pumped into the atmosphere each working day by these
 commuters' cars alone. The equivalent figure for nitrogen oxides is
 14.4 tonnes and for hydrocarbons 28 tonnes.18 Nitrogen oxide emissions
 increase very little at slow speeds, but a car moving at 6.4 km per
 hour (4 mph) produces five times as much hydrocarbon, soot, and carbon
 monoxide as it does at 48 km per hour (30 mph),18 so cyclists may
 perhaps be forgiven if they overtake rather quickly.
 
 It has been calculated that the average car puts out four times its
 own weight of carbon dioxide each year, 11 and admittedly the 170000 or
 so broad leaved trees in the royal parks will mop up a small
 proportion of this, as will the trees in the many other parks,
 commons, heaths, and squares of London. But they can do this only in
 summer, and only growing trees absorb carbon. Dying ones release it,
 and a site must be stocked with trees forever if the carbon initially
 sequestered on planting it is to be removed permanently from the
 atmosphere. Our terrestrial vegetation is patently not keeping pace,
 and atmospheric carbon dioxide continues to rise, as it has done since
 1900. 19
 
 Apart from the plant kingdom, the only other "sink" for carbon
 dioxide-- the oceans and the rocks--cannot possibly absorb it at the
 rate at which it is now being produced, 20 and forest clearance by
 burning not only drives up atmospheric carbon dioxide but, more
 seriously, reduces the total amount of vegetation available to mop it
 up.
 
 Before the industrial revolution the level of carbon dioxide in the
 atmosphere was 280 parts per million.21 Perhaps not surprisingly this
 was also the highest level reached 130000 years ago during the last
 interglacial period9--a nice, clean biological maximum. It is now 360
 parts per million and rising,20 an enormous proportional increase in
 one component of our environment. The level towards the end of each of
 the last two ice ages (150000 and 10000 years ago) was 190 parts per
 million.9 Throughout the Vostock ice core record, both carbon dioxide
 and methane levels correlated closely with temperature-- the colder,
 the lower.
 
 Fuelling the crisis
 
 Recent American work suggests that if we continue to burn up our

 fossil fuel reserves until they are exhausted, even if forest
 clearance is now halted, peak carbon dioxide concentrations of 1000
 parts per million are probable within the next few centuries.20 If the
 forests are destroyed this figure doubles to 2000 parts per million.
 
 The potentially devastating consequences of too rapid global warming,
 not to mention the air quality at ground level in cities, may force us
 to rein back on our use of fossil fuels long before the known reserves
 are exhausted. Catalytic converters do nothing to reduce emissions of
 carbon dioxide, and they have their own snags and imperfections.
 
 Diesel engines produce significant amounts of sulphur dioxide and
 carbon particulates, both of which, like nitrogen dioxide, are known
 respiratory hazards, especially it seems to people with asthma. "In
 the United States over one quarter of all the chlorofluorocarbons
 manufactured for refrigeration are used in vehicle air conditioning
 units, from which they eventually leak or are vented out,11 so even
 some motor cars are now sharing in the assault on the ozone layer.
 
 Bicycles need no catalytic converter and their riders would spurn the
 idea of air conditioning. Atomic power seems to be increasingly
 suspect and unsatisfactory for a whole variety of reasons. Our only
 basic source of energy and continuing life is, as it has always been,
 the sun and the radiation from it is colossal. The solar energy

 absorbed by the earth in a month is roughly equivalent to the world's
 estimated ultimately recoverable reserves of coal, oil, and gas (2 x
 1023J or 4.579 billion tonnes of oil equivalent).22
 
 Silicon based photovoltaic cells now exist with a conversion
 efficiency of 15%, and efficiencies of 30% are believed to be
 possible.5 Allowing 50% for spacing, even such present day cells
 placed on a land area only one fiftieth that of Lake Nasser could--in
 the hours of daylight--theoretically equal the electrical output of
 all 12 turbines of the Aswan High Dam--2.1 milion kilowatts. And the
 current energy demand of the whole world could be met if only 0.1% of
 the earth's total surface--land and sea--were used as a collector to
 convert incident solar energy at 10% efficiency.22 There is, after
 all, space and to spare. A twentieth of the earth's land surface can
 be classified as extremely arid,23 and many of these areas, which are
 not habitable, lie at relatively low latitudes where the sun's rays
 are strong.
 
 The Nile valley, a winding green ribbon of amazing beauty and cultural
 magic coursing through often staggering scenery, is bounded on each
 side by thousands of square miles of empty desert on which a positive
 torrent of solar energy--of the order of 400 watts per square
 metre24-- beats down year after year to sink unharnessed in the sand.
 A yearning for clean air in our cities and an end to exhaust pipes
 does not entitle us to too much wishful thinking about the future, but
 the thought of this country trading with Egypt for totally clean
 Saharan electricity does have its appeal, for the new non-carbon
 technologies may well become crucial. Economic factors will no doubt
 largely determine the outcome, but when London is clogged with
 electric cars the bicycle will still be needed by travellers who wish
 to move freely.
 
 Using their own language
 
 The officials of the NHS will not so far have found this article easy
 reading. Papers in a foreign tongue are always difficult, and we must
 therefore address some words to them in their own language. No civil
 servants with time on their hands and a need to fill their day or
 their page will say "use" if they can say "utilise" or "see" if they
 can say "visualise" or write "start" if they can spell "initiatise."
 So we must above all eschew monosyllables, even though some of the
 most charged and poignant lines in all Shakespeare consist of nothing
 else--but then he had something to say.
 
 The phrases which follow--though altered in sequence--have all come
 through our practice's letter box from official sources in the past
 four years, and the sighting of any one of them has regretfully meant
 the immediate despatch of the whole document to the waste paper
 basket, for our hours are long and our time precious, and our reading
 has to be selective. No working doctor can ever be entirely at ease
 when dealing with the defensive, stale inertia of a hostile,
 overmanned and self regarding bureaucracy, but we have to try, for it
 has the NHS by the throat.
 
 So, to our administrators: "The mediation of inter-personal
 expectancies in the helper-helpee professions has to be
 visualised"--you will agree --"in the context of a shared

 understanding of user empowerment. The a priori decision making
 process will be facilitated by an integrated approach using an optimum
 skill mix, and uniform guidelines will help to underpin and complement
 the process of change. There must be a perceived need to develop
 finely tuned coping skills within a problem solving and goal setting
 framework and a supportive environment. By a layered approach, ongoing
 in-depth strategies will be developed utilising a meaningful protocol
 of interactive and proactive progressive modules, in which the
 subjects will be taught didactically and analysed from the
 collaborative viewpoint. Unfortunately, due to slippage of some of the
 planned projects, the intended prioritisation of goals may not be
 achieved, and the necessary initiatisation and pump-priming may have
 to be postponed, particularly in the development of consumer-facing
 skills within the primary care setting and the understanding of
 children of their place and involvement within the
 dependency-independence continuum."
 
 Such turgid, pretentious, even preposterous prose has nothing whatever
 to do with the clinical care of patients and indeed interferes
 inexcusably with good medical practice, for it and its authors have
 deflected huge public funds from the known and often desperate needs
 of ill people and their families. Ask any general practitioner,
 hospital doctor, or nursing nurse.
 
 George Orwell was right: "The great enemy of clear language is
 insincerity . . . When the general atmosphere is bad, language must
 suffer." He further observed: "If thought corrupts language, language
 can also corrupt thought."25 In Utopia this essay on the writing of
 clear English is required reading for all government officers; the
 need for it in present day Britain is obvious.
 
 But to return to the real world in which we and our patients live. We
 too must look to our credibility, for if as general practitioners we
 tell our patients to exercise while we continue to crawl round central
 London cushioned in our cars--the repeated excuse for our late arrival
 being the traffic or the parking--we could well attract the fair
 retort: "Physician, wheel thyself."
 
 Concorde; bicycles; inertia
 
 The amount of petrol that I have saved in my 87000 km (54000 miles)
 and 22 years of cycling--an average of only 14.5 km (9 miles) per

 working day--is of course derisory, and hardly worth mentioning. It is
 approximately 10000 litres, and this volume of fuel is burnt up in 23
 minutes--at 7.3 litres a second--by Concorde flying supersonically at
 2172 km (1350 miles) per hour. Unlike the cyclist, Concorde's fuel
 consumption varies considerably--from 24 to 18 tonnes per hour--as it
 burns up its kerosene and gets lighter, and assuming a payload of 100
 passengers this averages out at just over 7.7 km (4.8 miles) per litre
 per passenger. At subsonic speeds, for aerodynamic reasons, the fuel
 consumption is considerably greater. Cyclists do rather better. In
 terms of petroleum equivalent their use of energy works out at well
 over 353 km (220 miles) per litre,26 and in fact is nearer 495 km (310
 miles) per litre. Yet my bicycle does have one thing in common with
 Concorde--nobody, so far as I know, has ever accused either machine of
 inertia.
 
 In general in this paper I have been discussing inertia in its current
 colloquial connotation of lethargy or sluggish movement, but its
 strictly scientific meaning is "that property of a body by virtue of
 which it opposes any agency which attempts to move it, or, if it is
 moving, to change the magnitude or direction of its velocity." It is
 ironic that this scientific definition applies most nearly to the
 least logical and rational of all the groups which we have discussed.
 The discerning reader will need no guidance as to which group this is,
 for it is clearly not the ancient Romans and neither is it Neanderthal

 man.
 
 But I have been fortunate. I entered medicine when the NHS was newly
 established, and for 35 years it was a privilege to work in it--though
 not any more. This civilised and once hopeful ideal was put in hand at
 the end of a long and punishing war by a country which had suffered
 much, and which, perhaps because of this, was less attuned to greed
 and more to pity than the generation that now lays down our
 priorities. It is sad to see its gradual strangulation and the
 pitiless downgrading of its services.
 
 The Romans, too, had assets of some value before their civilisation
 also declined, and not least an exemplary clarity of language. In a
 previous paper I suggested that modern Homo sapiens was beginning to
 evolve, at least in cities, into two distinct subgroups Homo se
 propellens and Homo (urbanus) vehiculo constrictus.4 Perhaps we should
 now widen this latter subgroup to Homo urbanus inertia constrictus, a
 term which time will quickly shorten to plain Homo constrictus.
 
 We do not yet know whether it will be the construction of their
 environment, their arteries, or simply of their outlook on life that
 finally does for these pitiable creatures. It could well be a
 combination of all three, but their enfeebling inertia will lessen
 their numbers and by end of the Holocene they will be gone. There is
 every reason to hope that Homo se propellens (and Puella se
 propellens) will survive, and they will deserve to. They will walk, or
 run, or ride their bicycles. They will not pollute. Their fuel will be
 endlessly renewable whether it be caviar, cucumber, or chips, and the
 key to the future will be in their hands.
 
 References
 1. Angrist SW, Hepler LA. Order and chaos. Laws of energy and entropy.
 New York: Basic Books, 1967.
 2. Wilkie DR. Muscle. London: William Clowes, 1968.
 3. Cooper KH. The new aerobics. New York: Bantam Books, 1970.
 4. Williams RE. De motu urbanorum. BMJ 1975;iv:25-7.
 5. Anderson D. Energy and the environment. Edinburgh: Wealth of Nations
 Foundation, 1991.
 6. Smith R. Overpopulation and overconsumption. BMJ 1993;306:1285-6.
 [Medline]
 7. Department of the Environment. Climate change--a discussion document.
 London: Department of the Environment, 1992.
 8. Cairncross F. Costing the earth. London: Economist Books, 1991:133.
 9. Jouzel J, Barkov NI, Barnola JM, Bender M, Chappellaz J, Genthon C, et
 al. Extending the Vostock ice-core record of palaeoclimate to the
 penultimate glacial period. Nature 1993;364:407-12.
 10. Shine KP. The greenhouse effect. In: Russell Jones RD, Wigley T, eds.
 Ozone depletion, health and environmental consequences. Chichester:
 John Wiley, 1989:72.
 11. Holman C, Fergusson M, Mitchell C. Road transport and air pollution,
 future prospects. Oxford: Oxford University Press, 1991:11-5.
 (Discussion paper 25.)
 12. Kasting JF. Earth's early atmosphere. Science 1993;259:920-5.
 [Medline]
 13. Bryant C. Doing without oxygen. Biologist 1993;40:58.
 14. Fowler D. In: Cannell MGR, Hooper MD, eds. The greenhouse effect and
 terrestrial ecosystems of the UK. London: HMSO, 1990:10-3.
 15. Harriss RC. In: Ozone depletion, greenhouse gases and climate change.
 Washington DC: National Academy Press, 1989:79-84.
 16. Riley F. The Settle district and north west Yorkshire dales. Settle:
 Caxton Press, 1923:29.
 17. Department of Transport. Transport statistics for London 1993. London:
 HMSO, 1993:10-1.
 18. Department of Transport. Design manual for roads and bridges.
 Environmental Assessment Vol 11. London: HMSO, 1993.
 19. Lamb HH. Climate, present, past and future. Vol I. London: Methuen,
 1972:45.
 20. Walker JCG, Kasting JF. Effects of fuel and forest conservation on
 future levels of atmospheric carbon dioxide. Palaeogeography,
 Palaeoclimatology, Palaeoecology 1992;97:151-89.
 21. Neftel A, Moor E, Oeschger H, Stauffer B. Evidence from polar ice
 cores for the increase in atmospheric CO(sub 2) in the past two
 centuries. Nature 1985;315:45-7.
 22. Archer MD. Photochemistry and photoelectrochemistry technology
 assessment. London: Department of Energy, 1991.
 23. New Encyclopaedia Britannica. Vol 17, 15th ed. Chicago: Encyclopaedia
 Britannica, 1974:1018.
 24. Archer MD. Prospects for solar energy. Futures 1974 (June):261.
 25. Orwell G. Politics and the English language. In: Collected essays.
 Journalism and letters. Vol IV. London: Secker and Warburg, 1968:
 127-40.
 26. Rice RA. System energy and future transportation. Technology Review
 1972;74(3):31-6.