Push and pull
I was very interested in the description of the Pathfinder solar-powered aircraft (“Sunshine Flights”, 17 June). However, I am puzzled by one aspect of the design. Both the text and the photograph (p 32) suggest that this “flying wing” aircraft has “tractor” propellers, on the leading edge of the wing. Yet where maximum lift for minimum power is needed, the aerodynamics favour the “pusher” configuration 鈥� with the propellers on the trailing edge.
This is because the airflow over the wing is much less disturbed with this arrangement. With the propellers in front of the wing the airflow over the section behind them is relatively turbulent, thus increasing drag. Note that the US Air Force’s long-range strategic bomber (B70, I think) had pusher propellers, range being very important.
With propellers along almost the whole of the wing of Pathfinder it would seem that a useful gain might well be made by adopting the pusher arrangement. I wonder whether there is some structural reason for this not being used.
AeroVironment, which built Pathfinder, says the propellers do create turbulence, but because they are mounted well in front of the wing, the airflow has time to smooth out before passing over the surface. It also points out that tractor propellers produce two advantages. First, they increase the airflow over the wing which increases the lift artificially. Secondly, they help to stabilise Pathfinder which has no tail 鈥� Ed.
Solvent misuse
It is the letter (22 July) from Mark Richmond, Trustee of the National Gallery, and not my Forum article (3 June) which “seriously misleads” your readers: I did not “cite as evidence” damage done to paint samples when immersed in cleaning solvent. Rather, I reported research in which paint, “when swabbed for only a few minutes with the solvent propane-2-01”, commonly used at the National Gallery, was found to be as “substantially leached” as samples which had been immersed totally in the solvent for 24 hours.
It can only be alarming, therefore, that Richmond should dismiss as a “chimera” the problem of leaching when so many experts elsewhere testify to its reality.
S茅gol猫ne Bergeon, the former head of painting conservation at the Louvre, the present head of the French national school of restoration (IFROA) and chair of the international conservation centre in Rome (ICCROM) has reported that in cases of direct contact between solvent and paint films, the latter is “always damaged by leaching” and that this damage “is at its maximum when [as at the National Gallery] the varnish is totally removed”. The damage comprises: “weakening of the binder when in direct contact with the solvent [leaching], tearing off the ‘patina’ [the thin grey veil which covers the picture] and, even, the fragile superficial glazes 鈥�”
Given that so many attempts are being made worldwide to find safer cleaning materials and methods, might it not be prudent for the Trustees of the National Gallery to consider drawing on a wider pool of expertise and research findings than that contained within the gallery?
Old gold
Identifying the sources of ancient gold by trace element “fingerprinting” (Technology, 8 July): if only it were that simple. Joan Taylor is quite right to point out that “we have never been able to identify the geological source or origin of the metal in artefacts before”, but quite wrong to imply that we can do so now.
Archaeological scientists have been trying to do this for over 30 years and there have been monumental analytical campaigns using various spectroscopic techniques to attempt to characterise both gold and copper-based artefacts and match them to potential ore sources. These studies have proved spectacularly unsuccessful and the results treated with indifference by archaeologists.
John Watling and Hugh Herbert apply a new generation of analytical instrumentation (laser ablation ICP-MS) to the provenance problem. Highly accurate data for a wide range of elements can be produced, but the same fundamental problems that bedevilled the earlier studies remain. For trace element provenancing of gold to be possible it is necessary to assume that all of the gold occurrences exploited in prehistory have a specific and unique trace element fingerprint, but this is highly unlikely 鈥� however many elements are measured.
Although it is possible for gold to incorporate trace elements from host rocks and associated mineralisation, there is no reason to assume that individual gold deposits will be homogenous in composition. This is especially true of the placer deposits which are highly likely to have been the principal gold sources for the ancient world. Placers are often formed from minerals with quite different sources and can themselves be mixed and redeposited.
Even if compositional fingerprints could be determined for individual deposits, how unique would they be? There are many gold sources, perhaps quite insignificant in modern economic terms, which might have appealed to ancient prospectors. All would have to be characterised or, at the very least, their geology examined. To suggest that entire regions like Scandinavia have gold of a single composition is surely premature.
Finally, it must be remembered that we are dealing with what must have been the world’s first recycled material. Gold has always been precious and is continually mixed and remelted. There is no reason to think that this was not so in prehistory.
Analysing a gold artefact and finding it similar in composition to a source which you have in mind can never be proof that the artefact was made of gold from that source. Australian defence attorneys take note.
No mystery
You recently reported that a research ship is to investigate the idea that methane bubbling up from beneath the seabed in the Bermuda Triangle may be “one cause of the mysterious disappearance of ships and planes” (This Week, 1 July).
What “mysterious disappearance of ships and planes” is being referred to? Kusche (The Bermuda Triangle Mystery Solved, Harper, New York 1975) demonstrated quite convincingly the nonexistence of a Bermuda Triangle. This has been amply demonstrated by subsequent researchers. Why are we being subjected to the spectacle of a serious scientific study apparently being supported by the need to explain a nonexistent phenomena?
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Shell shock
Fred Pearce seems to have joined the government and media bandwagon in accusing Greenpeace of being “unscientific” over Brent Spar (Forum, 15 July). Perhaps you should instead highlight how Shell and the government have ignored, or not sought, the facts that would refute their theory that the rig could be safely sunk.
Aberdeen University’s “independent” AURIS report 鈥� one of the key reports on which Shell based its decision to sink the Brent Spar 鈥� did not investigate the contents of the rig, but relied on information from Shell. The contamination on Brent Spar has not been fully or independently analysed.
Nor does Shell know precisely what is in Brent Spar. The AURIS report states that contaminated seawater in the storage tanks “had not been analysed fully”. Only two samples of sludge from six tanks were analysed. Oil floating on top of the seawater was not analysed. The radioactivity of the scale in the tanks was not tested 鈥� instead an “estimate” was made.
Shell and the government assumed the Brent Spar would settle on the deep seabed unharmed, and that it would take thousands of years for the contents to be released. The AURIS report states that “it is likely that some or all of the tanks would split”, with the immediate release of the contents.
In a leaked memo [on the implications of dumping in shallow water] of 6 December 1993, W McMinn of the Ministry of Agriculture’s Fisheries Laboratory concludes that on the data he had concerning the known contents, the waste water in the Brent Spar “has to be considered very toxic” to marine life and “discharge should be prohibited”.
At least two European companies (Smit Engineering of Rotterdam and HeereMac) provided Shell with detailed reports on onshore dismantling. This work has now become routine. Nine North Sea platforms and over 900 Gulf of Mexico platforms have already been dismantled. Yet in the papers they published on Brent Spar, Shell has not once referred to these reports.
Finally, the government wishes to approach dumping case-by-case. Where is the study to assess the cumulative effect from dumping between 50 and 60 oil installations?
So much for the pseudoscience of the corporation. Congratulations to Greenpeace for revealing the unscientific nature of Shell and the government.
It is unfair to say that the oil companies had “decided” there should not be public debate 鈥� and had therefore “paid the price”. The industry has been debating the issues openly for a number of years, for example, at many public conferences. The fact is that until a few months ago, decommissioning British North Sea oil and gas installations 鈥� three-quarters of which will be totally removed 鈥� was not at the top of many agendas.
Although the subject has a much higher profile now as a result of Greenpeace’s actions, it does not invalidate the industry’s argument, supported by the government, that installations in deep water that do not have to be totally removed should be looked at on a case-by-case basis. Environmental assessments of the impact of any proposal must be part of each case, but so too must safety, which is of paramount importance. The cost benefit of different options should also be a part of the equation.
Despite being cast in the role of the villain, it is the industry that has been working hard to come up with solutions for the decommissioning issue. It continues to do this even after the media attention has died down.
It would seem to me that more than a little “emotion” raises its head in Pearce’s article. As one of the millions of lay people watching this debate (many of whom, like me, are not members of Greenpeace), I need more than powerful invective to sway me from the view that Greenpeace has done us a favour.
We are told that the Brent Spar contains hundreds of tonnes of toxic chemicals. I cannot see how we can be sanguine about the effect of dumping these at sea. I also fail to understand how decommissioning the Brent Spar offers a greater risk to the planet than, say, trying to harness nuclear power 鈥� something in which we are willing to invest enormous amounts.
The whole thrust of Greenpeace’s argument is that Shell and other producers have sold oil at an artificially low price: one which failed to include the cost of cleaning up afterwards, and thus makes disposal on land seem prohibitively expensive.
To a simple mind like mine, Greenpeace’s logic seems inescapable. We have to start to pay the real price of using our planet’s finite resources. Am I really being duped by a less than honest Greenpeace or is Pearce just one of the many commentators who are offering wisdom after the event?
Floating farms
I suggest that redundant North Sea oil platforms like the Brent Spar could be turned into marine farms large enough to help solve major global problems. Adapted and towed to the tropics, they could convert large areas of deep ocean from the equivalent of barren deserts to productive assets. This would generate new resources as oil and fish decline, and help solve the global greenhouse problem by turning infertile tropical ocean areas into plankton-rich carbon dioxide absorbers.
All the nutrients required for ocean fertilisation are present in the cold water of the deeps but need pumping up to the warm ocean surface. The temperature differential available could be utilised by a cheap adaptation of the well-proven principle of ocean thermal energy conversion: the water can be made to pump itself from about 400 metres and slowly propel the platform as well.
Local marine life, including floating algae, will proliferate. The algae can be harvested and pumped down to 5 kilometres. There, high pressure exists, causing a conversion to occur when a relatively small amount of heat energy is supplied to raise the temperature to about 500 掳C for 25 minutes. Syncrude [oil] will emerge together with water, the latter carrying most of the nutrients. A recycling feature is thereby provided and is essential for swamping the effects of excess carbon dioxide in water upwelled by self-pumping.
A study I conducted, which included a computer simulation of the marine ecosystem, showed that the output as oil would be equivalent to between 14 and 33 times the (free) energy used for pumping. An upwelling pipe 10 metres bore, wound from polyethylene gas piping and costing 拢1.8 million, would cultivate 220 square kilometres of open ocean, yielding fish worth 拢110 million (at 拢10 per kilogram) as well as 350 000 barrels of syncrude per year. No crop-confining barriers would be needed.
Under contract
Tom Dalyell wrote in his always very readable Thistle Diary (8 July) that he was stunned by an assertion he thought I made “that the number of people [researchers] involved in short-term contracts soared from 5000 in 1989 to 22 000 in 1994”.
I think I must correct this postprandial slip (whether mine or his I do not know) and ask you to print the correct aggregates of contract researchers (all disciplines) and timespans which are for 1977-78, 7568 and 1993-94, 21 453. The total number of contract researchers in science, engineering, technology and sciences allied to medicine are now close to 18 000: almost exactly the same as the number of established academic staff in the same disciplines. Our report should be available in a matter of weeks.
Fuel of the future
In your article “Are oxyfuels good for us?” (15 July) you correctly state that catalytic converters have greatly improved air quality in the US since their introduction in 1975. However, you overlook the fact that their penetration in Europe is far lower. For at least the next ten years in Europe, there will be more emissions from cars without the converters than from newer cars. This problem can be addressed by raising the quality of fuels.
The benefits of petrol containing oxygenates have been demonstrated in many industry trials, and, more importantly, by improved air quality in many cities in the US and Europe. Thus, the products receive strong endorsement from the US Environmental Protection Agency.
Moreover, it is inaccurate to suggest that the health effects have not been thoroughly studied. More than fifty studies have been conducted, and there is ample evidence that these components are benign. It is frustrating that you compare these serious studies with work which is scientifically questionable.
ARCO Chemical is not seeking to have oxygenated petrol mandated in Europe. We would prefer to see it installed via market mechanisms. It represents an inexpensive (an increase of about 1 per cent of pump price), simple and proven means of improving air quality. Our choking cities need not wait for year-2000 European legislation.
Mercury matters
Ben Iannotta states correctly that liquid mirrors cannot be tilted (“Spinning images from mercury mirrors”, 15 July). But having created the mirror, why not freeze it and provide it with a support? It could then be tilted. Any defects in the frozen mirror could be polished out or eliminated by selective melting and refreezing. The cost of maintaining the mirror frozen would not perhaps be great when compared with the cost of manufacturing a conventional mirror.
Iannotta writes: “In theory, there is no limit to the size of liquid mirrors”, and then mentions some of the practical limitations. There is another important limitation, and that is the Coriolis effect (due to the spin of the Earth), which limits the size at which the mirror can remain a parabola.
This appears to have been recognised by physicists as long ago as 1857. The Scottish opticist David Brewster, having visited the French physicist Jean Bernard Foucault in Paris in June that year, wrote in his diary: “Mr Foucault told me that a mercurial surface in revolution 鈥� becomes a parabola only at the pole.”
The history of the liquid mirror telescope idea seems to begin in about 1850, when E. Capocci and W. C. Krecke were conducting experiments in Naples and Utrecht, respectively. Brewster referred to an American named Buchan as being the originator of the idea, but he awaits identification.
Iannotta mentioned in his piece the work by Robert Wood in Baltimore in 1909. Wood credited Richard Carrington, who had an observatory near Redhill in Surrey where he gained fame for his sunspot observations, as the inventor of the idea, but Carrington’s work on this does not seem to have begun until the 1870s.
An earlier attempt in England was made by Henry Skey in the 1850s; Skey emigrated to New Zealand in 1860, finding there a plentiful source of mercury to continue his experiments, since mercury was used in the goldrushes for dissolving the precious metal out of its ore body. His successful model was demonstrated to the New Zealand Institute in 1872, and written about in Nature in 1874.
After Wood’s experiments, the liquid mirror concept lay dormant for a decade or so, until in 1922 some excitement was engendered by the announcement of plans to build a 50-foot diameter mirror at the bottom of a mine shaft in Chile, with which it was expected that any inhabitants of Mars would be observable. That project was never completed, so it is gratifying to see the experiments of the past century and a half at last resulting in useful astronomical observations.
Not for minors
Morgan Holt indicates that censoring Internet pornography should be achieved by technology (“For adults only”, 22 July). However, the problem does not need high-tech solutions.
The fact that the information is electronic makes no difference. The same rules covering traditional media should apply to electronic media. It’s the information content that is the concern, and not whether it is printed or on a computer screen.
The public can’t be blamed for getting the impression that the Internet is seething with seedy material when articles such as Holt’s don’t make it clear enough that it’s actually quite hard to find anything erotic, and nearly impossible to inadvertently stumble upon anything distasteful.
In fact, it’s far simpler and cheaper to get pornography via public newspapers, top shelf magazines, sex phonelines, sex shops or cable TV. These are the sources that are readily available to children in spite of legislation 鈥� and if anything should be banned it is these rather than erotic newsgroups.
The only crime is making material too distasteful or easily accessible by juniors, and a mandatory warning screen would suffice. If users could send their age with their logon, then it’s up to the service provider whether they want to break the law by permitting access. Unmediated newsgroups should carry similar warnings and age checks. Then nobody is breaking the law if anonymous adult material finds its way there. Once standardised, only small changes are needed to implement this scheme.
People can still break the law, but can’t avoid detection and remain public. As soon as police start prosecuting, service providers will realise that forgetting warning screens doesn’t pay.