Letter: Left in the dark
I am surprised that John Feather, an energy consultant, believes that
he must keep his lights switched on for longer periods in order to make
the use of compact fluorescent lamps (CFLs) financially worthwhile (Forum,
9 December).
Assuming a cost of 6.5 pence per unit of electricity, an Pounds sterling
8 CFL consuming 18 watts over an expected life of 8000 hours should result
in a lifetime lighting cost of just over Pounds sterling 17. The equivalent
cost of 8000 hours of lighting using 75-watt incandescent bulbs (five 1600-hours
units at 50 p each) would be around Pounds sterling 41. Has Feather left
me in the dark regarding extra terms in the costing equation?
Alan Murta Manchester
Letter: Shapes of balloons
The short answer to John Maynard Smith’s question about the shape of
balloons (Letters, 11 November) is that the surface should have constant
mean curvature. This rules out ellipsoids other than perfect spheres. The
same condition governs the shape of a liquid surface, and the reason in
both cases is that the internal pressure must balance the effects of surface
tension, the latter being proportional to the mean curvature.
R. V. Taylor’s suggestion (Letters, 9 December) is to start with a curve
and form a tube that surrounds it at a constant distance. Unfortunately,
this construction doesn’t equalise the pressure: the assumption that it
does stems from considering curvature in one direction only and ignoring
any curvature at right angles to that. It’s approximately correct for long
thin tubes surrounding almost straight curves.
The longer answer is that the above is based on a number of possibly
inappropriate assumptions, for example, that the tension in the balloon’s
surface is the same at all points. In real balloons the thickness of the
skin, and its elastic properties, not only vary to begin with, but can change
in non-uniform fashion as the balloon inflates. The skin may be ‘prestressed’
by building in some initial curvature (for example, at the ends of sausage
shapes). Plastic deformation of the rubber skin, and temperature effects,
can also occur, making the question rather complicated for theoretical analysis.
The shortest answer is that a balloon can be any shape you want, provided
you design it that way.
Ian Stewart Mathematics Institute University of Warwick
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Letter: Save the fish!
I read with interest Colin Tudge’s article ‘Plenty more fish in the
lake?’ (Forum, 9 December). The situation facing the fish of Africa’s Great
Lakes is indeed serious, as increasing pressures from overfishing, alien
introductions and pollution continue to exert their effects, and the scenario
for freshwater fish in general is no better, with perhaps one quarter of
the world’s 8500 freshwater fish species currently considered to be threatened
with extinction.
So what can public aquaria do? Certainly they must devote more effort
to the captive breeding of carefully selected endangered fish, but we must
be pragmatic and realise that we will not be able to save more than a few
dozen or so species in this way. Public aquaria must continue to devote
more time and effort to conservation-orientated research, so that we can
better understand and tackle the problems around us.
Finally, of course, there is education. With attendances at all the
major public aquaria in the world increasing year by year, we have an audience
which we can inform, educate and motivate in the many and varied aspects
of aquatic conservation and yes, to do this properly we need not one national
aquarium but several large regional aquaria that can bring fish conservation
to the fore. Plans exist for major new aquaria in places like London, Plymouth,
Birmingham and Glasgow, but suitable financial backing is required to make
these plans a reality.
C. Andrews Assistant Curator, Aquarium and Insect House Zoological Society
of London Chairman, IUCN Freshwater Fish Specialist Group
Letter: Heated debate
David Slade (Letters, 16 December) seems to take a peculiarly one-eyed
view of his own statistics. On a scale from zero to 10 where 10 indicates
complete disbelief, he finds that 14 out of 22 ‘scientists engaged in greenhouse
research’ rate the evidence for an existing greenhouse warming lower (which
means higher, on this scale) than 7. Of the eight scientists who gave scores
in the range 7 to 10, it would be surprising, from what I recall of statistical
distributions, if more than two of them expressed ‘complete disbelief’.
So Slade’s own figures could be used to support the contention that 20 out
of 22 scientists (more than 90 per cent) agree that there is some evidence
that the greenhouse effect is upon us. A consensus if ever I saw one – and
evidence that statistics of this kind are meaningless.
John Gribbin Piddinghoe, East Sussex
Letter: Thailand's forests
I cannot let Dorothy Jackson’s letter on Thailand’s forests pass without
comment (Letters, 2 December). Thailand is the most stable and peaceful
country of the region, partly because of the deliberate policy of welcoming
all visitors and refugees. The hill tribes are welcomed, and positively
encouraged to join Thai communities. Many do. Some remain above the 3000-metre
level, below which opium cannot grow.
Recognising this, the King has sponsored many projects, including the
crop replacement schemes on Doi Inthanon, to encourage the hill tribes to
contribute to the Thai agricultural system.
Thai forests are in a sad state, but can be restored to a shade of their
former glory by the hard work and understanding of the Thais that live in
or near them. Thailand is fortunate in having some far-sighted conservationists,
in particular the local Bhuddist monks, as the World Wide Fund for Nature
has found in its excellent projects near Khao Yai National Park.
Barnaby Briggs London W2