Snail鈥檚 pace
Question: Everyone knows that conventional letter post is referred to by
e-mail users as 鈥渟nail mail鈥�. I wonder if this dismissive term may be really
accurate.
The other day I posted a letter to my parents who live about 25 kilometres
away from me. It arrived two days later. I calculated that the average speed of
this letter鈥檚 journey was around 0.5 kilometres per hour.
If I had used the cheaper second-class post, the letter would typically
travel at 0.25 kilometres per hour.
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How fast does a snail travel? Is snail mail now actually slower than a snail,
or does Britain鈥檚 Royal Mail still have some way to go before it is challenged
by a lowly mollusc?
Answer: Snails move by sliding on their single foot. Specialised glands in
the foot secrete mucus, which lubricates the snail鈥檚 path. The snail uses the
muscles in the foot to glide smoothly over the slimy surface. The foot is so
strong that the snail can move along the edge of a sharp razor without hurting
itself.
Snails have been measured at speeds of 0.048 kilometres per hour, which means
that they are a factor of 10 slower than the first-class letter quoted in the
question. The common land snail travels at around a quarter of this speed. This
is significantly slower than a sloth (0.24 kilometres per hour) or a giant
tortoise (0.27 kilometres per hour). The fastest speed achieved by a snail in
the Guinness Gastropod Championship, held over a 13-inch (330-millimetre) course
in the O鈥機onor Don pub in central London, is only 0.0085 kilometres per hour.
This record is held by a mollusc called Archie, which took 2 minutes and 20
seconds to cover the course.
Jon Noad
Den Haag, Netherlands
Answer: The answer to this question is yes and no.
During a series of experiments involving the marine gastropod Gibbula
umbilicalis, I measured a mean speed of 0.0065 kilometres per hour when it
was in the presence of a predatory starfish, Asterias rubens. This
equates to approximately a 100-fold drop in performance compared with the
reader鈥檚 observations of the Royal Mail. For a true snail-mail equivalent one
would have had to post a first-class letter that was only destined to travel a
distance of around 300 metres. Hardly worth the effort!
The fastest molluscs of all are squid, which use water-jet propulsion to
reach speeds of 40 kilometres per hour while swimming to escape from predators.
Has anyone patented a watertight mailbag?
Douglas Taylor
Nottingham
Answer: I have not had the opportunity to measure the speed of snails but I
have recorded speeds of two species of slug. From casual nocturnal observations
they seem to move at comparable speeds.
The large yellow slug, Limax flavus, is a vigorous climber. At
night, several individuals climb the brick wall of our house to feed on the
lichen growing on the roof. It takes them about 50 minutes to climb 5.25 metres,
which corresponds to a speed of 0.0063 kilometres per hour鈥攆ar slower than
the 0.5 kilometres per hour quoted for snail mail.
The yellow slug is highly nocturnal. It leaves its lair on the ground well
after dark and has to return before first light. In the height of summer, this
means the roof-feeding individuals have a journey time of about an hour,
followed by about three hours grazing on lichen before the hour鈥檚 descent to
ground level.
A student of mine has measured speeds of several other slug species over
short distances. The mean speeds for species ranged from 0.001 to 0.0069
kilometres per hour. The keeled slugs (Milacidae) and round-back slugs
(Arionidae) were slower than the semi-keeled species (Limacidae). The fastest
species was the very common pest Deroceras panormitanum. If you go to
the slug races, put your money on this slug. Not only is it fast, it is also
very quick out of the starting blocks.
Incidentally, in 1968 I sent a parcel by British Rail from Launceston in
Cornwall to Spalding in Lincolnshire, a distance of about 400 kilometres as the
crow flies. The journey by road and rail (the railway line through Launceston
had been closed) took two weeks, giving my parcel an average speed of a little
over 1 kilometre per hour. According to your questioner, the speed of mail has
reduced to half this over the past 33 years.
If this small sample is representative, and the speed continues to reduce by
50 per cent every 33 years, before the end of this century it will be quicker to
write your letters in microprint on the shell of a homing snail.
Chris du Feu
Beckingham, Nottinghamshire
Travel tips
Question: Why are the wing tips on modern passenger jets upturned?
(continued)
Answer: One of your previous correspondents pointed out that gliders possess
the highest aerodynamic efficiency of all aircraft. This is true but it is wrong
to suggest that they do not benefit from winglets. Most current designs use them
and many older gliders have had them retrofitted.
While winglets on gliders do confer benefits, there was initially some
opposition to them. One well-known tale tells of a pioneer of glider winglets
offering a flight in his newly modified craft to an experienced pilot who
responded: 鈥淔ly in it? I wouldn鈥檛 walk under it鈥︹��
Jerry Niman
Stockport, Cheshire
This week鈥檚 questions
The bowl tolls: I recently bought a singing bowl, used in Buddhist rituals.
When struck with a mallet it generates a pleasant, steady tone that can last for
more than 30 seconds. The seller in Kathmandu told me the tone lasts so long
because the bowl is made from an alloy of up to seven metals. I suspect, though,
that its properties are more likely related to the cooling or smithing process
when it鈥檚 made. Can anybody provide the answer?
David Sokal
Durham, North Carolina
Ghostly flock: I live in the countryside and I鈥檝e noticed that when walking
home in the twilight the spring blossom on trees鈥攁nd even a flock of white
sheep鈥攁ppear to almost glow in the near darkness. Is there something
special about the twilight that produces this effect or is it just me? It is
certainly very beautiful.
Joanna Tagney
Cornwall