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Q: Why do American drains steam? This is not just a feature of Hollywood
movies, because I have observed steam emerging from drains in several American
cities, but never in Europe. The temperatures involved seem quite high as
some drain covers are hot to the touch.

* * *

A: In New York, at least, there is no mystery. The steam does not come
out of the sewers, but leaks from underground steam pipes that serve industrial
and commercial users. The steam is a by-product of electricity production,
in particular for the underground trains, which use a specially generated
600 volt direct current.

Pierre Jelenc New York

* * *

A: There are two reasons why American drains appear to steam. The first
only applies when it is very cold and very dry. Under these conditions the
warm soiled water in the sewer may actually vaporise when it hits the outside
air. Having spent the first 25 years of my life in the very cold American
Midwest, I admit that I have only seen this once or twice.

The second, most common, reason is because of the existence of both
public and private steam companies in many American cities. Steam has been
sold as a source of heat in the US since the turn of the century. One would
assume that what you see coming out of the drain is steam escaping from
the supply network – most of which is carried in tunnels for ease of access.
I understand that London once had a similar system.

Jeremiah Sheehan London

Sleeping sickness

Q: Why is a yawn infectious? Why, as you read this, do you find yourself
beginning to yawn?

* * *

A: The yawn is an evolutionary part of our body language. It was first
developed among the earliest primates and may be seen today in tribes of
baboons, or even scientists relaxing after another exciting day in the lab.
Among apes, the yawn is also a necessary preliminary to the day’s conclusion.

Because of the tribal hierarchy with its dominant male, opportunist
immatures, willing females, revenge-seeking bullies, and baby-stealing deprived
mothers, none alone can trust the others to keep the peace. Constant vigilance
is required; so the yawn has to be signalled infectiously by all in an increasing
acquiescence to the group’s prevailing mood for sleep. If the last to yawn
wider and longer than the others is the dominant male, warningly exposing
his canines in anticipation of an untroubled forty winks, it may be assumed
that he is sealing the evening’s truce. However, it is more likely that
he has just been watching too much of the World Cup on late night TV.

Ray Heaton Solihull, West Midlands

* * *

A: In our distant past, when we lived and hunted in groups, every member
of the group had a decent night’s sleep. To ensure this happened, and there
was no coming home late from the pub and waking up the neighbours, we all
needed to go to sleep at the same time as each other. Yawning is a subconscious
signal to others that we are tired and want to go to sleep and so they must
do so too. This signal is particularly intended for the younger members
of the group who persist in remaining active long after their bedtime. So
remember, parents, next time Johnny is running around driving you mad, start
yawning.

Nik Honeysett Brighton, East Sussex

Aromatic pees

Q: What is the aromatic compound excreted in urine after eating asparagus?

* * *

A: The odour which appears in urine after eating asparagus has been
the focus of sensory, genetic and analytical chemical research. A number
of different sulphurous compounds have been implicated, though one of the
most thorough investigations indicates that the effect may be caused by
a cocktail which includes methanethiol, dimethyl sulphide, dimethyl disulphide,
bis (methylthio)methane, dimethyl sulphoxide and dimethyl sulphone (Xenobiotica,
vol 17, no 11, Waring et al).

Possible precursors in the asparagus may include S-methylmethionine
and asparagusic acid. The difficulty of det-ermining exactly which compounds
are responsible lies in part with the many variations in production or perception
of the odour. Several studies have indicated that production of the odour
is a genetically determined (dominant autosomal) trait, exhibited by 40
to 50 per cent of adults (Experientia, no 43, Mitchell et al).

However, at least one other report has suggested that all people produce
the odour, but the ability of a minority of people to smell it is the genetically
determined factor (British Medical Journal, vol 281, Lison et al). It may
be that different individuals produce a different array of compounds and
also have a differential sense of smell to these.

D. J. Mela Institute of Food Research Reading, Berkshire

* * *

A: The ability to smell the substances is also genetically determined
and present in about 10 per cent of people. Not everyone finds it offensive.
Juvenal Urbino, in Gabriel Garcia Marquez’s Love in the Time of Cholera,
‘enjoyed the immediate pleasure of smelling a secret garden in his urine
that had been purified by lukewarm asparagus’.

J. K. Aronson Department of Clinical Pharmacology University of Oxford

This week’s questions – Wind Assistance

Wind assistance: The noise from distant roads, railways, and so on,
is heard much more clearly when the wind is blowing from the sound source
towards us, than when it is blowing against the sound. If you shout in a
gale, people downwind can hear, those upwind can’t. The speed of a gale-force
wind is only a few tens of kilometres per hour. This is almost negligible
when compared with the speed of sound. So, as sound is propagated by vibration
in the air, how does the relatively slow air movement of the wind have such
a noticeable effect when carrying sound downwind or upwind?

Brian Richards Preverenges, Switzerland

This week’s questions – Bubbling down

Bubbling down: At one of my local pubs there is a juke box. In the display
on the front are columns of liquid containing rising bubbles. As the bubbles
rise they get smaller – some of the smallest even vanish before they reach
the top of the column. Why do the bubbles get smaller rather than larger
as they rise, as one would expect?

Owen Petchey Cambridge

This week’s questions – Hair rising

Hair rising: At the moment of exposure during a recent X-ray, I distinctly
felt the hairs on my legs move. The radiographer insisted it was not the
discharge of an electroscope by ionising radiation, even though my recently-removed
trousers were a polyester/wool mix and the atmosphere hot and dry. This
explanation is out by several orders of magnitude when the amount of radiation
used is taken into consideration. Can anyone provide an alternative explanation?

Mike Harrison Godalming, Surrey

This week’s questions – Clear day blues

Clear day blues: Why (on a clear day) is the sky blue?

Jasper Graham-Jones Southampton