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Bruce Stark is correct saying that a big advantage of tea is that it can
be drunk. On expeditions in arid climates this is a vital consideration.
However the other big advantage of tea is its weight compared to
mercury. Mercury on Australian expeditions was often packed in 2lb
flasks and they took several - a luxury on a pack horse where every
ounce was vital.

Also from practical experience in the Australian desert mercury is a
nightmare to handle. It attracts dust and leaves as soon as it is
decanted, immediately dulling the sheen. Cleaning mercury in the bush is
a time consuming and frustrating process. If spilt it is impossible to
recover which is a major advantage of tea. When a pannikin of tea is
kicked over, and it inevitably is at least once on every expedition, the
loss is not as great.

At night I find it particularly difficult to get mercury back in the
flask without spillage, especially if the pan is really full as it often
is to enable low altitude shots. This often requires two hands to hold
the pan and a lot of patience so it doesn't splash everywhere. Also
using two hands there is nothing with which to hold the torch. This has
been alleviated by head torches.

While Bruce is correct that there is no difference between tea and water
for sun sights I have not found water reflective enough for stars,
however bright stars can be captured with a tea artificial horizon.

Kieran Kelly






-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Bruce Stark
Sent: Wednesday, 9 July 2003 11:15 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Arificial Horizons and Tea


Kieran Kelly says that A. C. Gregory, the Australian surveyor and
explorer, preferred a pannikin of black tea for his artificial horizon.
I'd supposed the tea might prevent a reflection from the bottom of the
container, if that was bright. Got to wondering if perhaps tea had a
sheen that would make it show dim objects better. So, just now, I set
out two similar containers. One had water, the other the same amount of
strong black tea made with loose, rather than tea bag, tea.

With the sun's reflection I couldn't tell if there was a difference in
brightness. What I did notice was that the tea seemed to settle more
quickly after being ruffled by a breeze. Switching the places of the two
containers didn't change the effect, so it wasn't that the tea was less
exposed.

My quick experiment doesn't prove anything, and the effect, if there,
was modest. Perhaps others will examine the merits of tea more
carefully. According to Kelly, one of the reasons Gregory liked tea
better than mercury was that he could drink it after the observation.

Bruce

   

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