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    Re: Astrofixes and Len Beadell
    From: Kieran Kelly
    Date: 2004 Apr 29, 17:57 +1000

    Greg Gilbert wrote:
    "1. Do the levelling devices on theodolites provide a better "horizon" than
    an artificial horizon used with sextants?"
    No definitely not. After using a theodolite several times the frustrating
    thing is that every time you touch the instrument the bubbles seem to move.
    Also if there is an error in the bubble it is quite a complicated procedure
    to allow for. The great advantage of a theodolite is that it gives azimuth
    as well as altitude and it is free from the "wobbly hands" of an observer.
    A mercury pan horizon conversely gives a true horizon distorted only by the
    negligible impact of local gravitation on the mercury. It is impossible
    therefore for the pool not to provide a "true" horizon. The problem with a
    mercury artificial horizon however is that it needs a glass cover known as a
    cloche and therein lies a source of error if the glass in not ground
    optically flat. That said, like a theodolite, an artificial horizon cloche
    error can be eliminated by simply taking sights with the cover in one
    direction then reversing it for another round of sights. Don't underestimate
    that fact that Dolland et al knew a thing or two about instruments and in
    all the time I have been using an 1850 Dolland AH I have detected not the
    slightest error that I could attribute to the cover.
    "2. When did theodolites become generally accepted for land based exploring,
    and replace sextants and artificial horizons?"
    Don't know but sextants were still being used in Australia right up until
    Madigan's expedition across the Simpson Desert in circa 1934.
    "3. Why didn't more early explorers use theodolites for astronomical
    observations and navigation?  I understand that Thomas Jefferson used a
    theodolite in his own surveying practice, but did he equip the Lewis and
    Clark expeditions with one?"
    Don't know but may have been due to transportation difficulties. A
    theodolite is very difficult  to pack on the back of a horse or a camel. It
    has long, very heavy legs that would be difficult to accommodate on a pack
    animal. Also a sextant is a much more robust instrument than a theodolite.
    Break the bubbles on a theodolite and it is useless. Not so a sextant. If it
    gets bumped you simply adjust the index error and carry on.
    "A question to Kieran Kelly, whose two books on Gregory and the Tanami
    Desert I really enjoyed:  why did you use a sextant and artificial horizon
    when crossing the Tanami and not a theodolite?  Was it a matter of
    historical accuracy in replicating Gregory's observation methods, or just a
    matter of weight and convenience when packing camels?  Did Gregory use a
    theodolite on any of his Australian exploring expeditions?"
    A sextant is a very compact, rugged instrument as is an artificial horizon.
    During my last expedition, both were stored on the back of a camel for
    nearly six weeks and the index error on the sextant was the same at the end
    as it was at the start. Both sextant and artificial horizon fitted neatly
    into a hard plastic pannier used for transporting food and were surrounded
    by packing material such as dried peas, packets of beef jerky and freeze
    dried meals. There must be a way of tying theodolite legs on the back of a
    pack horse or camel but it would be a needless extravagance. Getting a
    shovel or shotgun to stay put is bad enough. I should imagine if you break
    the legs on a theodolite tripod the instrument becomes useless. (Or you
    carve more legs which sometimes happened in the field.)
    Furthermore there is the time factor. I would pull the AH out, pour in the
    mercury, slap the
    cover on, take the sights, pack it all up and put it away - no fuss - half
    an hour tops. Mucking around in the middle of the night, with no light, in
    the middle of a desert trying to get bubbles level and light the fire, cook
    a meal and tend to camels would have driven me nuts.
    Theodolites are for people who go exploring with vehicles not pack animals.
    Gregory used a theodolite on his professional surveying activities in
    Western Australia but never on his exploring trips and there is a big
    difference between surveying and exploration. To draw another analogy, Mason
    and Dixon were surveyors, L & C were explorers. They are two different
    professions  trying to achieve two different ends although they both use the
    celestial  bodies for navigation.
    Hope this helps and thanks for buying my books.
    Kieran Kelly

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