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Re: 1st type Hand held Bagnold Sun Compass
From: Geoffrey Kolbe
Date: 2020 Nov 28, 22:09 -0800
From: Geoffrey Kolbe
Date: 2020 Nov 28, 22:09 -0800
David Harrison wrote, "not sure how much she cropped from all of your contributions, but still looking for snippets about it...."
Sounds like somebody being overly anxious about GDPR and/or copyright issues. Anyway, here is the essense of the email exchange
My comment was,
"Firstly, and without looking through the books and papers of Bagnold or Shaw or others to remind myself how Bagnold and his chums navigated in the desert before the war, I should say I have no recollection of any record of a sun compass being produced before the war. This is certainly the first time I have seen this instrument or heard of its existence.
Secondly, it does not quite work on the same principle as a Bagnold sun compass, where the bearing to true North could be read directly off the compass. For the Bagnold sun-compass, a card with the sun co-azimuth for various times during the days scribed upon it is placed on the plate of the compass. The compass is rotated until the shadow of the gnomon aligns with the appropriate co-azimuth line for the time of day. You sight through the compass, aligning the gnomon with the object of interest and read the bearing off the compass edge. Or, if it is a vehicle compass, it will be fixed to the vehicle such that the bearing read off the edge is the heading of the vehicle.
Thirdly, the compass in question would not work in the Libyan desert, except in early morning or late evening when the sun is low on the horizon. You will note on Bagnold vehicle compasses and my own version with a telescopic antenna for a gnomon, the gnomon is very tall compared to the diameter of the compass plate so that at times near noon the short shadow will still fall on the edge of the compass plate. That instrument in question was never used in the desert.
It may be that E. R. Watts & Sons was tasked with producing a hand held sun compass, and they made something that could be used at our latitudes, but then it was found that it was no use in the desert, or the need for a tall gnomon was not realised for the specification.
The reason for needing the sun compass on a vehicle, by the way, was not the unknown magnetic variation (as David Pike suggested), but the way the steel in the vehicle altered the magnetic variation in unpredictable ways, particularly as in those days the chassis was used as the earth return for all the electric circuits and so created magnetic fields which varied every time something was switched on or off.
Too, there is a disconnect between a hand-held sun-compass, which one might use when hiking around the desert on foot, and needing to lug around Davies tables to get any sense out of the thing. The reason mine was hand-held was because I was a passenger in the vehicle and there was no way to mount the compass on the vehicle."
David Pike replied
"Just a thought; what about something like British Arctic Air Route Expedition 1930-31 or the East Greenland Expedition 1932-33? You’d have to wear gloves to hold it of course. They didn’t have many plastics in the 30s, so metal was probably next best despite the cold. Also, can you remember the name of the dear lady who gave us a talk at Greenwich I think about the RGS’s stock of instruments?"
And I responded
"Just because it is a "Bagnold" sun compass (which it is not, I contend), does not mean its intended use was in the desert. At least, not a hot desert. I seem to remember a lass called Jane Wess talking about instruments...?"
And at that point we ran out of ideas.
For those interested in my practical experience in, and comments on, navigating in the Libyan desert, I have written it up on my webiste at http://www.geoffrey-kolbe.com/C-Nav/index.htm
