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Dan Allen asked in NavList 3450:
> |
> | It is my understanding that earlier instruments, which did not have
> | the double reflecting principle, could only measure their arc's
> | worth, so to speak.
> |
>   
Then George Huxtable replied:
> Dan is absolutely right, that until double reflection came in, instruments 
> could only measure a range of angles which corresponded to the geometrical 
> length of their arc. So the astronomer's quadrant could measure only to 90 
> degrees. similarly with the mariner's quadrant, when this was no more than a 
> 90-degree arc with a sight-tube attached and a plumb-bob to show the 
> vertical. Other quadrants, such as the Davis, appeared, and these all had an 
> arc, or two combined arcs, 90 degrees in length, to measure altitudes up to 
> the vertical. No doubling.
>
>   
There was another single reflecting instrument, which was invented in 
1660 by the Dutchman Joost van Breen: the spiegelboog (mirror-staff in 
English). This instrument was capable of measuring backward in a range 
from 10 - 90 degrees using a single mirror. I made two reconstructions 
of this instrument which I tested in the field and described in the SIS 
Bulletin no. 90, 2006. You can also use this instrument in a forward 
manner, in which case it will measure in a range from 90 - 170 degrees, 
although I have to say there is no proof this was ever done, apart for 
calibrating the instrument. Somewhere soon I will try to shoot some 
lunars using this instrument, see what it is worth for that.

If you want to know more, visit my website: 
http://www.dehilster.info/instrumenten/spiegelboog/index.html
If you click on image 22 you will get the full article on this instrument.

Nicol�s


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