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    Re: Azimuth compass?
    From: William Allen
    Date: 2004 Feb 19, 11:11 -0800

    Jim,

     

    Jim, I actually own a couple of WWII vintage “azimuth compasses”, so this may or may not be helpful to your contact with regard to 18th century tools.  The real difference between this and a regular steering compass (I have one mid 19th century steering compass/binnacle and a couple of WWII vintage) is that there is a bronze ring (usually blackened) that sits on top of the compass that can be turned through 360 degrees in order to line up mirrors, shades, and sighting vanes between the sun (and stars) and the degrees on the compass card.  Like a pelorus in some ways, the navigator can take an azimuth sight on the heavenly body in relationship to the compass card.  Of course, the steering compass must be properly compensated for all forms of subpermanent and transitory vertical and horizontal deviations. 

     

    We actually use some of these instruments occasionally on board my little vessel during our navigation classes just for the fun of practicing azimuth and amplitude sights – although we continue to tweak our adjustments and compensation for our steering compass (we are now trying to re-do the Flinders bar calculations and change compensation as needed).  So the definition given below seems fairly accurate.  Note, the compass itself is not “aligned” or turned in anyway – it is only an alignment of a visual reading of the degrees on the card with the reflection or sight of the body through the mirrors (with shades for sun) or vanes on the rotating azimuth ring.

     

    You should have your contact look at Ebay sometime and search for “Azimuth Ring” under Antiques: Maritime.  There are usually a couple of WWII vintage rings for sale (ranging in price from $50 to $100) and the pictures of the item might give your contact a better idea of how they work.  I have not seen the rings to fit early compasses in quite a while and I suspect they are rare these days.

     

    Bill Allen

     

     

    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Jim Thompson
    Sent:
    Thursday, February 19, 2004 10:36 AM
    To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
    Subject: Azimuth compass?

     

    Can anyone provide an answer for this person, and I will forward the answers to them?

     

    Jim Thompson
    jim2@jimthompson.net
    www.jimthompson.net
    Outgoing mail scanned by Norton Antivirus
    -----------------------------------------

    -----Original Message-----
    Subject: A plea for help....

     

    Forgive the intrusion, but I am a writer researching a novel about the 18C Royal Navy . 

     

    I wonder if you could help?   I have come across a reference to an “azimuth compass” on a voyage of exploration in 1790.   Most sites seem to refer loosely to an azimuth compass as being simply one which marks the card in degrees rather than rhumb-lines, but I suspect there is more to it than that.  The following definition from  the National Maritime Museum seems to bear this out:

     

     “The azimuth compass was a step up from the standard mariners’ compass. These compasses incorporated a means of aligning the compass with a celestial body such as the Sun or the Pole star. The reading from this alignment would then give another reading for north which could be compared with that given by the compass needle thus allowing the variation to be easily read.”

     

    Can you please tell me:

    1.  If the above definition is broadly accurate?

    2.  If so, I take it that the azimuth compass would be carried aboard separately from the steering compass?

    3.  Hand-held like a sextant?

     

    I would be hugely grateful for any light you can throw on this, as it’s always satisfying to get things right.

       
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