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Eric
I promised a copy of the heading and speed change correcting tales if you don't use the Mears Slide.  These are attached.  They're also useful for showing that at high speed a tiny heading change can produce a massive effect on Ho.  Therefore, if you're applying the corrections, you must always follow the bubble.

Unless you're deep into 'enactment' you won't need to worry about any of this with your Mk2a.  Just connect it to two AA batteries; pop it on the window ledge;  chock it level with playing cards; set it to the values on Franks GPS Anti-Spoof Pro; and shoot away.  Unless you've a fitness class going on on the floor above, you don't even need need to use the averaging mechanism.  Oops! did I give the game away.  Sorry chaps. 

If you end up with a Mk2, I believe you can shoot daytime shots using the bubble without power.  If you end up with a Mk2a or later, you always need 3V to light up the 'London Undergound Sign' pendulous graticule.  You can make an adapter from one of the spare black and white power plugs which come with the sextant and a 2xAA battery box.  I can tell you which of the three teminals in the plug to connect your batteries to.  DaveP

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