NavList:

Your floating box works because the surface of the water is horizontal and, you hope, that the box will float so as to remain horizontal also. This will only work if the box is absolutely symmetrical so that its center of buoyancy is exactly above its center of gravity (COG) when the mirror is exactly horizontal, a tough thing to achieve. So by adding a rod with a weight mounted at its bottom to the bottom of your box at right angles to the mirror this weight will act like a pendulum and keep the box floating in the proper orientation. Since this construction might be hard to achieve you can make the position of the weight adjustable so as to get the weight directly below the proper center of buoyancy. I would mount three bolts or screws threaded into the sides of the weight that could be turned in or out to shift the center of gravity of the weight slightly to achieve the proper location of the COG.

You might be better off by eliminating the box and use a floating ball (which would be symmetrical) which would be free to float in any orientation. Mount the mirror on the top of the ball and have the weighted rod project down from the other side of the ball at right angles to the plane of the mirror and provide the same adjusting screws. This would then be a pendulous mirror with the weighted rod being the pendulum and the floatation provided by the ball acting as the pendulum pivot. The weight will hang straight down which will make the mirror horizontal. This set up is fundamentally different than yours in that the horizontal is determined my gravity acting on the pendulum and not by buoyancy effects due to the water surface being horizontal.

If this isn't clear, think of mounting a mirror on the top of a plumb bob, the bob will hang straight down and the top surface of the bob, with the  mirror attached, will be horizontal.

The MA-1 uses a pendulous mirror similar in function to this. A mirror is mounted on the top of a cone, perpendicular to the axis of the cone, and the bottom edge of the cone is weighted evenly which results in the COG of the cone being in the center of the cone on the axis perpendicular to the mirror. The cone is placed on top of a pivot point and the the center of gravity of the cone centers itself directly below the pivot point which puts the mirror in a horizontal position. This whole thing is immersed in liquid to provide dampening of the pendulum for use in flight. This whole assembly is only about one inch in size.


gl
Ken Muldrew wrote:

How about you put a bulb keel on it? A keel would provide a righting
moment and make it behave like a pendulous mirror horizon such as the
    

one in an MA-1 sextant.
    

Sounds interesting, but can you translate to English?

BTW, I tried it again last night (tonight is full on clouds and rain) and I got
much closer results, but at the end of my sights I shook the mirror box and
heard water inside it. So perhaps much of the error is due to my usual half-
assery in construction. I'm going to try to rebuild it with more care and see
how it works,

Ken Muldrew



  

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