NavList:

Brad you wrote

I was reading HO551, Manual of Ice Seamanship.

After a discussion of abnormal refraction and the care a navigator should take in regards to it, the following interesting statement is made

"If the motion of the ship permits use of a bubble sextant, it can quickly be determined whether the apparent horizon is appreciably displaced"

The manual gives no further information. 
Am I to infer that a bubble sextant can be used as a dip meter.  Or is it merely a gross check, as in 'the horizon is far from normal'.

Has anyone tried this?  Can a bubble sextant be used as a dip meter?

I think it would depend upon the bubble sextant used and your definition of appreciable.  I’ve never used a marine sextant with a bubble attachment, so I can’t comment on that.  It is conceivably possible with an RAF MkIX or an RAF Periscopic Mk2.  The mirrors are set in ten degree steps and the counter reading is added to the number of tens.  Both start from a D (-10°) position.  I’ve not looked at my American sextants recently.  However, you’re unlikely to get closer than one minute accuracy and the slightest acceleration will reduce the accuracy considerably.  Three minutes is typical.  If three minutes accuracy is sufficient for your purpose, then you could use such a sextant as a dip meter.  I’ve never worked using the D position, but I believe it was incorporated, because at altitude it’s possible to see bodies below the surface horizon, Nories says 1° 38’ below from 10,000ft.  It was also quite useful over the sea for calculating a range from a distant coastal feature.  I vaguely remember there was a formula for this, but I’m afraid I’ve forgotten it long ago.  DaveP