NavList:

Just a couple of extra thoughts on the two horizon theme, if this was the C4, what about the C1 or even the A1.  Such a device would have been particularly useful in an airship if only a conventional marine sextant was available.  Although dip change v height change reduces with altitude, and dip tables go way up, the problem was knowing the surface pressure to set on a pressure altimeter in order to get an accurate height to enter the tables with.  A dip meter would skip all these problems.
Another possibility, with aerial ballistics there’s a climb and glide term in the foreword throw equation.  Is there a similar term at sea for the rise and fall of the ‘big guns’ in heavy weather?  Working backwards and with practice, this instrument could be used to calculate the rise and fall of the vessel.  German naval gunnery was frequently very accurate in both WW1 & 2 even after months in port.  DaveP