artificial horizons, can anyone help"

[alan2]

In latest edition, 1994 edition of Mary Blewitt’s Celestial Navigation for Yachtsmen, page 53 under Practice Sights, one finds the following, re the Davis Type Artificial Horizon, essentially a dish of water or other liquid, with it‘s own sun shields, side panels and clear wind shields. “Corrections for index error and refraction must be applied to arrive at observed altitude.”
Fair enough until one looks at page 22 where one finds the following. "Nautical Almanac tables already contain refraction correction". Also, index correction is obtained from “zeroing” one’s sextant, then viewing the natural or sea horizon through the sextant’s optics. If there is a “break” in the horizon line, turn the micrometer wheel until it’s leveled out, and read the vernier. The number of minutes shown, if any, is the index correction. “If it’s on, take it off, if it’s off, put it on”, as the saying goes. Unless one has previously taken shots with this sextant, how does one arrive at the referenced index correction, or might this be a printing or editing error that got by?

Re the Celestaire Practice Bubble Horizon, page 13 of Celestaire’s 2009 catalog, see page 12 for Davis unit, one finds the following instruction for determining the BC (Bubble Correction). Using the Known Position Method, “take several observations from a known geographic position (GPS does serve a purpose), and compute the lines of position normally. You may attribute the average error to the bubble, and subtract as a correction (BC) for future use”. My questions follow.

Re the several observations mentioned, I assume the following. One takes several sets of sun shots. Reduce each set of shots, and plot them. You have already plotted your Known Position. Measure the displacement between calculated position plots and the plot of Known Position. Say you took three sets of shots, the average error, displacement between their plotted positions and that of Known Position being say 12 nautical miles, the BC would be 12 minutes of arc, 1 minute of arc equaling 1 nautical mile. Next question is, re “subtract this as a correction (BC)", subtract the obtained BC from what number. I use the USPS SR 96a form for reducing sights.

If it turns out that I’m way off base here, having seriously misunderstood the directions, someone please explain, so that I might correct the error of my ways. Thanks for reading through the above, and for the benefit of your wisdom, whomever you might be.