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Marvin Sebourn wrote, about using large "engineering" spirit levels to
adjust the plate of an artificial horizon-

> .... If you  read my post, you
>will see nothing suggesting that I used a level longer than 15  inches.
>
>Your suggestion that there is an inherent uncertainty  of setup using larger
>levels (because there removal might disturb the plate  alignment) is in error
>because of the incorrect assumption that the  levels were removed after
>alignment and prior to sighting use. They were  not, and they served to
>indicate the
>continued alignment of the  plate.
>
>Were you "horrified" (that is, shocked, made fearful, or disgusted) by
>considering the use of an eight inch spirit level? Really?

===============

Reply from George-

I was certainly surprised that Marvin got that arrangement to work.

Yes, if there's room on the plate to accept the continued presence of two
such large spirit levels, at right angles, and still leave room for a
reflected light path, then Marvin's argument holds water. But in that case,
just how big did the plate have to be, and did it then allow use over a
full range of altitudes and azimuths, without the light rays clanging on
the spirit levels with which they have to share the surface of the plate?
It would be interesting to know what the geometry was, and what
restrictions, if any, it imposed.

In general, artificial horizons were designed to be small, and therefore
portable, to be used by explorers and travellers. Marvin may have had no
such reason to restrict the size of his reflecting plate. I've no wish to
get into a big argument with Marvin about this matter: with no portability
limits on plate size, I accept that an arrangement such as he describes
could be made to work.

George.

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