Adjustable Levels: What's Under The Hood?

I had hopes of making an adjustable, rather than fixed level, but after taking apart one of the antiques I bought, I see that I lack the necessary skills to do so.

This E. Preston and Sons level (foreground in the first image) includes plumb and level vials, each of which are housed in a "vial casket" (which sounds more like a shoddily constructed sarcophagus than it does an encasement for a spirit level).

The adjustment for both vials relies on a pinned hinge, spring, and machine screw.

The screw slides through the spring and threads into a tapped metal cup that supports the spring. By tightening or loosening the screw, the casket is pulled closer to or pushed further away from the brass top plate. 

It's a clever way to maintain the accuracy of the level. But how do you determine level in the first place?

Several commenters in the last post offered good suggestions. I had planned to find the most level surface in my shop by using manufactured levels and then shimming accordingly. Of course, who's to say that they're accurate?

Instead, I've decided to do as the Romans did by using a trough (in this case, a long glass baking pan), marking a line on each end at the same height from the bottom of the pan, filling it with water, and sitting it on a flat surface in my shop. Then, I'll shim as needed.

After that, I'll lay a large sheet of brass on top of the pan on which to place my shop-made level.  (A sheet of metal rather than wood because it's more likely to be perfectly flat.)

Old vials which did not have adjustment mechanisms relied on plaster to seat the vials.  I'm thinking of using silicone sealer instead because it will remain flexible as the wood expands and contracts with the seasons.

And speaking of vials, you may wonder where I got those two lovely glass ones at the top of the page. Thank you to my friend, Charles Davis, for sending these original, unused Stanleys to me.

They have black lines which help determine when the bubble is centered. Vials that did not have these lines required a center strip of brass on the brass top plate, which can be seen on the Preston level.

The glass vials that Charles sent are curved in a shallow arc to help the bubble find center.

Vials that were not curved made it nearly impossible to center the bubble. The most minute movement in the angle of the level caused the bubble to shoot toward one end of the glass or the other.

That's what I call a vial with a vile temperament.