Lindbergh's panel-- Ryan NYP-- what is this instrument and how was it used?
See this link-- it should allow you to zoom in by mousing around
Consider the "T" shaped instrument at the lower center--
The top (horizontal) part is a bubble level -- it acts exactly like a slip-skid ball except the bubble deflects in the opposite direction as a ball would--
The lower (vertical) part is some sort of pitch level. It is calibrated to show pitch attitude. The glass tube may possibly have a part that we can't see, that disappears into the panel.
So why how was the vertical glass tube intended to be used? Was it intended to provide useful information in cloud? If so, would it actually have provided any useful information, or not? Or was it just intended to display a precise measure of the aircraft's pitch attitude and thus angle-of-attack, during unaccelerated straight-and-level cruising flight in visual conditions, for the purpose of optimizing the airspeed and angle-of-attack for long-range cruising flight?
The calibration suggests that the instrument was intended more to detect large deviations than for fine-tuning the angle-of-attack in cruising flight in visual conditions, but it is hard to see how this would be of much practical use during actual cloud flying. It's pretty much analogous to a simple pendulum, but with more damping.