Lindbergh's panel-- Ryan NYP-- what is this instrument and how was it useful?
Its an Inclinometer which provided pitch and roll information


  In the early 1920's, the US Government contracted Rieker Instruments
  Company to produce bubble style pitch and roll indicators for the fast
  emerging aircraft market.
  
  One of the most famous inclinometer fluid filled vial type
  installations - if not the most famous - was the use of a pair of
  Rieker glass tube instruments on the panel of the Ryan NYP "The Spirit
  of St. Louis".  In 1927 Charles Lindbergh chose the lightweight Rieker
  P-1057 Degree Inclinometer to give him climb and descent angle
  information.




(source)

This was useful to the Spirit of Saint Louis as it had effectively no forward visibility with the exception of the periscope. He may have also chosen this in the interest of weight as it would be much lighter than an early gyroscopic instrument.