What a first sight seems to be a clock with a missing hand and 12 hours dial continued by a second 24 hours round, is actually a tachometer.
This Instrument used to measure engine revolutions per minute, has in it´s interior a clock.
The propeller spin movement is transmitted to this instrument and the rotating movement charge the power of the clock just for a several seconds, that is inside the the piece in the photo below.
It has 4 jewels and a swiss pallet escapement, the rest is the mechanism for the indication per minute.
This instrument was used before and in several WW I aircrafts as:
BLERIOT XI
NIEUPORT 28
MORANE SAULNIER A-I
