Antique Air Meter details

An antique brass machine for measuring wind speed.

The text in the label in the box reads (as much as I can read of it) as follows:

International Inventions Exhibition Awarded Silver Medal

Correction for Air-Meter No. 20

Add 30 per minute to the observed reading of the Air-Meter.

Note—Each measurement[?] is regulated so as to reduce the co[missing] to a constant quantity per minute, which must be[missing] in the example given.