This is an antique treen searce made from Maple wood and dating from the late 18th century. The top section consists of a semi circular bowl with  a metal perforated mesh in the centre. When the powder is sieved it will pass through into the base. The base is hollow and unscrews so that the powder can be retrieved.
Antique treen searces were used for sifting powders mainly for apothecaries, the sieved powder would then go on to be made into a paste for pills. 
These antique treen searces nowadays are particularly difficult to find.
This example measures 12.5cm w x 4.8cm h.