These are side panniers worn under various Georgian style gowns to keep the fabric of the gown away from the legs. They were worn with a petticoat over to hide the ridges of the boning.
These panniers are made from cotton twill fabric. The boning is plastic-coated continuous steel. 
These panniers fasten with ribbon ties. They will expand from a size 8 to fit up to a UK size 18

White Cotton Side Panniers