This still was in the ownership of Mr William Glen of Mains, Linlithgow with a licensed capacity of 52 gallons (roughly 236 litres).
The Wash Act of 1784 distinguished how distillers were taxed based on location, with a line drawn between the Lowlands and Highlands. In the Highlands, the capacity of the still determined the level of duty, but in the Lowlands, the wash was taxed per gallon.
Mr Glen has come up with a cunning system to increase the capacity of his Lowland still resulting in a vastly reduced tax bill. Not cunning enough for the eagle-eyed excise man who uncovered this secret compartment.