 The tower is 14 metres (46 ft) high and has massive masonry walls about 3 metres (9.8 ft) thick with a number of apertures to let in air. Inside there is a massive column made of concrete. At night, the whole structure is allowed to cool, and during the day warm moist air enters the structure via the high apertures, cools, descends, and leaves the building by the lower apertures. Knapen ’ s intention was that water should condense on the cool inner column. In keeping with Chaptal ’ s finding that the condensing surface must be rough and the surface tension must be sufficiently low that the condensed water can drip, the central column's outer surface was studded with projecting plates of slate. The slates were placed nearly vertically to encourage dripping down to a collecting basin at the bottom of the structure. Unfortunately, the aerial well never achieved anything like its hoped-for performance and produced no more than a few litres of water each day.
