 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 . 
