To maintain a comfortable temperature inside such a large glass building was another major challenge, because the Great Exhibition took place decades before the introduction of mains electricity and air-conditioning. Glasshouses rely on the fact that they accumulate and retain heat from the sun, but such heat buildup would have been a major problem for the Exhibition, and this would have been exacerbated by the heat produced by the thousands of people who would be in the building at any given time. Paxton solved this with two clever strategies. One was to install external canvas shade-cloths that were stretched across the roof ridges. These served multiple functions: they reduced heat transmission, moderated and softened the light coming into the building, and acted as a primitive evaporative cooling system when water was sprayed onto them. The other part of the solution was Paxton's ingenious ventilation system. Each of the modules that formed the outer walls of the building was fitted with a prefabricated set of louvres that could be opened and closed using a gear mechanism, allowing hot stale air to escape. The flooring consisted of boards wide, which were spaced about apart; together with the louvres, this formed an effective passive air-conditioning system. Because of the pressure differential, the hot air escaping from the louvres generated a constant airflow that drew cooler air up through the gaps in the floor. The floor too had a dual function: the gaps between the boards acted as a grating that allowed dust and small pieces of refuse to fall or be swept through them onto the ground beneath, where it was collected daily by a team of cleaning boys. Paxton also designed machines to sweep the floors at the end of each day, but in practice, it was found that the trailing skirts of the female visitors did the job perfectly.