Natural ventilators

A natural ventilator surmounting a ventilator gap 14 in the roof of a building has convex outer side walls 30 and an upwardly divergent sided central structure 32 above spaced ventilator throat walls 38, all following the axis of the gap 14. Upper edges 50 of the throat walls 38 are outwardly flared to train air to each side of the central structure 32 and curved or bent upper baffles 64 guide air past upper edges 52 of the central structure. Those upper edges 52 are also preferably curved to assist and train air flow in avoiding turbulence. Also curved guides 60, 62 can be provided in from the ventilator throat into airways to each side of the central structure 32. The latter guides 60, 62 are shown pivotted to be capable of blocking-off the ventilator gap. The ventilator may be for a foundry. <IMAGE>