Dutch engineer who found fortune in Italy. He worked for Clement IX (1600-1669, Pope from 1667), notably on designing waterworks to regulate the Tiber. These projects were discussed in L'Arte di restituire a Roma la tralasciata navigazione del suo Tevere [The Art of restoring to Rome the neglected navigation of her Tiber] (Rome, 1683). Earlier, in Pisa, Mejjer had worked on the regulation of the Arno at the request of Cosimo III (1642-1723), to whom he dedicated the Nuovi ritrovamenti [New findings] (Rome, 1696). He was a member of the Physico-Mathematical Academy of Rome.