In 1400 the Harde surrendered to Queen Margaret I of Denmark and remained within Ribe County. Until 1864 the western part of Föhr, together with Amrum, belonged to the Danish Enclaves in North Frisia while Osterland and Wyk belonged to the Duchy of Schleswig since it had seceded from the Danish Kingdom in the 1420s. Together with the Wiedingharde, the Bökingharde, the isle of Strand and Sylt, Osterland in 1426 signed the "Compact of the Seven Hundreds" (German: Siebenhardenbeliebung) with Duke Henry IV of Schleswig, which stated that the Hundreds intended to keep their judicial autonomy. In 1523 the northern marshlands of Föhr were shut off against the sea by dikes and 22 hectares of new farming land were won. Beginning in 1526, the Protestant Reformation began to introduce the Lutheran confession on Föhr, which was completed in 1530. In the 17th century a private navigation school was established in Süderende by pastor Richardus Petri which was the first of its kind on the island. It improved the situation of the seafaring population considerably and soon other navigators opened their own schools across Föhr. Although Petri led the Süderende school successfully for many years he never went to sea himself. Eschels (1757–1842) writes, however, that in his youth, learning to navigate was still unpopular among many young sailors from Föhr because it smacked of elitism. Eventually though, these navigation schools enjoyed a high reputation far beyond the island. Subsequently, whaling brought about a Golden Age for Föhr. During the 17th and 18th century most Dutch and English whaling ships would have a crew of Frisians from the islands. Around the year 1700 Föhr had a total population of roughly 6,000 people, 1,600 of whom were whalers. At the height of Dutch whaling in the year 1762, 1,186 mariners from Föhr were serving on Dutch vessels at Greenland and Svalbard and 25% of all shipmasters on Dutch whaling vessels were people from Föhr. In the late 18th century a thousand sailors, 150 Commanders among them, were living on Föhr. Still today the exquisitely decorated houses of the Commanders can be seen in Nieblum and Süderende. The so-called "talking tombstones" in the cemeteries of the three churches on Föhr account for their vitae. Yet with the decline of the whale populations ever fewer men would go sailing and the people of Föhr focused on agriculture again. Notable seafarers from Föhr include Matthias