(now in Kenya), including Mombasa and Dar es Salaam. Over the next few years, however, almost all of these mainland possessions were lost to European imperial powers. The sultans developed an economy of trade and cash crops in the Zanzibar Archipelago with a ruling Arab elite. Ivory was a major trade good. The archipelago, sometimes referred to by locals as the Spice Islands, was famous worldwide for its cloves and other spices, and plantations were developed to grow them. The archipelago's commerce gradually fell into the hands of traders from the Indian subcontinent, whom Said bin Sultan encouraged to settle on the islands. During his 14-year reign as sultan, Majid bin Said consolidated his power around the east African slave trade. Malindi in Zanzibar City was the Swahili Coast's main port for the slave trade with the Middle East. In the mid-19th century, as many as 50,000 slaves passed annually through the port. One of Majid's brothers, Barghash bin Said, succeeded him and was forced to abolish the slave trade in the Zanzibar Archipelago by the British. He largely developed Unguja's infrastructure. Another brother of Majid, Khalifa bin Said, was the third sultan of Zanzibar and furthered the relationship with the British which led to the archipelago's progress toward abolishing slavery. British protectorate Control of Zanzibar eventually came into the hands of the British Empire; part of the political impetus for this was the 19th century movement for the abolition of the slave trade. Zanzibar was the center of the east African slave trade. In 1822, Captain Moresby, the British consul in Muscat pressed Sultan Said to end the slave trade by signing Moresby treaty. The Moresby Treaty was the first of a series of anti-slavery treaties with Britain. It prohibited slave transport south and east of the Moresby Line, from Cape Delgado in Africa to Diu Head on the coast of India. Said lost the revenue he would have received as duty on all slaves sold, so to make up for this shortfall he encouraged the development of the slave trade in Zanzibar itself. Said came under increasing pressure from the British to abolish slavery entirely. In 1842, Britain told Said it wished to abolish the slave trade to Arabia, Oman, Persia, and the Red Sea. Ships from the Royal Navy were employed to enforce the anti-slavery treaties by capturing any dhows carrying slaves, but with only four ships