William Dillwyn, a Quaker, who had asked to see evidence that was critical of the slave trade. The London Yearly Meeting of the Society of Friends decided shortly after to begin campaigning against slavery and a petition signed by 273 Quakers was submitted to parliament in July 1783. Sharp also sent letters to Anglican bishops and clergy and to those already sympathetic to the abolitionist cause. The immediate effect of the Zong massacre on public opinion was limited, demonstrating—as the historian of abolitionism Seymour Drescher has noted—the challenge that the early abolitionists faced. Following Sharp's efforts, the Zong massacre became an important topic in abolitionist literature and the massacre was discussed in works by Thomas Clarkson, Ottobah Cugoano, James Ramsay and John Newton. These accounts often omitted the names of the ship and its captain, thereby creating, in the words of Srividhya Swaminathan, "a portrait of abuse that could be mapped onto any ship in the Middle Passage". The Zong killings offered a powerful example of the horrors of the slave trade, stimulating the development of the abolitionist movement in Britain, which dramatically expanded in size and influence in the late 1780s. In 1787, the Society for the Abolition of the Slave Trade was founded. Parliament received numerous petitions against the slave trade and examined the issue in 1788. With strong support by Sir William Dolben, who had toured a slave ship, it passed the Slave Trade Act 1788 (Dolben's Act), which was its first legislation to regulate the slave trade. It restricted the number of slaves that could be transported, to reduce problems of overcrowding and poor sanitation. Its renewal in 1794 included an amendment that limited the scope of insurance policies concerning slaves, rendering illegal such generalised phrases that promised to insure against "all other Perils, Losses, and Misfortunes". (The Zong owners' representatives had highlighted such a phrase in seeking their claim at the King's Bench hearing.) The act had to be renewed annually and Dolben led these efforts, speaking frequently to parliament in opposition to slavery. The Slave Trade Act of 1799 was passed to make these provisions permanent. Abolitionists, notably William Wilberforce, continued their effort to end the slave trade. Britain passed the Slave Trade Act 1807, which prohibited the Atlantic slave trade and the Royal Navy enforced the Blockade of Africa. The United States also prohibited the Atlantic slave trade in 1808 and helped intercept