1783, an anti-slavery movement began among the British public to end slavery throughout the British Empire. After the formation of the Committee for the Abolition of the Slave Trade in 1787, William Wilberforce led the cause of abolition through the parliamentary campaign. Thomas Clarkson became the group's most prominent researcher, gathering vast amounts of data on the trade. One aspect of abolitionism during this period was the effective use of images such as the famous Josiah Wedgwood "Am I Not A Man and a Brother?" anti-slavery medallion of 1787. Clarkson described the medallion as "promoting the cause of justice, humanity and freedom". The 1792 Slave Trade Bill passed the House of Commons mangled and mutilated by the modifications and amendments of Pitt, it lay for years, in the House of Lords. Biographer William Hague considers the unfinished abolition of the slave trade to be Pitt's greatest failure. The Slave Trade Act was passed by the British Parliament on 25 March 1807, making the slave trade illegal throughout the British Empire. Britain used its influence to coerce other countries to agree to treaties to end their slave trade and allow the Royal Navy to seize their slave ships. Britain enforced the abolition of the trade because the act made trading slaves within British territories illegal. However, the act repealed the Amelioration Act 1798 which attempted to improve conditions for slaves. The end of the slave trade did not end slavery as a whole. Slavery was still a common practice. In the 1820s, the abolitionist movement revived to campaign against the institution of slavery itself. In 1823 the first Anti-Slavery Society, the Society for the Mitigation and Gradual Abolition of Slavery Throughout the British Dominions, was founded. Many of its members had previously campaigned against the slave trade. On 28 August 1833, the Slavery Abolition Act was passed. It purchased the slaves from their masters and paved the way for the abolition of slavery throughout the British Empire by 1838, after which the first Anti-Slavery Society was wound up. In 1839, the British and Foreign Anti-Slavery Society was formed by Joseph Sturge, which attempted to outlaw slavery worldwide and also to pressure the government to help enforce the suppression of the slave trade by declaring slave traders to be pirates. The world's oldest international human rights organization, it continues today as Anti-Slavery International. Thomas Clarkson was the key speaker at the World