One of the first duty acts (1710) laid a restrictive duty of 40_s._ on slaves, and was eventually disallowed.[27] In 1712 William Southeby petitioned the Assembly totally to abolish slavery. This the Assembly naturally refused to attempt; but the same year, in response to another petition "signed by many hands," they passed an "Act to prevent the Importation of Negroes and Indians,"[28]--the first enactment of its kind in America. This act was inspired largely by the general fear of insurrection which succeeded the "Negro-plot" of 1712 in New York. It declared: "Whereas, divers Plots and Insurrections have frequently happened, not only in the Islands but on the Main Land of _America_, by Negroes, which have been carried on so far that several of the inhabitants have been barbarously Murthered, an Instance whereof we have lately had in our Neighboring Colony of _New York_,"[29] etc. It then proceeded to lay a prohibitive duty of L20 on all slaves imported. These acts were quickly disposed of in England. Three duty acts affecting Negroes, including the prohibitory act, were in 1713 disallowed, and it was directed that "the Dep^{ty} Gov^{r} Council and Assembly of Pensilvania, be & they are hereby Strictly Enjoyned & required not to permit the said Laws ... to be from henceforward put in Execution."[30] The Assembly repealed these laws, but in 1715 passed another laying a duty of L5, which was also eventually disallowed.[31] Other acts, the provisions of which are not clear, were passed in 1720 and 1722,[32] and in 1725-1726 the duty on Negroes was raised to the restrictive figure of L10.[33] This duty, for some reason not apparent, was lowered to L2 in 1729,[34] but restored again in 1761.[35] A struggle occurred over this last measure, the Friends petitioning for it, and the Philadelphia merchants against it, declaring that "We, the subscribers, ever desirous to extend the Trade of this Province, have seen, for some time past, the many inconveniencys the Inhabitants have suffer'd for want of Labourers and artificers, ... have for some time encouraged the importation of Negroes;" they prayed therefore at least for a delay in passing the measure.[36] The law, nevertheless, after much debate and altercation with the governor, finally passed. These repeated acts nearly stopped the trade, and the manumission or sale of Negroes by the Friends decreased the number of slaves in the province. The rising spirit of independence enabled the colony, in 1773, to restore the prohibitive duty of L20 and make it perpetual.[37] After the Revolution unpaid duties on slaves were collected and the slaves registered,[38] and in 1780 an "Act for the gradual Abolition of Slavery" was passed.[39] As there were probably at no time before the war more than 11,000 slaves in Pennsylvania,[40] the task thus accomplished was not so formidable as in many other States. As it was, participation in the slave-trade outside the colony was not prohibited until 1788.[41]