Massachusetts consequently passed an Act for the regulation of navigation and commerce, whereby they prohibited the exportation of any American produce or manufacture from their ports in vessels owned by British subjects after the 1st of August, 1785; with a provisional exception in favour of those British settlements whose governors should reverse their proclamations against the admission of American vessels into their ports. They also levied several extra duties to be paid by vessels belonging to foreigners, and especially by British subjects. There was, however, a proviso, containing a permission for newly-built vessels constructed in Massachusetts, though partly or wholly owned by British subjects, to take in cargoes upon equal terms with the citizens of the United States, but only for their first departures. Several States, following the example of Massachusetts, levied duties of various kinds on foreign tonnage. In some of the States 1_s._ per ton was imposed, while in others foreign vessels were subjected to a tax of from 3_s._ to no less than 5_s._ per ton, counterparts, in many respects, to the ancient navigation laws of England. However prejudicial to other nations, these high and conflicting rates led to a general misunderstanding among the States themselves, which contributed about as much as any foreign competition would then have done to check the progress of American navigation. But a common interest soon made it manifest to the people of the United States that these differential, or rather protective duties could not be maintained, and that some general regulations were essential to the safety and welfare of the Union, and to the development of its trade and navigation. In short, the different States found it absolutely necessary to part with a portion of their individual liberty in order to secure the combined and wholesome action of the entire Union. Indeed they soon perceived the necessity of confiding to Congress alone the power of regulating and controlling their intercourse with foreign nations; and, with this object in view, a convention was called to revise the articles of the confederation. [Sidenote: Constitution of the United States.]