All persons possessed of old rights, unsatisfied warrants, or other grants from the late proprietaries, be and they are hereby confined, in locating the same, to the lands already purchased from the Indians; and in order to prevent disputes touching the same, it is hereby enjoined on the commissioners appointed for making a further purchase, that they ascertain, in their negotiations with the Indians, with the greatest possible precision, the line between the lands already purchased and those that shall be by them purchased.