Any person or persons who have received a patent or patents on mortgages, for any land lying north and west of the rivers Ohio and Allegheny and Conewango creek, under the law of March 1, 1811,¹ entitled “An act to encourage the warranting and patenting of lands north and west of the rivers Ohio and Allegheny and Conewango creek,” and any person or persons who shall hereafter receive their patents under the said law, shall only be required to pay interest on the principal due on such mortgage, or upon such lands, from the expiration of five years after the date of the first improvement made on the lands therein described.